HVAC sizing calculators

Wrong tonnage is the single most expensive HVAC mistake a homeowner can make. An oversized AC short-cycles. The compressor stops before the coil pulls humidity out of the air, so the house ends up at 72 degrees and clammy. Energy bills go up because the unit starts and stops constantly. The compressor wears out years early because every restart pulls locked-rotor amperage that is roughly seven times the running current.

An undersized system has the opposite problem. It runs nonstop on design days, never reaches setpoint when the outdoor temperature climbs, and burns out coils and compressors trying to keep up. Either way, the homeowner pays for it twice: once on the install and again on the replacement.

The six calculators in this category replace the old "one ton per 500 square feet" rule of thumb with a real load calculation. Square footage matters, but so does climate zone, attic insulation, window orientation, ceiling height, infiltration rate, and how many people live in the house. Get those inputs right and the answer is usually a half-ton smaller than what a quick-quote contractor will recommend, which is exactly the point. ACCA estimates that two-thirds of residential systems in the field are oversized by at least 25 percent.

Heat pumps need their own sizing logic. A heat pump rated 3 tons at AHRI 95 degrees outdoor pulls roughly 65 percent of that capacity at 17 degrees outdoor, which is the design temperature for most of the northern half of the country. Sizing a heat pump on its cooling capacity alone is what created the "heat pumps do not work in cold climates" myth. Size on the heating load at design temperature and the math works.