HVAC Guide for Everyone - Key Terms, Systems, and Refrigerants Explained

A Note from Mike Haines: Hi there, folks. Mike Haines here, and if you’ve ever stood in front of a buzzing condenser unit on a July afternoon wondering what exactly makes that thing tick – you’re in the right place. Whether you’re just getting started in HVAC, brushing up after a few years in the field, or simply curious about how your home stays cool in summer and warm in winter, this guide is for you.

I’ve spent decades teaching HVAC to techs, students, and curious homeowners alike, and one thing I’ve learned: understanding how heating and cooling systems really work gives you a massive edge. You stop guessing. You start diagnosing. And honestly? You start appreciating the sheer brilliance behind how these systems move heat around to keep us comfortable.

In this guide, we’re going to break things down the Mike Haines way – clear, straightforward, and packed with practical takeaways. No fluff, no unnecessary theory. Just solid explanations, real-world analogies, and helpful memory tricks to lock in the basics.

So grab a cup of coffee, get comfortable, and let’s dive into the world of compressors, coils, and cold air. Trust me – by the end of this, you’ll be thinking like a seasoned tech, whether you’re on your first week of HVAC school or just trying to keep your house running smoothly.

Let’s get to it.

– Mike Haines

Heating, Ventilation, and Air Conditioning – better known as HVAC – is all about controlling indoor climate and air quality. If you’ve ever adjusted your thermostat on a hot day or huddled near a heater in winter, you’ve benefited from HVAC technology. But how do these systems actually work? This guide breaks down the fundamentals of HVAC in a friendly, conversational way. It’s designed for HVAC students, curious beginners, and even seasoned technicians looking for a refresher. We’ll explore essential terminology, from components like condenser, evaporator coil, compressor, air handler to technologies like inverter and concepts like SEER ratings. We’ll also get into different system types – central split systems, package units, ductless mini-splits – and how they operate in cooling, heating, and even defrost mode. Along the way, we’ll share handy mnemonic devices to help you remember key terms and principles.

Importantly, we’ll discuss the refrigerants that make modern air conditioning possible – including today’s common refrigerants like R-410A and R-32 – and why environmental regulations are driving changes in those chemicals. By the end of this extensive guide, you’ll have a solid grasp of HVAC basics and beyond. Whether you’re training to become an HVAC technician or simply want to understand the system that keeps your home comfortable, this evergreen HVAC knowledge hub has you covered. Let’s start our journey into the world of compressors, coils, and comfort!

HVAC Basics: How Does Heating & Cooling Work?

At its core, an HVAC system moves heat around to create a comfortable environment. It may sound surprising, but an air conditioner doesn’t actually “create” cold air out of thin air – instead, it removes heat from inside your home and dumps that heat outside. The process is based on some fundamental physics: when a special fluid called refrigerant changes from liquid to gas, it absorbs heat, and when it changes from gas back to liquid, it releases heat. HVAC systems harness this principle to provide cooling in summer and (in heat pumps) heating in winter.

Think of your home’s air conditioner like a refrigerator or even like your body’s circulatory system. There’s a cycle in which refrigerant flows through various components – we’ll cover each one in detail soon – to pick up heat from inside and carry it outside. In a cooling cycle, warm indoor air passes over a cold coil filled with low-pressure refrigerant; the refrigerant absorbs heat (and also moisture, dehumidifying the air) and evaporates into a gas. Then a pump called the compressor drives that hot gas to the outdoor coil, where the refrigerant releases its heat to the outside air and condenses back into liquid. Finally, the refrigerant flows back and repeats the cycle. The result: cooler, drier air inside your space, ahhhh!

For heating, the process can run in reverse (if you have a heat pump) – drawing heat from outside (even chilly air contains some heat energy) and releasing it indoors to warm the house. If you have a furnace instead of a heat pump, heat is generated by burning fuel or using electric resistance, but it still uses the HVAC system’s air movement to distribute that warmth through the house. We’ll delve deeper into heating modes later. The key point is that HVAC is all about moving heat in the direction we want it to go.

One handy way to remember this: “Cold” is just the absence of heat. An air conditioner works by taking heat out, not by adding cold in. So whenever you feel cool air from a vent, it’s because heat was extracted and sent somewhere else. The scientific principles behind this come from thermodynamics and the refrigeration cycle, but don’t worry – we’ll keep things straightforward. Just know that an HVAC system is basically a heat transporter, shuttling energy from one place to another to make you comfortable.

Essential HVAC Components and Terms

Now that we have a high-level idea of what an HVAC system does, let’s zoom in on the individual parts that make it all happen. A typical air conditioning system (the cooling side of HVAC) relies on a handful of key components working together. Each component has a specific job in the refrigeration cycle we just described. We’ll also mention some other important elements found in HVAC systems and a few related terms. By understanding these, you’ll be speaking the HVAC language in no time!

Below are the most essential components and concepts, explained in plain language. We’ll throw in some mnemonic tricks along the way to help you remember them.

Thermostat

Every HVAC journey begins with the thermostat – the device on your wall that you set to your desired temperature. The thermostat is essentially the control center or brain of your HVAC system. It constantly measures the indoor temperature and decides when to turn the heating or cooling on or off to maintain the temperature you set (the "setpoint"). For example, on a hot day, if you set 72°F, the thermostat will signal the air conditioner to kick in whenever the room goes above that. Likewise, in winter, it tells the heater when to fire up if the temperature drops below your setpoint.

Modern thermostats can be simple dials or advanced programmable and smart thermostats that learn your schedule or can be controlled via phone. Regardless of type, the thermostat’s job is the same: keep the indoor climate where you want it by cycling the HVAC system on and off appropriately. If you think of the HVAC system as a team of components, the thermostat is the coach calling the plays.

Mnemonic: To remember what a thermostat does, think of "thermo-stat" as in "thermal status." It monitors the thermal status (temperature) of your space and takes action to change that status when needed. Another way: the thermostat is the "temperature boss" – it bosses the HVAC system around to keep you comfy!

Evaporator Coil

The evaporator coil is a vital part of the cooling system, typically located inside the indoor unit (like inside an air handler or attached to a furnace in a home). It’s called “evaporator” because this is where liquid refrigerant evaporates into a gas, absorbing heat from the surrounding air in the process. In cooling mode, warm air from your house is blown over the cold evaporator coil; the refrigerant inside the coil picks up the heat from that air (and also pulls out moisture, which condenses on the coil). By the time the air comes out the other side of the coil, it’s much cooler and less humid – ready to be circulated back into your rooms.

If you peek inside an air handler, the evaporator coil looks like a network of copper or aluminum tubes bent into a coil shape, often with many thin metal fins to increase surface area. It might form an A-shape (so sometimes called an “A-coil”) or be a flat “slab” style coil. When the AC is running, this coil is very cold (often around 40°F surface temperature) and will even be dripping water from the humidity it removes from your air – that water drains away via a condensate drain.

A healthy evaporator coil is crucial for effective cooling. If it gets dirty or if airflow is reduced (say by a clogged filter or a failing blower), the coil can get too cold and freeze into a block of ice – which means no cooling until it thaws. That’s why filter changes and maintenance are important (more on that later).

Mnemonic: Evaporator = indoor cold coil. Remember that it evaporates refrigerant. Also remember that the evaporator evaporates heat out of the indoor air (absorbing that heat into the refrigerant). Some folks use the phrase “Evaporator Evaporates (to take heat)” to keep it straight. It’s the component making your inside air cold by sucking up heat.

Blower (Fan) and Air Handler

Moving air through the system is just as important as cooling or heating it, and that’s the blower’s job. The blower is a fan – usually a centrifugal fan that looks like a hamster wheel – that pushes air across the coils and through your ducts. The blower is typically housed in the air handler, which is the indoor unit that also contains the evaporator coil (and possibly a heating element or furnace). In a system with a furnace, the furnace’s blower does double duty in the summer, blowing air over the evaporator coil for AC.

The blower draws warm return air from your home (through return ducts and the air filter), forces it over the cold evaporator coil to cool it down, and then sends the cooled air back into the rooms via the supply ducts. The strength of the blower is measured in airflow (like CFM – cubic feet per minute). Many modern systems have variable-speed or multi-speed blowers so they can ramp up or down to fine-tune comfort and efficiency.

The term air handler often refers to the whole indoor unit when it’s not a furnace – basically the combination of the blower fan, the evaporator coil, and its enclosure, plus any control electronics. Air handlers are common in heat pump or straight-electric systems. If you have a gas furnace, that acts as the air handler when the AC is running. Either way, without a blower to move air, the coils can’t do their job. No airflow means no cooling (or heating) distribution!

Mnemonic: Air handler = handles air. It’s literally handling (moving) the air around your home. And the blower blows air – easy enough to remember. Picture a hand fan; an air handler is the powered equivalent that “fans” the air throughout the house.

