HVAC · Envelope heat-flow arithmetic

Break heat loss into visible components.

Measure a rectangular enclosure, enter verified U-factors and design temperatures, and compare conductive and infiltration terms.

← Enclosure length →↕ Wall height↕ Enclosure widthOpaque wall · ceiling · floor

← Total window area →↕ Total door areaSubtract openings from gross wall area

1 Measure the enclosure and openings

Enclosure lengthfeetEnclosure widthfeetWall heightfeetTotal window areaft²Total door areaft²

2 Enter temperatures and verified U-factors

Indoor design temperature°FOutdoor design temperature°FOpaque wall U-factorBTU/h·ft²·°FWindow U-factorBTU/h·ft²·°FDoor U-factorBTU/h·ft²·°FCeiling U-factorBTU/h·ft²·°FFloor U-factorBTU/h·ft²·°FEntered infiltration rateACH

Transparent calculation

Conductive loss = U-factor × area × temperature difference.

Opaque walls exclude entered window and door areas. The separate infiltration term uses 0.018 BTU per cubic foot per °F multiplied by room volume, entered ACH, and temperature difference.