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A
A2L RefrigerantA class of mildly flammable, low global warming refrigerants such as R-454B and R-32, now standard in new systems.
AFUEAnnual Fuel Utilization Efficiency. The percentage of fuel a furnace turns into usable heat. A 96% AFUE furnace delivers 96 cents of heat per fuel dollar.
AHRIThe Air-Conditioning, Heating, and Refrigeration Institute, which certifies the rated performance of matched HVAC systems.
Air HandlerThe indoor unit that moves conditioned air through your ducts. It pairs with a heat pump or air conditioner and holds the blower and coil.
B
BlowerThe fan inside the furnace or air handler that pushes air through the ductwork.
BTUBritish Thermal Unit, the basic measure of heat. HVAC capacity is rated in BTU per hour, and 12,000 BTU equals one ton of cooling.
C
CoilTubing where refrigerant absorbs or releases heat. A system has an indoor evaporator coil and an outdoor condenser coil.
CompressorThe pump at the heart of an air conditioner or heat pump that circulates refrigerant and raises its pressure and temperature.
CondensateThe water that drips off the indoor coil as it removes humidity. It drains away through a condensate line.
CondenserThe outdoor unit that releases heat removed from your home. It holds the compressor and the outdoor coil.
COPCoefficient of Performance, a measure of heating efficiency. A COP of 3 means three units of heat for each unit of electricity.
D
DamperA plate inside ductwork that opens or closes to control airflow to different areas, used in zoned systems.
Defrost CycleA heat pump's automatic routine that briefly melts frost off the outdoor coil in cold weather.
Dual FuelA system that pairs a heat pump with a gas furnace, running the heat pump in mild weather and the furnace when it gets cold.
DuctlessA system that delivers conditioned air straight into rooms with no ducts, such as a mini split.
DuctworkThe network of channels that distributes conditioned air through a home and returns it to the system.
E
EER2Energy Efficiency Ratio under the 2023 standard, a measure of cooling efficiency at a single high outdoor temperature.
Evaporator CoilThe indoor coil where refrigerant absorbs heat and humidity from your home's air.
F
FurnaceA unit that heats air by burning gas or using electric elements, then sends it through the ducts.
H
Heat ExchangerThe part of a furnace that transfers heat from combustion into the air without mixing the two.
Heat PumpAn electric system that both heats and cools by moving heat instead of burning fuel. In winter it runs in reverse to heat.
HSPF2Heating Seasonal Performance Factor under the 2023 standard, a measure of a heat pump's heating efficiency over a season.
I
InverterA variable-speed drive that lets a compressor ramp up and down instead of running full-on or off, improving comfort and efficiency.
L
Line SetThe pair of insulated copper lines that carry refrigerant between the outdoor and indoor units.
Load CalculationAlso called Manual J. The room-by-room calculation of a home's heating and cooling needs, the correct way to size a system.
M
MERVMinimum Efficiency Reporting Value, a 1 to 16 scale for how well an air filter captures particles. Higher catches more.
Mini SplitA ductless heat pump with an outdoor unit and one or more indoor heads that condition rooms directly.
P
Packaged UnitA system with all components in one outdoor cabinet, used where there is no indoor space for an air handler or furnace.
PlenumThe central air chamber on the furnace or air handler where supply or return air collects before distribution.
R
R-32A single-component A2L refrigerant with low global warming potential, used in many new systems.
R-410AThe refrigerant used in most systems sold from 2010 to 2025, now being phased down in favor of A2L refrigerants.
R-454BA blended A2L refrigerant with low global warming potential, the most common replacement for R-410A in new systems.
RefrigerantThe fluid that carries heat through an air conditioner or heat pump as it changes between liquid and gas.
Return AirThe air pulled from your rooms back to the system to be filtered and conditioned again.
Reversing ValveThe component that lets a heat pump switch between heating and cooling by reversing refrigerant flow.
S
SEER2Seasonal Energy Efficiency Ratio under the 2023 standard, the main measure of cooling efficiency over a season. Higher means lower running cost.
Setback ThermostatA thermostat that automatically lowers or raises the temperature on a schedule to save energy.
Short CyclingWhen a system turns on and off too often, usually from oversizing, which wastes energy and wears out parts.
Split SystemThe common setup with a separate outdoor unit and indoor unit connected by a line set.
Static PressureThe resistance to airflow inside the duct system. Too much strains the blower and cuts performance.
SubcoolingHow far refrigerant is cooled below its condensing temperature, used to check a system's charge.
SuperheatHow far refrigerant is heated above its boiling point, used to check a system's charge.
Supplemental HeatBackup heat, electric or gas, that helps a heat pump keep up on the coldest days.
Supply AirThe conditioned air the system delivers into your rooms through the supply ducts.
T
ThermostatThe control that senses room temperature and tells the system when to heat or cool.
TonA unit of cooling capacity equal to 12,000 BTU per hour. A 3-ton system removes 36,000 BTU per hour.
Two-StageEquipment with high and low settings for steadier comfort and better humidity control than single-stage.
V
Variable SpeedEquipment that runs at many output levels for the most even temperatures, quietest operation, and best efficiency.
Z
ZoningDividing a home into areas with independent temperature control, using dampers or separate units.
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