Compressor

If the thermostat is the brain, the compressor is the heart of an air conditioning or heat pump system. The compressor is a pump (usually located in the outdoor unit) that pressurizes the refrigerant gas and keeps it moving through the loop of coils and pipes. When refrigerant comes back from the indoor evaporator as a low-pressure gas carrying the heat it absorbed, the compressor “squeezes” that gas to a much higher pressure. Compressing the gas also makes it very hot (hundreds of degrees – think of how a bicycle pump gets warm as you inflate a tire).

The hot, high-pressure refrigerant gas then flows into the next component (the condenser coil) where it can dump that heat. In essence, the compressor maintains the pressure difference between the cold side (evaporator) and the hot side (condenser), which is what drives refrigerant around and enables heat transfer. It’s the workhorse of the system and typically the component that uses the most electricity.

There are several types of compressors – reciprocating (piston-like), scroll (spiral-shaped – very common in modern units), rotary, etc. Most residential systems have a hermetically sealed compressor motor, so you usually don’t see the compressor itself (it’s sealed within the outdoor unit). You’ll just notice the outdoor fan spinning and hear the compressor’s hum when it’s running.

Because the compressor is so crucial, a failure here is a big deal – it can cripple the AC. That’s why maintaining proper refrigerant charge and keeping the outdoor condenser coil clean is important: it prevents the compressor from overheating or overworking. Many systems have safety controls to shut off if pressures get too high or too low, to protect the compressor.

Mnemonic: Compressor = pump + pressurizer. It compresses (squeezes) the refrigerant. Think of it as the "heart" pumping refrigerant through the system’s arteries. One trick: both "compressor" and "heart" have an R near the end – stretch it a bit and say the compressor is the heart. Or just remember the obvious from the name: it compresses the refrigerant gas.

Condenser Coil

The condenser coil is the counterpart to the evaporator coil. Found in the outdoor unit (that big box outside, usually with a fan on top), the condenser coil is where the hot, pressurized refrigerant gas from the compressor is cooled and condensed back into a liquid. As the name suggests, it “condenses” refrigerant by removing heat. The outdoor fan blows outside air through the condenser coil. Since the refrigerant in the coil is super hot (hotter than the outside air), heat flows from the refrigerant into the outside air, which the fan helps disperse. You’ll feel very warm air blowing off the outdoor unit when the AC is running – that’s the heat removed from inside your house!

As the refrigerant loses heat, it changes from a gas back to a liquid (like steam condensing into water when cooled). By the time refrigerant leaves the condenser coil, it’s a high-pressure liquid. It then goes toward the evaporator coil again, passing through the expansion valve on the way. The condenser coil typically looks like a “radiator” wrapping around the outdoor unit (with lots of metal fins to dissipate heat) and a fan that pulls air through it.

Keeping the condenser coil clean and unobstructed is important. If it gets coated with dirt or leaves, it can’t release heat effectively, and your AC will struggle (and the compressor could overheat). That’s why it’s good practice to gently wash off your outdoor coil annually and make sure there’s no debris blocking it.

Mnemonic: Condenser = outdoor hot coil. It condenses refrigerant from gas to liquid, dumping heat outside. An easy way to remember: “Evaporator chills, condenser kills (the heat).” Or simply pair them: evaporator absorbs heat indoors, condenser releases heat outdoors. Evaporator evaporates; condenser condenses.

Expansion Valve (Metering Device)

Between the condenser and evaporator in the refrigerant loop lies a small but crucial component: the expansion valve (also known as a metering device, TXV/thermostatic expansion valve, or fixed orifice). After refrigerant has condensed into a high-pressure liquid in the condenser, it needs to become a low-pressure, cold liquid again so it can evaporate in the evaporator coil. The expansion valve creates a pressure drop that meters the flow of refrigerant into the evaporator.

When the high-pressure liquid refrigerant passes through the expansion valve’s tiny opening, its pressure plummets. This sudden drop causes some of the liquid to instantly flash into vapor (which cools it) and lowers the temperature of the remaining liquid. The result is a cold, low-pressure mix entering the evaporator coil, ready to soak up heat again. Essentially, the expansion device resets the refrigerant from “high-pressure hot” back to “low-pressure cold” to continue the cycle.

There are different types: thermostatic expansion valves adjust flow based on temperature, while simpler systems might use a fixed-size orifice or capillary tubes that allow a set amount of refrigerant through. If an expansion valve malfunctions (sticks open or closed), it can flood the evaporator with too much refrigerant (hurting cooling and risking compressor damage) or starve it (causing poor cooling and coil freeze-ups).

Mnemonic: Expansion valve = pressure dropper. It expands the space (pressure-wise) for refrigerant, which cools it down. Think of an aerosol can: pressurized liquid inside expands and cools when it sprays out. A memory phrase: “Expanding (pressure) lowers the temp.”

Refrigerant

We keep mentioning it, so let’s highlight refrigerant – the special fluid that circulates through all these components to transfer heat. Refrigerants go by names like R-22, R-410A, R-32, etc. They’re chosen for their ability to easily change from liquid to gas and back within the temperature ranges the HVAC system operates, effectively carrying heat with them.

Older systems often used R-22 (commonly called Freon), which has been phased out due to ozone layer damage. Modern systems use R-410A (brand name Puron) which is ozone-friendly but does contribute to global warming if released. Newer systems are adopting R-32 or other blends with lower environmental impact (more on refrigerants in a later section). But no matter the type, the job of refrigerant is the same: carry heat. It’s the blood of the AC system, flowing in a closed loop of copper lines, never getting “used up” unless it leaks out.

If your system is low on refrigerant, it means there’s a leak (refrigerant isn’t consumed like fuel). Handling refrigerant requires training and certification (EPA Section 608 in the U.S.) because venting it harms the environment, and refrigerants can be hazardous if mishandled (e.g., suffocation or frostbite dangers, and some are flammable). As a beginner, just recognize refrigerant as the working fluid that makes refrigeration possible.

Mnemonic: Think of refrigerant as the “heat carrier” or “thermal messenger.” The word refrigerant looks like “refrigerate” (to cool something) – because refrigerant is what makes cooling happen. You might recall old names: R-22 (Freon) = ozone bad, R-410A (Puron) = current standard, R-32 = next-gen. But core idea: refrigerant = the fluid that moves heat around.

Ductwork

Many HVAC systems (especially central air) use a network of ducts to distribute cooled or heated air throughout a building. Think of ductwork as the arteries of your home’s HVAC, delivering air to rooms and bringing it back for reconditioning. There are supply ducts (carrying conditioned air to rooms through vents/registers) and return ducts (bringing air from rooms back to the HVAC unit through return grilles).

Ducts can be made of sheet metal, fiberglass board, or flexible plastic tubes, running through attics, crawlspaces, walls, etc. Proper duct sizing is important – too small restricts airflow, too large can reduce air velocity and affect temperature control. Also, ducts should be well-sealed and insulated; leaky ducts waste energy by dumping air where it isn’t needed (like an attic), and uninsulated ducts can lose coolness or heat as air travels through them.

You’ll hear terms like plenum (the large air distribution box connected to the HVAC unit where ducts branch off), registers or grilles (the covers where air enters a room; registers usually refer to supply outlets and often have adjustable louvers), and returns (the grilles where air is drawn back in). Ventilation – the “V” in HVAC – often involves ducts too, like bringing in fresh outside air or exhausting stale air out.

Mnemonic: Ductwork = air highways. They direct traffic (airflow) around the house. Think of the saying “get your ducts in a row” (a play on ducks in a row) to remember the importance of well-organized ducts. Also, “what goes out must come back” – supply and return ducts form a loop.

Air Filter

Last but not least is the humble air filter. Before your HVAC system’s blower draws air into the evaporator coil, it passes through a filter that traps dust, pollen, and other particles. This protects the equipment (keeping the coil and blower clean) and improves indoor air quality by removing particulates.

Filters are usually made of pleated fabric or fiberglass and slide into a slot in the return air path. They have a MERV rating (Minimum Efficiency Reporting Value) – higher MERV means finer filtration. A typical home might use a MERV 8 to 11 filter that balances good filtration with not overly restricting airflow. High-end HEPA filters catch more, but most residential systems aren’t designed for HEPA (which requires special fan considerations).

Regular filter changes are critical. A clogged filter chokes off airflow, which can lead to a frozen evaporator coil (in cooling mode) or overheating (in heating mode), and generally makes the system inefficient. Depending on filter type and usage, you might change it every month or every few months – some thicker media filters last 6-12 months. But it’s good to check the filter monthly.

Mnemonic: Filter = system’s nose. It catches all the “gunk” so the system can breathe clean. A fun one: “filter filters filth.” And remember, a dirty filter is like a stuffy nose for your HVAC – it can’t breathe or circulate air properly.

HVAC Performance Metrics and Technologies

Understanding parts is one thing – understanding how we rate performance and the cool (pun intended) technologies used in HVAC systems is another. Let’s talk about efficiency metrics like SEER, as well as modern features like inverter drives and two-stage compressors that make systems more effective and efficient.

