MANUAL J HEAT LOAD
MANUAL J HEAT LOAD
q = ft² × HTM (climate × construction)
ft²
ft
RESULT
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FILL IN ABOVE
Whole-home rule-of-thumb sized to ACCA Manual J HTMs. Doesn't replace a room-by-room Manual J for permit submittal. Estimate only — verify with a licensed HVAC contractor running full ACCA Manual J/D/S before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This Manual J heat load calculator gives a whole-home approximation of cooling and heating BTU/hr using the ACCA Manual J approach: floor area × Heat Transfer Multiplier (HTM), with HTMs adjusted by climate zone, insulation level, window quality, and air-tightness. It is a planning tool — equipment selection, zoning, and duct design require a full room-by-room Manual J / Manual D / Manual S package run by a credentialed HVAC contractor (ACCA RSDI, Wrightsoft, or similar). Oversizing is the #1 reason heat pumps short-cycle, dehumidify poorly, and die early. ESTIMATE ONLY — verify with a licensed HVAC contractor and a full Manual J before equipment purchase.
Common questions
Why is oversized AC bad?
An oversized AC short-cycles — runs for 5 minutes, hits the thermostat, shuts off, repeats. Short cycles never run long enough to wring humidity out of the air, so the house feels clammy at the right temperature. The compressor also wears out fast from constant restarts. Manual J prevents oversizing.
Is this calculator a substitute for a real Manual J?
No. A real Manual J is room-by-room: every wall orientation, window U-value, door, infiltration rate, internal gains, ducts inside vs outside conditioned space — all entered separately and run through Wrightsoft, Cool Calc, or Elite Software. Use this calculator for a sanity check or a rough budget; permits and equipment selection need the full ACCA package.
What if my old AC has been working fine on a rule-of-thumb size?
"Working fine" usually means cooling but not dehumidifying — most older systems are oversized 30-50% and the homeowner has just accepted clammy summers. Replacement is the right time to size correctly: a heat pump (variable-speed) handles the design load and modulates down for partial loads, fixing the humidity problem.