The Manual J load calculation
If you take one thing from this site, take this: the size of your system should be decided by a calculation of what your particular house loses and gains, not by its square footage and not by what is currently sitting there.
What it is
Manual J is a residential load calculation procedure published by the Air Conditioning Contractors of America. It works through your house room by room — orientation, window area and type, insulation levels, infiltration, ceiling height, local design temperatures, and the heat from people and appliances — and produces the heating and cooling loads in BTU per hour. Those loads are what the equipment should then be selected against.
The Department of Energy's Building Science guidance puts it plainly: "ACCA's Manual J, Eighth Edition, enables contractors to perform load calculations for homes. Equipment, including ducts, should be selected based on these loads."
Why the square-footage rule fails
DOE's consumer guide addresses the common estimate directly. One ton per 400 to 500 square feet "fails to take into account local climate and the efficiency of a house", and "one ton can condition 800 to 1,000 square feet of a very efficient house."
That is a spread of more than two to one. A rule that can be off by a factor of two is not a sizing method — which is why DOE also says, of rules of thumb generally, that they are "too often used to size comfort systems, which results in excessively oversized systems."
Why oversizing is the specific danger
Homeowners tend to assume that if a system is too big, the worst case is that it cools quickly. The failure is subtler and worse:
- Short cycling. The system satisfies the thermostat fast, shuts off, and restarts. Starting is the hard part of a compressor's life.
- Poor dehumidification. Removing moisture takes run time. A system that never runs long leaves the house cold and clammy, which feels worse than warm and dry.
- Cost, twice. You pay more for the equipment, then pay again in efficiency losses and shortened life.
- Duct mismatch. Larger equipment moves more air than existing ducts were built to carry, which brings noise and pressure problems of its own.
DOE lists it as: "Oversized cooling systems can cost too much, provide inadequate comfort, and lead to mold and poor indoor air quality."
How to tell whether yours was really done
Ask for the output. A genuine Manual J produces a report — usually several pages, room by room, with the design conditions stated and a total load in BTU/h. If what comes back is a single number, or "we've done this a thousand times in this neighbourhood", you have not been shown one. It may still exist; you simply cannot check a figure nobody has put in front of you. Ask to see it.
Some contractors charge for it, often crediting it against the job. It is worth paying for on an $8,000 to $25,000 purchase.
What this site does — and firmly does not
We do not perform Manual J calculations and we do not sell them. We cannot: a load calculation needs someone in your house measuring windows and duct runs, and it is graded against a published standard we have no business claiming to meet.
What we do is ask whether one was done, print your square-feet-per-ton next to DOE's two published figures as context — while saying plainly that neither figure grades your quotes — and put "can I see the load calculation?" at the top of the questions sheet you take to the conversation. If you get one good outcome from this site, that is the one.
Informational only. Not a load calculation, not an ACCA Manual J, and not an engineering report.