SEER (Seasonal Energy Efficiency Ratio)

SEER stands for Seasonal Energy Efficiency Ratio. It’s the most commonly cited efficiency rating for air conditioners and heat pumps in cooling mode. SEER represents the total cooling provided (in BTUs) during a typical cooling season, divided by the total electricity used (in watt-hours). In short, it’s BTU per watt-hour over a season.

Think of SEER like the MPG (miles per gallon) rating for your AC. A higher SEER means the system is more efficient – it gives you more cooling for each unit of electricity. For example, a 16 SEER unit is more efficient than a 13 SEER unit. As of the mid-2020s, new central AC systems in the US have to meet minimum SEER standards (varying by region, roughly 14 SEER and up). High-efficiency models can exceed 20 SEER. (There’s also a new SEER2 rating from 2023 onward, which is just an adjusted test procedure, but conceptually the same.)

Keep in mind, SEER is a seasonal average. The actual efficiency at any given moment can vary. It assumes a range of outdoor temps and a typical usage pattern. It basically answers: over a whole summer, how efficiently will this unit perform on average? So a unit with SEER 18 will generally use about half as much electricity to provide the same cooling as an old SEER 9 unit from decades ago, for instance.

High SEER units often achieve their efficiency through features like variable-speed compressors, larger coils, and advanced controls that optimize performance.

Mnemonic: SEER = Seasonal Efficiency Rating. Think of a “seer” (fortune teller) predicting your summer electric bills – a higher SEER is a better prediction (lower bills). Also, link SEER with Savings. Higher SEER → more savings on energy.

EER and HSPF

Two other acronyms you’ll bump into are EER and HSPF.

• EER stands for Energy Efficiency Ratio. It’s similar to SEER but measured at one specific operating condition (e.g., 95°F outside, 80°F inside, 50% humidity). It’s like a snapshot of efficiency at a high load. EER is useful for understanding how a unit performs on the hottest days. Some utility rebates or codes in hot climates specifically look at EER because SEER can sometimes mask poor peak performance.
  
  

• HSPF stands for Heating Seasonal Performance Factor. It’s basically SEER’s equivalent for heat pumps in heating mode (covering the heating season). It is the total heat provided (in BTUs) during the heating season divided by total electricity used (in watt-hours). A higher HSPF means a more efficient heat pump in heating. Older heat pumps might be HSPF 6-7, while newer ones are 8-10+. Starting around 2023, HSPF2 is the new testing metric, but again it’s similar just different test conditions.
  
  

For context: An electric resistance heater (100% efficient) has an HSPF of about 3.4 (since 1 watt-hour = 3.4 BTU of heat). Modern heat pumps with HSPF 8-10 are 2-3 times more efficient than straight electric furnace heat because they move heat instead of generating it.

Additionally, you might see COP (Coefficient of Performance) for heat pumps, which is an instantaneous measure (heat out divided by energy in, at a given condition). For instance, a COP of 3.0 is like saying 300% efficient in the moment (1 kW in gives 3 kW heat out). HSPF is basically COP averaged over a season and converted to that BTU/watt-hr metric.

Mnemonic: EER – think “Extreme Efficiency Rating” (because it’s at an extreme hot condition). HSPF – “Heating Season Performance Factor” (heating equivalent of SEER). Simplify it: higher = better for any of these metrics. If trying to recall numbers: SEER for cooling, HSPF for heating, EER for hot snapshots.

BTUs and Tonnage

We’ve mentioned BTUs a few times. BTU stands for British Thermal Unit, which is a measure of heat. Specifically, one BTU is the amount of heat needed to raise 1 pound of water by 1°F. It’s a small unit of energy. Air conditioners are rated in BTU per hour to indicate how much heat they can remove.

The term ton in HVAC confuses many folks. It doesn’t refer to weight of equipment; it’s a throwback to when ice was used for cooling. 1 ton of cooling = 12,000 BTU per hour. Why 12,000? Because one ton of ice melting over 24 hours absorbs about 288,000 BTUs, which is 12,000 BTU/hour. So when we say a house has a “3-ton AC,” it means it can remove about 36,000 BTUs of heat per hour from the house.

Residential central AC units often range from about 1.5 tons (18,000 BTU/hr) for a small home or condo up to 5 tons (60,000 BTU/hr) for a larger home (5 ton is usually the largest single split-system you’ll see, above that you use multiple systems).

Room air conditioners and portable ACs are usually rated just in BTUs – like “8,000 BTU window unit” or “14,000 BTU portable AC.” You match capacity to the space size; too little and it won’t cool adequately, too much and it cools too fast without dehumidifying (leading to clammy air and on/off cycling).

Mnemonic: 1 ton = 12,000 BTUs is a key figure. Perhaps visualize a ton of ice or remember that a pickup truck (which can carry a ton) could theoretically cool a house if it was full of ice! For BTU, just recall it’s a heat unit. A higher BTU rating = more cooling/heating power.

Inverter (Variable-Speed Technology)

 

Traditional HVAC systems have single-speed compressors and fans – they’re either 100% on or off. Imagine driving your car by either flooring the gas or hitting the brakes, nothing in between! Modern high-efficiency systems often use inverter technology to allow variable-speed operation, especially for the compressor.

An inverter in HVAC is an electronic control that can modulate the speed of the compressor motor by converting AC power to DC and adjusting frequency. In practice, instead of the compressor being just on/off, it can run at, say, 30% capacity, or 80%, or anywhere in between, based on demand. Same goes for fans in many cases.

This has big advantages: The system can run continuously at a low level to maintain a steady temperature, rather than the temp drifting up, AC blasting on to over-cool a bit, then shutting off, and repeat. Continuous or longer cycles at low speed improve dehumidification, reduce energy use (motors are more efficient when not constantly starting/stopping and can run in a more optimal range), and eliminate the indoor temperature swings.

Many ductless mini-splits and higher-end central systems use inverters. You might also hear “variable-speed compressor” or “modulating furnace” – similar concept of adjusting output smoothly.

There are also two-stage or two-speed systems which are sort of in between – they run at either low or high, nothing in between. More on that next.

Mnemonic: Inverter = variable speed control. Think of it as the cruise control for HVAC – it adjusts the power to maintain comfort smoothly. The word “inverter” comes from the electrical side (inverting AC to DC), but I remember it as “it inverts the traditional all-or-nothing approach.”

Two-Stage vs. Single-Stage

If you’re shopping for HVAC, you’ll see descriptions like single-stage, two-stage, variable-speed. We covered variable (infinite stages). A single-stage compressor or furnace is either on or off (100% or 0). A two-stage has two levels – usually a low stage (~60-70% capacity) for most days and a high stage (100%) for really hot or cold days.

Two-stage equipment runs on the lower stage most of the time, which is quieter and more efficient than high, and it can run longer cycles. When needed (thermostat not satisfied or outdoor temp extreme), it kicks into high stage. This gives some of the benefits of variable speed (longer, more stable operation) but at lower cost than fully variable.

For example, a two-stage AC might be 3 tons on high stage, but only ~2 tons on low stage. On a mild 85°F day, low stage suffices and it just cruises along. On a 100°F afternoon, it goes to high.

Similarly, a two-stage furnace might have a low fire that’s ~60% of full capacity and will use that the majority of time, only ramping up on very cold days.

Thermostats are usually staged to control this (they sense if second stage is needed based on how far the temp is from setpoint or how long first stage has run without catching up).

Mnemonic: Single-stage = one speed, Two-stage = two speeds. It’s like a fan with two settings vs just on/off. It’s simpler than full variable but still better than plain on/off.

Alright, now that we’ve covered efficiency metrics and modern features, let’s look at how systems can be configured in different physical setups.

Types of HVAC Systems

Not all HVAC setups are alike. The way components are packaged can vary. The three main system types you’ll encounter are split systems, packaged HVAC units, and ductless mini-splits. They all use the same principles (compressor, coils, etc.), but arranged differently to suit different building needs.

Split Systems (Central HVAC)

A split system is the most common type for central heating and cooling in homes. It means the system is split into an outdoor unit and an indoor unit. The outdoor part (often just called “the AC” outside) contains the compressor, condenser coil, and a fan. The indoor part (like a furnace or air handler) contains the evaporator coil and blower, and possibly furnace burners or electric heat strips if it’s a combined system.

Refrigerant lines connect the two: one line carries liquid refrigerant from outdoor to indoor coil, and a larger line carries gaseous refrigerant back to the compressor.

In a central AC with furnace setup, the furnace (gas or electric) handles heating and contains the blower, and an evaporator coil is added to it for cooling. In a heat pump setup, the indoor unit is usually an air handler (with electric backup heat strips) and the outdoor unit is a heat pump that does both cooling and heating (via reversing valve).

