HVAC Calculators
This is the trade where the standard method is behind a paywall and the free method is the one the Department of Energy says causes the problem.
The calculators
- AC Tonnage Calculator12,000 Btu/h per ton by definition. DOE on why square feet per ton oversizes
- BTU CalculatorQ = U x A x dT over the whole envelope. The DOE on why rules of thumb oversize
Sizing heating and cooling equipment properly means ACCA Manual J. It is the standard, it is what a competent contractor uses, and it is not a free document. So the internet sizes equipment the other way, by multiplying floor area by a number between 20 and 30.
The Department of Energy's Building America programme, run by Pacific Northwest National Laboratory, is blunt about where that leads: rules of thumb are too often used, which results in excessively oversized systems, costing more, wasting energy and cycling on and off too frequently.
And oversizing is not the safe direction
This is the part that surprises people. With most things, too big is merely wasteful. With cooling it actively fails, because cooling and dehumidifying are different jobs and moisture only leaves the air while the coil is running. An oversized unit hits the thermostat setpoint fast, shuts off, and leaves the house cold and damp. A house that feels clammy at 72 degrees usually has too much equipment, not too little.
What these pages do instead
The conduction term, properly: area over R-value, times the design temperature difference, for every surface of the envelope, added up. It is one line of physics, it is the largest single term in a real load calculation, and it responds to the building rather than to its floor area. Two houses of identical size with different glazing come out a third apart.
And then they say what is missing. Infiltration, solar gain by orientation, internal gains, duct losses through unconditioned space, and the latent load for moisture. All five are named on the page rather than quietly omitted, because a number that looks complete and is not is worse than one that admits its edges.
The figure that is local and stays local
Design temperature difference drives everything linearly and it is a jurisdiction-level value from IRC Table R301.2(1). These pages ask for it. They also will not tell you it is the coldest night on record, because design conditions are deliberately exceeded a small share of hours a year: sizing for the extreme means running badly for the rest of the time, which is the same mistake as oversizing wearing a different hat.