Heating Load
Calculator
Combine verified envelope-transmission and entered infiltration inputs for a steady-state heat-loss sensitivity. This is not a complete load calculation or equipment selection.
Who it's for: mechanical engineers and building designers sizing heating systems for residential or light-commercial projects — you'll need the exposed envelope area, an area-weighted average U-value, and the ASHRAE 99% winter design temperature for the project location, typically at schematic design.
How to use this calculator
The formula
Qtotal = U · A · ΔT + 1.08 · CFM · ΔT
where CFM = ACH × Volume ÷ 60
U = average envelope U-value (BTU/h·ft²·°F) — the reciprocal of total R-value
A = exposed envelope area (ft²) — walls, roof, windows, and doors combined
ΔT = indoor setpoint − outdoor 99% design temperature (°F)
CFM = infiltration airflow in cubic feet per minute
1.08 = sensible heat factor for air at standard conditions (BTU/h per CFM per °F)
kW = Qtotal ÷ 3,412
Worked example
When this estimate will be off
- Solar and internal gains are intentionally excluded — this gives a conservative peak-loss result, which is standard for heating equipment sizing. Including gains would reduce the calculated load.
- Assumes one uniform U-value for the entire envelope. Real buildings have walls, windows, and roofs at very different U-values; compute the area-weighted average before entering or the result will be inaccurate.
- The ACH infiltration estimate is a rough whole-building figure. ACCA Manual J uses infiltration class tables tied to construction quality and local wind exposure — blower-door test results give a more accurate ACH.
- Covers only the two displayed steady-state terms. Ventilation, zoning, thermal bridges, ground coupling, transient effects, controls, distribution losses, equipment performance and selection are outside the worksheet.
Frequently asked questions
A common mistake: Solar and internal gains are intentionally excluded — this gives a conservative peak-loss result, which is standard for heating equipment sizing. Including gains would reduce the calculated load.
It is the sensible heat capacity of air at standard conditions: air density (~0.075 lb/ft³) × specific heat (0.24 BTU/lb·°F) × 60 min/h ≈ 1.08 BTU/h per CFM per °F. Multiplying by CFM and ΔT converts the infiltration airflow into a heat-loss rate.
ASHRAE recommends the 99% heating design dry-bulb temperature — the value exceeded 99% of hours in the heating season for that city. It is listed in ASHRAE Fundamentals Table 1 (climatic design conditions) for hundreds of locations. Using a warmer value under-sizes equipment; a colder value adds safety margin.
For each surface type (wall assembly, roof, window, door) calculate U = 1 ÷ R-total. Multiply each U by its area, sum across all surfaces, then divide by the total exposed area. For reference: code-minimum walls are roughly U-0.064–0.080; double-pane windows around U-0.30–0.40. A composite below U-0.10 indicates a well-insulated envelope.
It gives the design load the system must meet — a reliable starting point. Equipment is typically selected at 1.15–1.25× the calculated load to account for duct or piping losses and allow warm-up recovery. ACCA Manual J is the standard method for final residential equipment sizing; this calculator is a preliminary screen.
Sources
- ASHRAE Handbook of Fundamentals Ch. 18 — design heating load calculation method: U·A·ΔT for transmission, 1.08·CFM·ΔT for sensible infiltration
- ACCA Manual J (8th ed.) — residential load calculation standard; defines design conditions, infiltration classes, and construction factors
This free Heating Load Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.