Your air conditioner is one of those things you hope you never have to replace, but when that time  inevitably comes, choosing the right size air conditioner isn’t going to be as simple as matching a unit to your home’s square footage. Modern residential HVAC design relies on a certain calculation that determines just how much cooling your house actually needs under peak conditions. It’s called the “115% rule,” and it helps you know just how closely an air conditioner’s capacity should match your home’s calculated cooling demand.

First thing’s first: Your home’s cooling load has to be determined through ACCA Manual J. That is, the standardized residential load calculation method referenced by the International Residential Code and the International Energy Conservation Code. Manual J measures how much heat an air conditioning system has to remove from a home during the hottest expected weather. It also accounts for factors like climate, insulation, windows, air leakage, duct characteristics, occupancy, and indoor humidity. The resulting cooling load (measured in British Thermal Units per hour, or BTU/h) is then used as the basis for selecting equipment through ACCA Manual S.

Here’s where the 115% rule comes in: Manual S says an air conditioner’s rated capacity can’t be more than 115% of the calculated sensible cooling load. That way, equipment can be appropriately matched to the home instead of using excess tons of cooling capacity as a safety cushion.

The 115% rule is there to help you balance cooling with efficiency. Manual J is basically calculating worst-case outdoor conditions, but Manual S is more like a reality check: How often is your house really going to be in those worst-case conditions and need the maximum amount of cooling? The conditions determined through Manual J will be relatively rare throughout the year. Most of the time, your AC is going be running under much lighter demands. Hence the 115% rule.

Then there’s the problem of short-cycling. Using oversized equipment can cool your house faster, no doubt about it, but rapid cooling can make your system shut off before it’s run long enough to remove a meaningful amount of moisture from the indoor air. That can leave your home feeling humid, even when the actual temperature is perfectly comfortable. In especially humid climates, that’s a problem. If indoor humidity goes above 60%, you run the risk of mold and other moisture-related problems. Short-cycling also puts more mechanical wear on the system, which inevitably shortens the lifespan of your AC unit.