Home Heat Loss (HVAC) Calculator
Estimate residential design heat loss in BTU/hr from walls, roof, and windows using R-values and design temperatures for HVAC system sizing.
Results
What is it?
Design heat loss is the rate at which heat escapes a building under design (worst-case) winter conditions, measured in BTU/hr. It determines the minimum heating system capacity needed to maintain indoor comfort. The formula Q = A/R x DeltaT calculates heat flow through each building envelope component. This is a simplified Manual J approach covering walls, roof, and windows.
How to use
1. Enter your home floor area and ceiling height. 2. Enter R-values for walls, roof/ceiling, and windows. 3. Enter your outdoor design temperature (99th percentile coldest day for your location — find via ASHRAE data). 4. Enter your target indoor temperature. 5. The result shows total BTU/hr heat loss and equivalent tonnage for system sizing.
Example scenario
A 1,500 sq ft home with 9 ft ceilings, R-13 walls, R-38 roof, 200 sq ft of double-pane windows (R-2). Design temperatures: outdoor 0 F, indoor 70 F. DeltaT = 70 F. Estimated results: wall loss ~4,200 BTU/hr, roof ~2,800 BTU/hr, windows ~7,000 BTU/hr. Total ~14,000 BTU/hr = 1.2 tons. A 1.5-ton heat pump would provide adequate capacity.
Pro tip
This calculator omits infiltration (air leakage), which can add 20-40% more heat loss in older homes. For a full Manual J calculation (required for permit-based HVAC in many jurisdictions), use professional HVAC software like Wrightsoft. Upgrading windows from R-2 to R-5 often reduces window heat loss by 60% and can be the single highest-impact retrofit in a cold climate.