Split systems are popular because separating the noisy/hot part (compressor & condenser) outside keeps indoor quieter, and allows the indoor unit to be tucked in a closet, attic, or basement attached to ducts. They come in various sizes (common residential splits: 1.5 to 5 tons).

The term central air usually implies a split system with ducts distributing the air.

Mnemonic: Split = split in two (inside & outside). Just picture the two boxes. One cold inside, one hot outside, connected by pipes.

Packaged Units

A packaged unit combines all the components into a single cabinet, which is usually situated outside the building (like on the roof or a concrete slab). Ducts connect from this all-in-one unit into the building’s ductwork.

Packaged units often serve commercial buildings (rooftop units on flat roofs) or some homes without space for a split system inside. Types include:

• Packaged AC: Contains compressor, coils, blower in one, for cooling (with optional electric heat strips for heating or paired with separate furnace).
  
  

• Packaged Heat Pump: Same but can reverse for heating, with backup heat strips.
  
  

• Packaged Gas/Electric (Packaged Unit with Gas Heat): An AC and a gas furnace in one box (burner and heat exchanger included). Often called a "gas pack."
  
  

For example, many restaurants or stores have multiple packaged units on the roof handling different zones (you often see them as boxy units up there). Some homes (especially in warmer climates or in manufactured housing) might have a package unit on the side of the house that blows into a crawlspace duct system.

The advantage: no separate indoor unit needed, saves indoor space, and installation is simplified (just place the unit, hook up ducts, power, and fuel if gas). Also service is all outdoors.

The disadvantage: if it’s on the roof, you need roof access for service; and having everything outside could mean more exposure to weather or noise outside (though indoor noise is low since there’s no inside equipment running, aside from ducts noise).

Mnemonic: Packaged = all packed together. One-stop shop unit. Think of it as the self-contained AC solution. If split is “split apart,” packaged is “package deal – all in one.”

Ductless Mini-Splits

Ductless mini-split systems are like smaller-scale split systems that don’t use ducts to distribute air. Instead, the indoor unit is the air handler for a specific room or area. A basic mini-split has one outdoor unit (compressor + condenser) and one indoor unit (evaporator + blower) mounted on a wall or ceiling in the room it serves.

They’re called mini because the indoor units are compact, and split because, like standard splits, the noisy compressor is outside and quiet cooling coil is inside.

Each indoor unit only cools/heats the room or zone it’s in, but you can have a multi-split setup where one outdoor unit connects to multiple indoor units (e.g., one in each of 3 rooms). Each indoor unit has its own thermostat/remote, so you get independent zone control.

Mini-splits are very popular for retrofits (add AC to a house without ducts), additions, older homes, or even new efficient homes that want zoned control. They typically use inverter compressors, making them very efficient (SEER ratings in high 20s aren’t uncommon). Also, since no ducts, you avoid duct losses.

Indoor units styles: most common are wall-mounted (high on the wall, blown out from the unit). There are also ceiling cassettes (recessed in the ceiling), floor-mounted consoles, or even slim ducted units that hide above a ceiling and feed a couple small ducts (useful if you want hidden but in one zone).

Many mini-splits are heat pumps, giving both cooling and heating.

Pros: high efficiency, zonal control (heat/cool rooms individually), no duct losses, relatively easy install (just need to run refrigerant lines and power to units).

Cons: cost per BTU is higher than central systems (especially multi-zone systems get pricey), and some people don’t like the look of the wall units in the room. Also, in very cold climates, some older models might struggle (though modern cold-climate mini-splits work even below 0°F pretty well).

Mnemonic: Ductless = no ducts, mini-split = small split system. So just remember, it’s a tiny AC per room. When I see a wall unit, I think “that room has its own mini HVAC system.” Multi-split? “One outside, many insides.”

Other System Types

Some other configurations worth knowing:

• Window AC Units: These are basically packaged units for a single room that sit in a window (or wall slot). The evaporator face is inside cooling the room, the condenser side is outside getting rid of heat. They’re inexpensive and common for apartments or individual room cooling.
  
  

• Portable AC Units: Freestanding units on wheels that cool a room by using a small built-in evaporator and condenser, venting heat out via a hose through a window (more on these later). Useful if a window unit isn’t possible, but usually less efficient.
  
  

• Hybrid / Dual Fuel: A setup where you have both a heat pump and a furnace and can use either depending on what’s more efficient (heat pump in mild cold, furnace in very cold). This is a variant of a split system – essentially two heating sources sharing the same ducts.
  
  

• Geothermal Heat Pumps: These use underground coils (loops in wells or horizontal trenches) to exchange heat with the earth (which stays around 50°F). The heat pump unit is often indoors connected to that ground loop. Very efficient, but high install cost.
  
  

• VRF Systems: Variable Refrigerant Flow systems are like mega multi-splits for commercial buildings. They allow many indoor units (zones) on one or more outdoor units with sophisticated refrigerant routing. They can even heat some zones while cooling others by redistributing refrigerant. Big in office buildings, hotels, etc.
  
  

• Evaporative Coolers (Swamp Coolers): Not actually refrigeration AC – they cool air by evaporating water into it (adds humidity, works only in dry climates). You might see these on roofs in dry regions. They blow cooled humidified air through ducts. Mentioned to distinguish from true AC.
  
  

• Radiant Systems: Some homes/buildings use hot water pipes in floors or radiators for heating (boiler systems), and separate AC (or no AC). Those aren’t “air conditioning” via refrigeration (for heat, anyway) but are part of HVAC world. They often still need an AC for cooling unless relying on fans or something.
  
  

Alright, we’ve identified various system layouts. Now, let’s talk operation modes.

HVAC Operating Modes: Cooling, Heating, and Defrost

Your HVAC doesn’t always do the same thing – in summer it’s cooling, in winter (if a heat pump) it’s heating, and heat pumps have a funky defrost mode in winter. Understanding the differences in operation will help make sense of why a unit might sound or act differently in January vs July.

Cooling Mode (Air Conditioning)

This is the mode we’ve mostly been describing so far. The goal: remove heat from the indoor air and dump it outside, thereby cooling the indoors.

Let’s recap the cooling cycle step-by-step in a narrative:

1. Call for Cooling: It’s a hot day, house got warmer than thermostat setting, so the thermostat switches the cooling on. It typically turns on the indoor blower and the outdoor unit (compressor + condenser fan).
   
   

2. Compressor Pumps: The compressor starts up, drawing in low-pressure, cool refrigerant vapor from the indoor coil via the suction line. It compresses this vapor to high pressure, which also raises its temperature dramatically.
   
   

3. Heat Rejection at Condenser: This hot, high-pressure refrigerant gas flows into the condenser coil (outdoor). The outdoor fan is pulling ambient air through the coil. The refrigerant, being much hotter than the outside air, gives off heat to the air. The refrigerant cools and condenses into a liquid inside the coil. By the coil outlet, it’s a high-pressure liquid (still warm).
   
   

4. Throttling at Expansion Valve: The high-pressure liquid refrigerant goes through the expansion valve next. Think of this like moving from a narrow pipe to a wider one; the pressure drops. Some refrigerant flashes into gas due to the drop (cooling the mixture). Now we have a low-pressure, cold liquid/vapor mix.
   
   

5. Heat Absorption at Evaporator: This cold mixture enters the evaporator coil (indoor). The indoor blower is moving warm house air over this coil. Because the refrigerant is cold, it absorbs heat from the air (and the refrigerant boils/evaporates completely into a gas as it picks up that heat). The air, now minus some heat (and moisture, which condenses on the coil), leaves the coil cooler and drier, and goes back into the rooms via ducts.
   
   

6. Return to Compressor: The now warm (from absorbing indoor heat) low-pressure refrigerant vapor travels back through the suction line to the compressor… and the cycle repeats until the thermostat is satisfied and turns things off.
   
   

During cooling, a couple of notable things:

• Condensate: Because the evaporator is below the dew point of the indoor air, water condenses on it. That water collects in a pan and drains out (you might see a PVC drain line near your indoor unit). On humid days, a lot of water can come out (thus dehumidifying your home).
  
  

• System Controls: Most systems cycle fully off when setpoint is reached (unless you have a variable system that might just ramp way down). Some thermostats allow the indoor fan to run continuously if desired (for air circulation), but if you do that in humid climates while the compressor is off, that extra moisture on the coil can re-evaporate into the air, raising indoor humidity slightly. Many keep fan “Auto” to stop it when cooling stops.
  
  

• High Outdoor Temps: The ability of the condenser to dump heat is affected by outdoor temperature. The higher it is, the harder it is to cool the refrigerant. So efficiency drops some in extreme heat, and if it’s really hot, the AC may struggle to keep up because the differential is less. (This is where proper sizing and adequate condenser surface area matter).
  
  

Mnemonic (again): Compressor → Condenser → Expansion → Evaporator (and back). A loop with four steps: 1) compress (add pressure/temp), 2) condense (remove heat outside), 3) expand (drop pressure/temp), 4) evaporate (absorb heat inside). I sometimes remember it as “Pressurize, Dump Heat, Depressurize, Grab Heat”.

