Project Inputs
Results
ΔT (inside–outside) —
Envelope load —
Infiltration load —
Space heating subtotal —
DHW load —
Combined sizing load (before efficiency) —
Required boiler output (with pickup) —
Estimated boiler input (accounts for efficiency) —
Recommended nominal size —
All results shown in both kW and BTU/hr for convenience.
Quick Tips
- Rate per area is an overall envelope indicator.
- ACH depends on air-tightness and ventilation strategy.
- Use “Priority DHW” when an indirect tank is prioritized.
- Pickup covers cold starts and distribution warm-up.
Example Scenarios
| Scenario | Units | Area | Height | ACH | Rate | Inside | Outside | DHW Flow | DHW Rise | Efficiency | Pickup | Mode | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tight modern home | SI | 160 | 2.5 | 0.3 | 30 | 21 | -5 | 12 | 35 | 95 | 10 | priority | |
| Typical house | US | 2000 | 8 | 0.5 | 0.6 | 70 | 10 | 2.5 | 70 | 90 | 15 | priority | |
| Older leaky house | US | 2400 | 9 | 0.9 | 0.9 | 70 | 0 | 3.0 | 70 | 82 | 20 | simul |
Rate units: SI = W/(m²·K), US = BTU/hr·ft²·°F. Flow units: SI = L/min, US = gpm.
Formulas Used
Let ΔT = Tindoor − Toutdoor.
- Envelope loss:
SI: Qenv (W) = rate·Area·ΔT, where rate in W/(m²·K).US: Qenv (BTU/hr) = rate·Area·ΔT, where rate in BTU/hr·ft²·°F.
- Infiltration loss:
SI: Qinf (W) ