Heat Loss Calculation: Formula, Components & What It Means
What Is Heat Loss?
Heat loss is the rate at which a building bleeds warmth to the outdoors. It travels three ways: conduction through solid surfaces (walls, windows, roof, floor), convection via air leakage (infiltration), and radiation surface-to-surface. Every heating design starts by quantifying these paths so equipment can be sized correctly and energy waste cut at the source.
The Heat Loss Formula
Envelope loss is the sum of each surface's conduction, plus air leakage through cracks and openings:
Q = Σ(U × A × ΔT) + 0.018 × ACH × Volume × ΔT
| Symbol | Meaning | Unit |
|---|---|---|
| Q | Heat loss rate | BTU/h |
| U | U-value (thermal transmittance; lower = better) | BTU/(h·ft²·°F) |
| A | Surface area | ft² |
| ΔT | Indoor − outdoor design temperature | °F |
| ACH | Air changes per hour | 1/h |
| Volume | Indoor air volume | ft³ |
Heat Loss Components at a Glance
| Component | Typical U-value | Share of loss |
|---|---|---|
| Walls (R-13/R-19) | 0.077 / 0.053 | Large in old homes |
| Windows (single/double) | 1.1 / 0.5 | Often the weakest link |
| Roof / attic (R-38) | 0.026 | High (warm air rises) |
| Floor / basement | 0.03–0.05 | Moderate |
| Infiltration | ACH 0.5–2.0 | Up to 1/3 in leaky homes |
Heat Loss vs Your Energy Bill
A loss of 1 BTU/h running continuously through a 5-month heating season is roughly 0.036 therms/year. At $1.20/therm, a 10,000 BTU/h whole-house loss costs about $430/year just to replace. That is why sealing air leaks and upgrading windows pays back fastest — they attack the highest-U paths directly.
When a Full Calculation Matters
- New construction or permit: a full ACCA Manual J is required.
- Equipment replacement: a real calculation beats the 20–25 BTU/ft² rule of thumb.
- Single retrofit (one window or wall): a component estimate using the formula above is enough.
Standards & References
- ASHRAE Fundamentals — Ch. 18 (nonresidential), Ch. 19 (residential) load calculation
- ACCA Manual J — 8th ed., residential load
- EN 12831 — European heating load standard
- GB 50736-2012 — China civil-building HVAC design code
Frequently Asked Questions
What are the main components of heat loss?
Five: walls (conduction), windows (conduction + leakage), roof/ceiling (conduction upward), floor (conduction to ground), and infiltration (air leakage through cracks). Together they set the total BTU/h a heating system must replace.
What does U-value mean in heat loss?
U-value is thermal transmittance — heat flow per unit area per degree. Lower is better insulation: R-13 wall ≈ 0.077, R-19 ≈ 0.053, double-pane window ≈ 0.5, single-pane ≈ 1.1 BTU/(h·ft²·°F).
Is heat loss the same as heating load?
No. Heat loss is the physical rate (BTU/h). Heating load is the design input used to size equipment — it applies a safety factor to the loss and adjusts for internal gains. See our heating load calculation guide.