Load calculation guide
Heating Load Calculation Explained
Heating load is the rate of heat input needed at the selected winter design condition, not a square-footage rule.
Start with temperature difference
Conduction is driven by the difference between the indoor target and outdoor design temperature. Use the local design condition chosen for the project; do not mix weather data sources or apply a cooling condition to a heating calculation.
Document the heat-loss path
| Path | Data needed | Why it matters |
|---|---|---|
| Walls, roof, floor | Area and thermal performance | Determines conductive heat loss |
| Windows and doors | Area, U-value, exposure | Can dominate a small room |
| Infiltration and ventilation | Airflow and temperature difference | Adds outdoor-air sensible loss |
| Internal gains | Schedule and reliability | May offset loss only when present |
Useful check, not a complete method
For preliminary sensible outdoor-air heat loss in U.S. customary units, 1.08 × CFM × ΔT is a common check. For example, 100 CFM with a 30°F difference is 3,240 BTU/h. It does not replace a complete envelope, humidity, equipment, or distribution analysis.
Common mistakes
- Counting lights or occupants as guaranteed heating capacity when schedules do not match the winter peak.
- Ignoring the outside-air path because the building is described as “tight.”
- Selecting a furnace or heat pump only from a BTU/h result without checking manufacturer performance at the design condition.