Heating Mode (Heat Pump)

If you have a heat pump, this same refrigeration cycle can run backwards to heat the home. This is achieved with a device called a reversing valve that switches which coil is the evaporator and which is the condenser. Essentially, in heating mode:

• The outdoor coil becomes the evaporator (absorbing heat from outside air).
  
  

• The indoor coil becomes the condenser (releasing heat into indoor air).
  
  

Step-by-step heating mode for a heat pump:

1. Call for Heat: Thermostat says it’s cold, need heat. It energizes the reversing valve to the heating position (or de-energizes, depending on brand – different philosophies: O/B terminals on thermostats control this). It also turns on the compressor and outdoor fan (and indoor blower).
   
   

2. Compressor Pumps: It always pumps from the suction side to discharge side. But now, thanks to the reversing valve, the suction side is connected to the outdoor coil output, and the discharge sends hot gas to the indoor coil.
   
   

3. Heat at Indoor Coil: So now the compressor is sending hot, high-pressure refrigerant vapor to the indoor coil first. The indoor coil acts like a condenser: as the hot refrigerant passes through, the indoor air blower is moving cool house air over it, that air absorbs heat (warming up), and the refrigerant condenses to liquid, releasing its heat into your home. You feel warm air from the vents.
   
   

4. Expansion and Cold Outdoor Coil: The refrigerant, now a high-pressure liquid, goes through the expansion valve (or a bypass specifically for heating mode) and drops in pressure/temp. This cold, low-pressure refrigerant then goes into the outdoor coil. The outdoor coil is now acting as an evaporator. It’s colder than the outside air (even if it’s chilly outside, the refrigerant might be, say, 0°F inside the coil, so it can pick up heat). The outdoor fan draws outside air over the coil, and the refrigerant absorbs heat from that air and boils into vapor.
   
   

5. Back to Compressor: That now warmed (from outside air) vapor goes back to the compressor, and the cycle repeats.
   
   

Effectively, the heat pump is pumping heat from outside to inside. Yes, it can do this even in winter – the limiting factor is that the colder it gets outside, the less efficient and less capacity the heat pump has, because there’s less heat to grab and it has to work with a bigger temperature difference.

At some point (varies by system, maybe around 30°F for older ones, or 0°F for newer cold-climate ones), the heat pump can’t pull enough heat and might need help. That’s where auxiliary heat comes in – typically electric resistance coils in the air handler (or a furnace if it’s a dual fuel system) kick on to supplement.

Heat pumps are great because they provide 2-4 times the heating per watt as straight electric heat (since you’re moving heat, not making it). But one quirk: when it’s cold and the heat pump runs, the outdoor coil is below freezing and will accumulate frost (since it’s cooling that moist outside air). We handle that with defrost cycles (next section).

Mnemonic: For heat pumps, recall “reverse in winter.” The key parts: reversing valve flips the direction of refrigerant flow, so indoor coil gets hot. I like to remember “in winter, indoor coil = condenser (hot), outdoor coil = evaporator (cold).” Also think of it like this: an AC moves heat outside, a heat pump moves heat inside.

Defrost Mode (Heat Pump)

If you live in a colder climate with a heat pump, you might notice your outdoor unit sometimes does weird things on winter days: maybe it’s steaming, or you hear a swoosh sound, or the fan stops for a bit while the compressor runs. That’s the heat pump going into defrost mode.

Why defrost? As mentioned, when the heat pump runs in heating, the outdoor coil is cold – often below 32°F – so moisture in the outside air freezes on it, forming frost/ice. A thin layer is fine, but if it keeps building, it insulates the coil and blocks airflow, hurting efficiency. So the system needs to melt it periodically.

How defrost works: The heat pump temporarily switches back to cooling mode to heat up the outdoor coil and melt the ice.

• The reversing valve flips back to AC mode (so now the outdoor coil is hot).
  
  

• The outdoor fan typically stops (so it’s not cooling the coil with cold air while we’re trying to heat it).
  
  

• The compressor keeps running, pumping hot gas into the outdoor coil to warm it.
  
  

• The indoor unit, during this time, would be blowing cold air (because it’s essentially air conditioning the house) – which isn’t good for comfort – so to avoid that, most systems will turn off the indoor blower or turn on auxiliary electric heat to temper the air. Many just run the aux heat and still blow air, so you don’t feel cold air, you may actually not notice much except maybe the air isn’t quite as hot for a brief period.
  
  

• This lasts just a few minutes, until sensors detect the outdoor coil is above freezing and ice is gone (or a timer assures a minimum/maximum duration).
  
  

• Then it flips back: reversing valve back to heat mode, outdoor fan on, aux heat off, normal heating resumes.
  
  

During defrost, you often see steam rising from the outdoor unit – that’s the warm coil evaporating the melted frost quickly (it can look like smoke, but it’s just water vapor). You might hear the refrigerant changeover “whoosh” which can be a noticeable sound.

Defrost cycles typically occur only when needed. Modern heat pumps monitor coil temp and run time to optimize it. On a cold humid day, you might get a defrost every 30-90 minutes of runtime. On a dry cold day, maybe none for many hours.

Auxiliary heat is important with heat pumps not just for when it’s super cold, but also for defrost – since effectively the system cools the house for a couple minutes, the aux heat covers that so you don’t get a temperature dip.

Mnemonic: Defrost = self-defreezing mode. I think: “heat pump must thaw itself.” It reverses to melt ice. The key sign is steam puffing off the outdoor and the brief change in sound.

So, a heat pump in winter alternates between heating and occasional defrosting. As a user, you might only notice shorter heat pump cycles with aux heat kicking in during defrost.

Now that we’ve covered core operations, let’s talk about some specialized scenarios and system types, like HVAC in RVs, rooftop commercial units, wall ACs, and portable units.

RV Air Conditioning

Keeping an RV (recreational vehicle) cool in the summer is a challenge of its own – you’ve got a tiny home on wheels, often baking in the sun at a campsite. RV air conditioners are a specific breed of HVAC designed to meet these challenges.

Most RVs use rooftop AC units. If you glance at the roof of a camper or motorhome, you’ll see one or more boxy units – those are the ACs. They are essentially packaged air conditioners (compressor, condenser, evaporator, fans all in one box) that sit on the roof blowing cool air down into the RV.

Key points about RV ACs:

• They often come in standard sizes like 13,500 BTU or 15,000 BTU. A small trailer might have a 13.5k BTU unit, a larger RV might have two 15k BTU units.
  
  

• They usually run on 120V electricity. When at a campground (“shore power”), you plug in the RV and can run these units. A 30-amp RV hookup usually supports one AC. A 50-amp hookup is usually for RVs with two ACs (and other appliances).
  
  

• If camping without external power, you need a generator (or a hefty inverter/battery setup) to run the AC, because they draw a lot of power (often 12-16 amps each when cooling).
  
  

• RV AC units can be ducted or non-ducted. Non-ducted means the unit just blows air out of its own vents right there (good for small RVs where one unit can handle the whole space). Ducted means the AC feeds into ducts in the ceiling that distribute air through vents across the RV (common in larger RVs or ones with multiple AC units).
  
  

• They have controls either on the unit (knobs on the ceiling assembly) or through a wall thermostat (fancier ones).
  
  

• Many RV ACs have only cooling mode (they don’t reverse to heat). Some have optional heat strips – an electric heating element that can provide a little bit of warmth (not very efficiently, basically like a space heater). A few newer ones are true heat pump models that can provide moderate heating by reversing cycle (useful for cool but not frigid weather).
  
  

• Maintenance for an RV AC: similar concept – keep the filters (inside ceiling intake) clean, and periodically get on the roof safely and clean the condenser coils (and evaporator coils if accessible) of debris. Also ensure the gasket that seals it to the roof isn’t leaking rainwater.
  
  

• Noise: RV ACs can be loud, since the blower and compressor are right above (especially non-ducted ones, where the air is rushing right out the unit). Ducted ones disperse the air more quietly but you’ll still hear the compressor overhead. Newer RV AC models are focusing on being quieter and more efficient (some now use inverter compressors too, which also helps start-up current issues).
  
  

• If it’s extremely hot out, RV ACs can struggle – RVs are not insulated like houses, and lots of sunlight can heat them up. Many RVers use tricks like putting reflective foil in windows, using awnings, parking in shade if possible, or even supplemental fans to circulate air. Also, it helps to start the AC early in the day before the RV turns into an oven.
  
  

A typical scenario: You arrive at a campsite, plug in the RV to the pedestal giving you electricity, and turn on the roof AC. In a few minutes, cool air is blowing. If the RV is big and has two units, you might have one for the front living area and one for the rear bedroom, etc., possibly controlled separately.

There are also 12V DC RV air conditioners emerging – these can run directly off a battery bank (often with help of solar panels) and use very efficient inverter compressors, aimed at off-grid RV use. They’re still quite expensive and not mainstream yet for larger RVs (mostly in van conversions or trucking).

