Ask around and you will hear a lot of rules of thumb for sizing an air conditioner. One ton per 500 square feet. Just match what was there before. Go a half-ton bigger because it gets hot here.
Those shortcuts are how a lot of Northshore homes end up with a system that is too big, and a too-big system causes real problems.
What oversizing does
- Short cycling. The system runs a few minutes, satisfies the thermostat, and shuts off. Then it does it again.
- Poor moisture removal. Short run times mean the coil never removes much water, so the house stays damp.
- Uneven rooms. Rooms near the unit cool fast while rooms at the end of the duct run never catch up.
- More wear. Starting up is the hardest part of a compressor's life, and an oversized system starts far more often.
- Higher cost. You pay more for bigger equipment up front, and often for bigger ducts and electrical too.
What a load calculation is
A Manual J load calculation is the industry-standard method, published by the Air Conditioning Contractors of America (ACCA), for figuring out how much heating and cooling a specific house needs. It accounts for the size and direction of every window, insulation levels, ceiling heights, air leakage, the local climate and more.
The catch is that a load calculation is only as good as the measurements that go into it. Measuring a whole house by hand takes hours, which is why many contractors skip it.
How we do it
We use a LiDAR scan from a tablet to build an accurate 3D model of your home in a fraction of the time. Every room, window and wall is measured, and the load calculation runs from that model using ACCA-approved software.
The result tells us how much capacity your home really needs, room by room. That shapes not just the size of the equipment, but how the ducts should be designed so each room gets the right amount of air.
Ask before you buy
If you are getting quotes for a new system, ask each contractor how they sized it. "We matched your old unit" is not an answer. A room-by-room load calculation is.