It’s called the Mills Rule, and it makes a lot more sense than the modern 500 square feet per ton rule. Yesterday, I opened my weekly newsletter from and learned about a rule of thumb method for calculating heating loads that actually worked pretty well. A common one for air conditioning here in the southeastern US is one ton of capacity for each 500 square feet of conditioned floor area. ![]() ![]() Often it’s just a ton of heating or cooling capacity for some amount of floor area in the house. A heck of a lot of homes get their HVAC systems sized by rules of thumb. Of course, just because we can calculate the loads accurately doesn’t mean it happens all that often. They didn’t have air conditioning (not as we know it today anyway), and the engineering of heating systems wasn’t as well developed. Nowadays, we know the physics behind heat transfer, so we can come up with fairly accurate predictions of how much heating and cooling a building will need even before it’s built. Sizing a heating or cooling system should start with an assessment of the heating or cooling load.
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