Mnemonic: RV AC = rooftop cooling. I remember that RV ACs are literally above you (on the roof), unlike home AC which is usually outside and unseen. So think roof = RV AC domain. The challenge is power – hooking up or having generator to run them.

Rooftop Systems (Commercial HVAC)

We discussed packaged units, and specifically rooftop units (RTUs) are a big category of those, especially in commercial buildings. You’ve likely seen big metallic boxes on top of supermarkets, schools, office buildings – those are HVAC units handling those spaces.

A typical rooftop unit in a commercial setting is a packaged system that provides cooling and often heating. For heating, many have a gas furnace section inside (burning natural gas or propane) or electric heating coils.

What makes rooftop systems notable:

• Placement: They sit on the roof (usually flat roofs). They connect to ductwork that goes down into the building. Often they’ll have a downflow design (blowing air downward into a curb/duct) for ease of duct connection through the roof.
  
  

• Economizers: Many RTUs have an economizer damper system – this is essentially an large vent that can let outside air in to cool the space when outdoor conditions are favorable. For example, if it’s 60°F outside and you need cooling inside, the AC compressor can stay off while the economizer opens to bring in that cool outside air (so-called “free cooling”). It also allows fresh air for ventilation per code (bringing in a bit of outside air even when recirculating).
  
  

• Zoning: In a single large building, you might have multiple RTUs each handling different zones. For instance, a shopping plaza might have one RTU per store. Or a school might have one per wing or large room. They are self-contained so it’s a modular approach.
  
  

• Maintenance Access: Roof units mean techs can service them without going inside (good for not disturbing occupants). On the flip side, it means working on the roof (exposed to weather, heights).
  
  

• Residential Rooftop Units: In some parts of the country (like the Southwest), it’s not uncommon for homes to have packaged units or swamp coolers on the roof. This saves yard space and can make duct routing easier in certain designs (blowing down from roof through ceiling registers).
  
  

• Durability: RTUs are built to endure outdoor conditions – heavy rain, sun, even some snow/ice (though in areas with snow, you have to ensure they aren’t buried or accumulating snow too deeply around them).
  
  

• Zoning inside: Some RTUs are VAV (variable air volume) systems feeding multiple zones via VAV boxes (dampers) but that gets into advanced commercial design. Simpler is one RTU per zone.
  
  

Example: Go into a big-box store on a hot day – you might hear a slight humm of big units on the roof. Those are likely like 10-ton or 20-ton package units cycling on/off to keep that warehouse space cool.

For technicians: rooftop units are a staple – you go on the roof, might see a row of units. They often have easy access panels. Many modern RTUs also have sophisticated control boards, maybe even connectivity to building management systems.

Mnemonic: RTU = Roof Top Unit. Easy. It’s a packaged unit on the roof. I sometimes think of them as the “Swiss Army knife” of HVAC – they do it all in one and can handle an entire building’s needs in a box.

Wall-Mounted Units

Wall-mounted AC/heating units come in a few varieties:

1. Ductless Mini-Split Indoor Units: We talked about these – those sleek wall units high on the wall inside rooms for mini-split systems. They’re quiet, efficient, and directly cool/heat that room.
   
   

2. PTAC (Packaged Terminal Air Conditioner) Units: These are common in hotels, motels, and some apartments. They’re the units you see typically under the window, often covered by a grille, blowing directly into the room. Each unit goes through the exterior wall – so half of it sticks out the building (condenser side) and the inside part has the evaporator and controls. They plug into a 208/230V outlet usually.
   
   

• PTACs often provide both cooling and heating. Heating is usually either electric resistance or a heat pump function. In many cases, even heat pump PTACs have resistance backup or supplemental because of limited efficiency in cold snaps.
  
  

• They’re essentially a self-contained packaged unit for one room. Easy to replace (slide out the old, slide in a new of same size).
  
  

• Downside: they can be a bit noisy, and not as efficient as split systems.
  
  

• You’ll know them as the thing you adjust with a knob or digital control in your hotel room under the window.
  
  

3. Through-the-Wall ACs: Similar to window units but permanently installed through a wall sleeve. Many apartments in big cities have these. They are basically like a PTAC but usually only cooling (some have electric heat). Instead of being low and having a heating strip, sometimes they are higher on the wall or wherever an opening was made. These usually are user-installed or builder-installed where central air conditioning isn’t present.
   
   

4. Floor Mounted or Console Units: In some settings (like some mini-split indoor options or old schools/buildings), you have floor consoles or through-wall console units (like big radiators but AC units).
   
   

Focus on common: The PTAC style is important. If you’ve been in a hotel, you know the experience. They often have a thermostat on the unit itself or on the wall connected to it. They’re cost-effective for individually conditioning rooms without central system.

One emerging thing: some newer high-rise apartments use VTAC (Vertical Terminal Air Conditioners) – basically a small packaged unit in a closet connected to short ducts for a one-bedroom or studio, but that’s more specialized.

Mnemonic: For wall units, think “hotel AC” for PTACs (that box under the window that hums). And “mini-split wall” for those modern wall hung units at home. They both mount on or through walls but are very different beasts (one is standalone package, one is part of a split).

Portable Systems

Portable air conditioners are a unique category. These are the ones you buy at a home improvement store on wheels that you can roll into a room, vent out a window with a hose, and plug into an outlet.

How portable ACs work:

• Inside the unit, they actually have both an evaporator and a condenser coil (like any AC). But both are in the same box. They take air from the room to cool (pass it over evaporator, send it back cooled). But they also have to cool the condenser coil. Depending on design:
  
  

• Single-Hose Portable: They pull air from the room itself to pass over the condenser and then blow that hot air out the single exhaust hose to the outside. This unfortunately creates a slight vacuum in the room, which then draws in warm air from adjacent rooms or cracks to equalize pressure, somewhat reducing efficiency.
  
  

• Dual-Hose Portable: One hose brings in outside air to blow over the condenser and then out the other hose goes the now-hot air. This way they’re not sucking conditioned room air to cool the condenser. Dual hose units are generally more efficient and can cool a room faster.
  
  

• Most portables are on casters, around 8,000 to 14,000 BTU (advertised, often actual effective BTU is lower for single hose). They plug into standard 115V outlets (~10-12 amps draw typically).
  
  

• Condensate handling: Many portable ACs are designed to evaporate the condensate water they collect and expel it out the hose (they kind of fling water onto the hot condenser to evaporate it, which even slightly improves efficiency). These are often advertised as “no drain” or auto-evap. However, in very humid weather, they might not evaporate all of it, so many have a backup reservoir and will shut off if it gets full (or have a connection to attach a drain hose to let water drip out continuously into a bucket or floor drain).
  
  

• Use cases: They are used in situations where you can’t have a window unit (maybe the window won’t fit one, or building rules prohibit them for exterior appearance), or you want to be able to move the AC between rooms. They’re also used in server rooms or offices where you can’t modify the building to add AC but need some cooling (just vent out a drop ceiling or window).
  
  

• Performance: They generally are less efficient than window units because of the whole indoor unit and especially single-hose issues. Also, the exhaust hose can radiate some heat back into the room (it gets quite warm). To help, keep the hose short and straight, and some people even insulate the hose.
  
  

• Noise: They have the compressor and fans right in the room, so they’re about as noisy as a window unit, sometimes more because they often have a higher pitched exhaust fan for the hose.
  
  

• The convenience is you didn’t have to lift a heavy unit into a window; you just wheel it and use a window kit (a board or slider that the hose fits into in a small opening of the window).
  
  

Portable AC vs Evaporative Cooler:

• Some folks confuse them. A true portable AC has a compressor and refrigerant and must vent out hot air. An evaporative cooler (swamp cooler) might also be portable but it only blows air over water to cool by evaporation (works only in dry climates and doesn’t need venting, but it adds humidity). So if it has a big hose, it’s a real AC.
  
  

There’s another niche: portable heat pumps / dual-purpose units that can be used for heating (some can reverse or just use resistive heat). But generally, portables are primarily for cooling.

Mnemonic: Portable = on wheels, needs a vent hose. I remind people: it’s only portable in the sense you can move it around, but it must vent out a window (or you’re just recirculating hot air). Think “portable = plug and play (and vent).” And single vs dual hose: single sucks room air to cool itself, dual uses outside air to cool itself.

Refrigerants and Environmental Regulations

Now let’s talk more about those mysterious refrigerant numbers (R-22, R-410A, R-32, etc.) and why they change. HVAC isn’t just thermodynamics; there’s chemistry and environmental science in play too.

From CFCs to HFCs: A Brief History

The story of refrigerants involves improving technology and fixing environmental mistakes:

• Early ACs (1950s-1980s) used refrigerants like R-12 in fridges/cars and R-22 in home ACs. These were CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) types. They were great at refrigeration but had chlorine atoms that, if leaked, would reach the upper atmosphere and destroy ozone. Ozone layer depletion was a huge issue (more UV hitting earth, not good).
  
