HVAC

Whole-House Heat Load Calculator

A free block-load estimator for residential heating and cooling loads, built on the same public ASHRAE Fundamentals physics that ACCA's Manual J procedure uses — transmission loss, infiltration, solar gain, and internal gains. It is not a reproduction of ACCA's copyrighted Manual J worksheets and is not room-by-room or duct-design accurate.

Whole-House Load Estimator

Inputs stay in your browser until you unlock the result. See methodology & limitations below before using this for equipment sizing.

Winter design temp (°F)Local 99% heating design temp — check your jurisdiction's mechanical code or an ASHRAE lookup toolSummer design temp (°F)Local 1% cooling design tempIndoor heating setpoint (°F)Indoor cooling setpoint (°F)Conditioned floor area (sq ft)Ceiling height (ft)Wall assembly R-valuee.g. R-13 for standard 2x4 fiberglass battCeiling/attic R-valueFloor/foundation R-valueTotal window area (sq ft)Window U-factor0.20–0.35 typical for double-pane low-E, 0.9–1.2 for single-paneWindow SHGCSolar heat gain coefficient, typically 0.20–0.40Blower-door result (ACH50)If unknown, 7 is a reasonable estimate for an average-tightness existing homeOccupants

Methodology & limitations — read before using this for equipment sizing

Not ACCA-certified. This is a whole-house block-load estimate, not ACCA's full Manual J procedure. It does not reproduce ACCA's copyrighted tables or worksheets. Where a jurisdiction requires certified Manual J software for a permit, use that instead.
Not room-by-room. Manual J's real value for duct design and equipment sizing comes from room-by-room loads. This tool estimates the whole building only.
Design temperatures are user-supplied. We don't embed a proprietary ASHRAE climate database. Pull your local 99% heating / 1% cooling design temps from your jurisdiction's mechanical code, a certified Manual J program, or a public ASHRAE data lookup tool.
Solar gain is simplified. Window solar heat gain uses a flat average peak factor (~150 Btu/h per sq ft of glass at the given SHGC), not ASHRAE's full CLTD/SHGF tables broken out by orientation, latitude, and hour. Homes with significant west or south glass exposure may see meaningfully higher actual peak loads than this estimate shows.
Infiltration uses the ACH50 ÷ 20 rule of thumb to convert a blower-door result to natural air changes — a widely used approximation in energy auditing, not Manual J's full shielding-class and leakage-area method.
Wall area is estimated from a square footprint assumption based on floor area and ceiling height, not measured. For an irregular footprint, actual wall area will differ.

Sources: transmission and infiltration formulas follow standard ASHRAE Fundamentals heat-transfer equations (Q = U × A × ΔT; Q_sensible = 1.08 × CFM × ΔT; Q_latent = 0.68 × CFM × Δgrains), as summarized at engineeringtoolbox.com. Occupant sensible/latent gain figures (230 / 200 Btu/h per person) are standard ASHRAE design values.

Want the operating problems this feeds into? See the HVAC measurement guide.