TradesHow this works
Once you have a cooling load from a Manual J, the equipment should meet that load without going much over it. ACCA Manual S sets the ceiling: about 115% of the load for an air conditioner or a cooling-dominant heat pump, and up to 125% for a heat pump in a heating-dominant climate. This tool takes your load and system type, compares every nominal size against those limits, and recommends the smallest size that meets the load while staying under the ceiling.
When the load lands between two nominal sizes and the next size up would be oversized, the tool says so and points you to two-stage or variable-speed equipment, which sizes to the peak but runs low most of the time. The goal is always the same: meet the load, avoid short-cycling.
Common questions
How big can an air conditioner be over the load?
ACCA Manual S caps cooling equipment at about 115% of the calculated cooling load. A heat pump in a heating-dominant climate is allowed up to 125%, because the extra cooling capacity helps cover more of the heating load. Past those limits the equipment is oversized.
Why is a bigger AC worse?
An oversized unit cools the air fast, hits the thermostat, and shuts off before it has run long enough to pull humidity out. That short-cycling leaves the house cool but clammy, wears the compressor, and costs more to buy and run. Right-sized equipment runs longer, quieter, and dehumidifies properly.
What if my load falls between two sizes?
That is common. If the only nominal size that meets the load is above the Manual S ceiling, the right answer is usually a two-stage or variable-speed unit. It is rated for the peak but runs at low capacity most of the time, so it matches the load without short-cycling.
Do I still need a Manual J and Manual S?
Yes. This tool checks a nominal size against the Manual S sizing limits using the load you enter. It does not calculate the load (that is Manual J) and does not select equipment from its expanded performance data (that is a full Manual S). Get both from your contractor before you buy.
Sources: ACCA Manual S residential equipment sizing limits (115% AC / cooling-dominant HP, 125% heating-dominant HP) · 1 ton = 12,000 BTU/h. Engine version 1.0.0. Risk tier 2.