  

• The world responded with the Montreal Protocol (1987), phasing out CFCs/HCFCs. R-12 (CFC) was phased out first (1990s). R-22 (HCFC) production for new equipment stopped in 2010, and total phaseout (no new production even for service) by 2020 in many countries.
  
  

• Replacements with no chlorine came: HFCs (hydrofluorocarbons) like R-134a (used in car AC after R-12), and R-410A (blend of R-32 and R-125) to replace R-22 in home AC. R-410A became standard in new AC units from about 2006 onward (in the US).
  
  

• These HFCs don’t harm the ozone (yay), but they have high Global Warming Potential (GWP). For example, R-410A has a GWP of 2088 (meaning pound for pound, it traps 2088 times more heat in the atmosphere than CO₂ over 100 years). Not a big deal if contained, but leaks happen, and the sheer amount worldwide adds up. So attention turned to HFCs as contributors to climate change.
  
  

• The Kigali Amendment to the Montreal Protocol (2016) globally plans to phase down HFCs. In the US, the AIM Act (2020) authorized phasedown of HFC production by certain percentages over the next 15 years or so.
  
  

Today’s Refrigerants: R-410A and New Alternatives

 

As of the mid-2020s, R-410A is still the standard in residential and light commercial AC. But the transition to new refrigerants is underway:

• R-410A: Widely used, non-ozone-depleting, but high GWP (~2000). Will be phased down in new equipment by around 2025-2026 in many regions.
  
  

• R-134a: Used in many older car ACs and some large chillers. Also high GWP (~1300). Cars have largely moved to R-1234yf now.
  
  

• R-407C, R-404A: Other HFC blends used in some systems (407C in some older drop-in retrofits for R-22, 404A in commercial refrigeration). Also being phased down for high GWP.
  
  

• R-32: This is actually one of the components of R-410A, but on its own, it has a GWP of ~675, about one-third of R-410A. It is mildly flammable (A2L safety rating). Many room ACs and mini-splits globally already switched to R-32, and some central AC manufacturers plan to use R-32 in new units. It’s efficient and you need less charge compared to R-410A (because of better heat capacity).
  
  

• R-454B: A newer blend (R-32 + R-1234yf) with GWP ~466. Many North American manufacturers have chosen this as their R-410A replacement for unitary AC systems. Also an A2L.
  
  

• R-1234yf: This is an HFO (hydrofluoro-olefin) with extremely low GWP (~4). It replaced R-134a in car AC (all new cars use it now). It’s also mildly flammable (A2L). It could be used in blends (like above) or in some systems by itself (though usually more in cars).
  
  

• CO₂ (R-744): GWP = 1 (baseline). Used in some commercial refrigeration and increasingly in heat pump water heaters, etc. But CO₂ operates at very high pressures (1000+ psi), and efficiency drops at high ambient temps (transcritical cycle issues). Likely not coming to residential central AC soon, but who knows in specific uses.
  
  

• Propane (R-290): GWP ~3, fantastic thermodynamic properties, but it’s propane (A3 highly flammable). Used in very small charge systems like some small refrigerators, and allowed in small window units in some places (with charge limits like 150 grams). Unlikely for central AC due to flammability in large charge amounts and code restrictions.
  
  

• Ammonia (R-717): Great refrigerant (GWP 0), but toxic and somewhat flammable, so it’s confined to industrial systems (large cold storage warehouses, etc).
  
  

• Others: R-454A, R-455A, R-466A (an attempt at a nonflammable low-GWP), etc., but the frontrunners for standard AC seem to be R-32 and R-454B.
  
  

Safety: The shift is a bit complicated by the fact these new refrigerants are mostly A2L (mildly flammable). That means codes need updating (you may need leak detectors in enclosed spaces, ventilation, etc., though the risk is low, it’s not like propane, but precautions are there). Techs need to be aware (no brazing near leaked refrigerant without venting, etc., because flame + A2L could ignite).

Functionally, R-32 and R-454B units will work similarly to R-410A ones, maybe even with a boost in efficiency. But they won’t be backwards compatible – you can’t just drop R-32 into an R-410A unit; whole new equipment is designed for them (different lubricants, optimized coils, pressure, etc.).

If you’re a student tech: you’ll be learning about handling A2Ls, getting proper training and possibly new manifold gauges (some old ones not rated for flammable), vacuum pumps rated for it, leak detectors that can sense HFOs (they have different properties).

EPA Regulations and What They Mean for Techs and Consumers

Some key regulatory points (using the US as example):

• Section 608 Certification: To work with refrigerants, techs need EPA 608 certification. Type I for small appliances (e.g., small window ACs), Type II for high-pressure systems (most ACs, heat pumps), Type III for low-pressure (centrifugal chillers, etc.), or Universal for all. This ensures knowledge of handling and rules. It’s basically a must-have if you’re going into the field.
  
  

• No Venting Rule: It is illegal to knowingly vent refrigerants during service. When you need to open a system, you must recover the refrigerant into a recovery cylinder using a certified recovery machine. This applies to all CFC, HCFC, HFC, HFO refrigerants (even the new ones – because while the new ones are climate-friendlier, they still can form harmful byproducts or are flammable).
  
  

• Refrigerant Sales Restriction: You can’t just buy big jugs of R-410A or R-22 as a do-it-yourselfer. Sellers require proof of certification or that it’s for resale to certified persons, etc. (Small cans of R-134a for auto use are still sold OTC, though that’s being reconsidered too).
  
  

• Production Phaseouts/Ins: The EPA is phasing down HFCs. By 2025, expect that you can’t get new equipment with R-410A (manufacturers are already rolling out R-454B or R-32 models). They’ll still make R-410A refrigerant for servicing existing units for a long while, but decreasing amounts.
  
  

• Servicing Legacy Systems: R-22 is very expensive now if you need a recharge, since it’s only from recycled/reclaimed sources. Many old R-22 systems are being retrofitted to alternative refrigerants (with varying success) or just replaced. For R-410A, supply will be ample for a while, but 10 years down, it could get pricier. (However, the EPA phasedown is of production/import, not use – existing equipment can still use it, and stockpiles/recovery will supply service demand).
  
  

• State/Local Rules: Some states (like California) have their own aggressive rules. E.g., California banned R-410A in new ACs starting Jan 1, 2025 outright (so they’ll definitely use alternatives). Also, building codes regarding flammable refrigerants (A2L) might vary in adoption speed.
  
  

• Handling Flammable (A2L) Refrigerants: Training is being ramped up. For example, if you detect a leak of R-32 in an enclosed space, you’d ventilate well before brazing, because brazing torch + concentrated refrigerant could ignite. Also, certain leak detectors and ventilation fans may be required in mechanical rooms with large charges. For small residential split systems, the safety risk is minimal (they use something like 5-10 pounds of refrigerant, which if all leaked in a house would likely not reach flammable concentration except maybe in a very small room). But codes will still specify things like maximum charge based on room size, etc.
  
  

• Disposal: When retiring equipment, refrigerant should be recovered (and either reclaimed or destroyed). It’s illegal to cut the lines and let it go.
  
  

For consumers:

• It means new ACs they buy might have a different refrigerant listed (and maybe a big flammable warning sticker, which might be surprising but it’s mildly flammable).
  
  

• Servicing older units might get pricier if refrigerants are scarce.
  
  

• Environmentally, it’s better in the long run – these new refrigerants significantly cut down greenhouse impact if leaked.
  
  

For techs:

• It’s an ongoing learning process. Many who went through R-22 to R-410A now doing R-410A to R-32/R-454B.
  
  

• New tools maybe: e.g., a special manifold set or gauges for R-32 (to keep it from mixing with other oils, and spark-proof if using digital ones perhaps), recovery machines certified for A2L (since you don’t want an electrical spark in a machine with flammable gas).
  
  

• Work safe, follow best practices (which you should already, like using nitrogen to purge when brazing, proper leak checking, etc.).
  
  

The good news, these changes are manageable and the industry adapts. And who knows, in a few decades maybe we’ll even move beyond vapor compression cycles? (CO₂ transcritical, magnetic refrigeration, etc. are being researched).

Mnemonic: CFC Bad (Ozone), HFC Better (Ozone) but Bad (Climate), HFO/NatRef Best (Ozone+Climate). And always “Recover, Don’t Vent!” Tech mantra. Maybe timeline: “R-22 past, R-410A present, R-32/454B future.”

Maintenance and Best Practices

No HVAC guide would be complete without talking about maintenance. Proper maintenance can keep systems running efficiently, prevent breakdowns, and ensure longevity. Here are core maintenance points and good practices for HVAC systems:

• Regular Filter Changes: We’ve harped on filters – because it’s truly critical. Check your filter monthly. At minimum, before heavy-use seasons (spring and fall). Dirty filter = bad. Keep spares on hand. If you have pets or a dusty environment, you might need to change more frequently. This is often a homeowner’s responsibility, but as a tech you should always check it when you service a system and advise accordingly.
  
