How Do I Size an Air Conditioning System?

What Sizing Means

Sizing is the process of selecting an air conditioning and heating system with the right capacity for your space. A system that is too large will short cycle, meaning it turns on and off too often. Short cycling does not give the equipment the run time it needs to remove humidity, and a home that holds low humidity feels much cooler at any thermostat setting. A system that is too small will run constantly and still struggle on the hottest days. The goal is a system that is neither too large nor too small.

How To Size Your System

Software Load Calculation

Running a sizing program with all of your construction data is by far the most accurate method, because it removes guesswork and rules of thumb. The result reflects insulation, windows, orientation, and other details that square footage alone cannot capture. Many municipalities require a software load calculation when permitting new construction, since no best guesses are allowed for a code correct installation. AC Direct provides a free sizing program with any equipment purchase.

Local Contractor Estimate

We sometimes recommend a local estimate before you purchase so a professional can look at your floor plan and confirm the size. Contractors do not always agree, so ask what method each one used. If an estimate relies only on square footage, there is no assurance the system will perform properly. Asking good questions and expecting clear answers helps you avoid an oversized unit.

Read Your Existing Model Number

If you are replacing a system, the outdoor unit model number usually codes the BTU capacity. There are 12,000 BTU per ton, so a number such as 24 means 24,000 BTU, which divided by 12,000 equals 2 tons. Use the model number, not the serial number.