Inputs
Example Data
| Room | Size (L×W×H) | Insulation | Windows % | ACH | ΔT | Estimated BTU/h |
|---|---|---|---|---|---|---|
| Bedroom | 12×10×8 ft | Average | 12% | 0.5 | 50°F | 5,400 |
| Living | 20×15×9 ft | Poor | 18% | 0.8 | 50°F | 16,900 |
| Office | 14×12×9 ft | Good | 10% | 0.4 | 50°F | 7,600 |
Values are illustrative; your building and climate will differ.
Formulas Used
- ΔT = Indoor setpoint − Outdoor design.
- Envelope loss = Uwall·Awall·ΔT + Uwin·Awin·ΔT + Uceil·Aceil·ΔT + Ufloor·Afloor·ΔT (BTU/h).
- Walls area ≈ 2·(L+W)·H. Window area = % of walls.
- Infiltration loss = 1.08 · CFM · ΔT (BTU/h), where CFM = ACH·Volume/60 and Volume = L·W·H (ft³).
- Hydronic mode (optional): BTU/h = 500 · GPM · ΔTwater.
- Required capacity = (Envelope + Infiltration [+ Hydronic]) × Safety / Efficiency.
- Metric: 1 BTU/h = 0.000293071 kW; 1 W = 3.412142 BTU/h.
How to Use This Calculator
- Select units and enter indoor and outdoor design temperatures.
- Enter room dimensions. Toggle ceiling and floor losses if relevant.
- Choose insulation quality, window percentage, and window U-value.
- Provide ACH. Typical tight homes: 0.3–0.5; older: 0.7–1.0.
- Set system efficiency and safety factor for equipment selection.
- Optional: enable Hydronic mode to size water-side heat delivery.
- Click Calculate. Export results as CSV or PDF immediately.
Rule‑of‑Thumb BTU/ft² by Insulation & Climate
| Insulation | Mild (ΔT 25–35°F) | Cool (ΔT 35–45°F) | Cold (ΔT 45–60°F) | Very Cold (ΔT 60–75°F) |
|---|---|---|---|---|
| Good | 15–20 BTU/ft² | 18–25 BTU/ft² | 22–30 BTU/ft² | 28–38 BTU/ft² |
| Average | 18–25 BTU/ft² | 22–30 BTU/ft² | 28–40 BTU/ft² | 35–50 BTU/ft² |
| Poor | 22–30 BTU/ft² | 28–40 BTU/ft² | 35–55 BTU/ft² | 45–65 BTU/ft² |
Guidance only; use your specific inputs for precise results.
Design ΔT Quick Reference (Illustrative)
| Indoor Setpoint | Outdoor Design | ΔT (°F) | Comment |
|---|---|---|---|
| 70°F | 45°F | 25 | Mild coastal climates |
| 70°F | 35°F | 35 | Cool temperate zones |
| 70°F | 20°F | 50 | Cold continental climates |
| 70°F | 0°F | 70 | Very cold/wintry regions |
Pick a design outdoor temperature aligned with local standards.
Equipment Sizing Bands (Average Insulation)
| Input Capacity (BTU/h) | Typical Coverage at ΔT≈50°F | Notes |
|---|---|---|
| 10,000 | 250–400 ft² | Small bedroom, office |
| 20,000 | 500–800 ft² | Large bedroom, suite |
| 30,000 | 750–1,100 ft² | Apartment, open plan |
| 40,000 | 1,000–1,500 ft² | Small home level |
| 60,000 | 1,500–2,200 ft² | Average single‑family |
| 80,000 | 2,000–3,000 ft² | Larger home level |
Ranges depend on ΔT, ACH, U‑values, and efficiency.
Results
Envelope loss
—
Infiltration loss
—
Total space load
—
Required output with safety
—
Estimated input capacity (accounts for efficiency)
—
Tip: Choose standard sizes above the input capacity shown.
Reference U-Values
- Poor walls/ceiling: U≈1.2 BTU/h·ft²·°F (≈6.8 W/m²·K)
- Average walls/ceiling: U≈0.7 BTU/h·ft²·°F (≈4.0 W/m²·K)
- Good walls/ceiling: U≈0.4 BTU/h·ft²·°F (≈2.3 W/m²·K)
- Floor to basement: U≈0.3; slab on grade: U≈0.6
Notes & Assumptions
- Rectangular room approximation; complex geometries introduce variance.
- Window percentage applies to wall area only.
- ACH represents infiltration at design conditions.
- Values are estimates; verify with local codes and professionals.
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