  

• Keep Coils Clean: The evaporator coil (indoors) and condenser coil (outdoors) both benefit from being clean. A dirty condenser coil (covered in cottonwood fluff, dirt, etc.) can raise head pressures and drive up energy use. A dirty evaporator coil (which can happen if filters were missing or not effective for a long time) will impede airflow and hurt performance (and can ice up). Cleaning evaporators often requires opening up the plenum; sometimes a foaming cleaner is used. Condensers can usually be rinsed from outside (power off, use a gentle stream of water, not high pressure which can bend fins, or coil cleaner if very greasy).
  
  

• Clear Condensate Drains: Ensure the condensate drain line from the evaporator is clear. Algae can grow and clog it, especially in humid areas. Many maintenance visits include pouring a little vinegar or bleach solution in the drain or a tablet to inhibit algae. If the drain clogs, water can overflow (causing water damage) or a float switch will shut off the system (no cooling until cleared). If you hear water sloshing or see water around the indoor unit, suspect a drain issue.
  
  

• Blower and Fan Check: The blower wheel should be relatively clean (dirty blades reduce airflow). Motor bearings in older units might need oil (many modern motors are sealed/no-oil). The outdoor fan motor similarly might need checking. Listen for any bearing noise. Clean the fan blades if there’s buildup.
  
  

• Refrigerant Charge: Ideally, a system won’t leak and the charge stays correct. But in reality, some systems leak slowly. During a tune-up, a tech will often connect gauges or (now more commonly) measure pressures/temps to ensure proper charge (often by checking superheat and subcooling). If low, they’ll find the leak and fix it if possible, then recharge to spec. Running a system undercharged or overcharged both hurt efficiency and reliability (undercharged = overheating compressor because not enough cooling return, overcharged = high pressures, possible floodback of liquid to compressor).
  
  

• Electrical Connections: HVAC units draw significant power; over time electrical terminals can loosen from vibration/heat. Checking/tightening connections at contractors, relays, and ensuring capacitors are within spec (microfarad rating) is part of maintenance. A weak capacitor can lead to motor starting issues. Replacing a $15 capacitor proactively is better than having it fail on a 100°F day.
  
  

• Thermostat Calibration & Settings: Make sure the thermostat is reading accurately and is programmed (if applicable) to customer’s preferences. Sometimes a simple homeowner complaint “it’s not reaching set temp” is because someone set an incorrect schedule or the thermostat location is getting a false reading (like a lamp heating it, etc.).
  
  

• Duct Inspection: Especially in homes, ducts can become disconnected or develop leaks (flex ducts sag or get torn, rodents can even chew through). Checking accessible ducts (attic/basement) for obvious leaks or disconnected sections can improve system performance a lot. Also making sure supply and return vents are not blocked by furniture or rugs helps.
  
  

• Outdoor Unit Environment: Advise keeping bushes or fences at least a couple feet away from the condenser for airflow. Also, in fall, keep leaves away, and before winter perhaps cover the top to prevent debris (though completely wrapping an outdoor unit is not recommended – can trap moisture; better to just cover the top or use a manufacturer cover that breathes).
  
  

• System Performance Check: Simple measure – what’s the temperature drop across the coil? Typically, in cooling, an AC should give about a 18-22°F drop between return and supply air (known as delta-T). If it’s much lower, could indicate an issue (not enough drop = maybe low charge or weak compressor; too much drop (like 30°) = could mean not enough airflow or coil very cold possibly freezing). In heating (heat pump), the rise might be similar or in furnace might be more like 30-60°F depending on furnace design.
  
  

• User Education: Remind people to give their system breaks if possible, e.g., don’t drastically set back the thermostat if
  ... performance a lot. Also making sure supply and return vents are not blocked by furniture or rugs helps.
  
  

• Outdoor Unit Environment: Keep bushes or obstructions at least a couple of feet away from the outdoor condenser unit so it has clear airflow. In the fall, clear away leaves or debris. In winter, if you get snow, make sure the heat pump isn’t buried (elevate it or clear snow around). Some people cover their AC in winter – if you do, cover only the top to keep leaves out; don’t wrap it tightly (trapped moisture can cause rust).
  
  

• System Performance Check: A quick diagnostic is measuring the temperature difference between return and supply air in cooling (called the delta-T). It’s usually around 16-22°F in a properly charged system. If it’s much lower, the system might be undercharged or have an airflow issue; if it’s much higher (say 30°F), airflow might be very low or the system overcharged. In heating, heat pumps will have a certain expected rise as well. Technicians check pressures and refrigerant charge if performance is off.
  
  

• Listen and Watch: Unusual noises or sights can warn of problems. Scraping sounds from a blower motor, buzzing from a failing contactor, hissing (could be a refrigerant leak), gurgling (low refrigerant), or short-cycling (unit turning on and off rapidly) are all red flags to investigate. Water where it shouldn’t be (around the indoor unit) means a drain issue usually. It’s always cheaper and easier to fix an issue early than after a major component fails.
  
  

• Regular Professional Check-ups: It’s often recommended to have a professional tune-up twice a year – once before cooling season and once before heating season. The tech will clean coils, test electrical components, verify refrigerant levels, and ensure everything is operating safely (for example, checking furnace combustion and venting or heat pump defrost operation). Think of it like servicing a car – a little maintenance can prevent breakdowns and keep efficiency up.
  
  

• Proper Operation Habits: Advise consistent thermostat settings or using programmable ones wisely. For instance, with heat pumps, huge temperature setbacks overnight can trigger inefficient auxiliary heat in the morning, so moderate setbacks are better. On really hot days, it’s often better not to turn the AC off – let it run to maintain, rather than the house turning into an oven that the AC can’t catch up with later. Little things: don’t close off too many vents (in a modern system it can upset airflow balance), and avoid constantly fiddling with the thermostat (systems work best when they can settle into steady operation).
  
  

In short, preventive maintenance is key. A well-maintained HVAC system not only runs more efficiently (saving energy) but also lasts longer and is less likely to fail when you need it most. For example, keeping a condenser coil clean can reduce compressor strain and extend its life, and changing filters can prevent blower motor overheat or coil freeze-ups.

Mnemonic: “Filter, Coil, Drain, Fan” – a simple checklist of the big four to remember: keep the filter clean, coils clean, drain clear, fans (blower and condenser fan) running smoothly. If those are in good shape, chances are your HVAC is happy.

Final Thoughts

Congratulations on making it through this comprehensive HVAC guide! We’ve covered a lot of ground – from the basics of how an air conditioner works, to the specifics of every major component; from different system types like splits, packaged units, and ductless systems, to special scenarios like RV air conditioning and portable units; from understanding efficiency ratings like SEER to bracing for new refrigerants and EPA rules; and finally, the importance of maintenance and best practices.

By now, you should have a solid grasp of HVAC terminology and concepts. You know that an evaporator coil chills the indoor air while a condenser coil dumps heat outside, and that a compressor is the hardworking heart pumping refrigerant through the system. You’ve learned some handy memory tricks (hopefully they stick!) like remembering the cycle of refrigerant or what SEER means. And you’re aware of how systems can be configured differently – whether it’s a big rooftop unit handling a supermarket or a tiny mini-split quietly conditioning a home office.

For HVAC students or aspiring technicians: use this knowledge as a foundation. There’s no substitute for getting your hands on equipment and seeing these principles in action, but understanding the “why” and “how” behind the system will make you a far better tech. When you encounter a problem unit, think back to basics: refrigeration cycle, airflow, electrical controls – diagnose systematically. And keep learning – technology in this field evolves (as we see with inverter drives and new refrigerants). Perhaps soon you’ll be explaining these concepts to customers or junior techs in the field!

For homeowners or curious readers: you now can impress (or mildly annoy) your friends by pointing out the condenser unit at a restaurant and saying, “That’s the condenser – it’s releasing heat to the outdoors.” You’ll also be more in tune with your home’s system. You’ll know to change that filter and not block that return grille with a bookshelf. And if a technician comes to service your unit, you’ll understand what they mean when they say “Your capacitor is weak” or “the TXV might be sticking.”

In the end, HVAC is about comfort and health. It’s the reason we can live and work comfortably in climates that would otherwise be unbearable. It’s a field that combines physics, engineering, environmental science, and plain problem-solving grit.

Thank you for reading this far. Hopefully you found this guide educational and engaging. Keep it as a reference – HVAC has a lot of jargon, and it’s normal if you don’t remember it all at once. As you continue to study or work with these systems, concepts will click into place.

Stay curious and cool (or warm, depending on the season!), and welcome to the world of HVAC – a blend of heat and cool, pressure and flow, science and service, all working together to keep our indoor environments just right. Enjoy the journey of learning, and may all your future troubleshooting be a breeze!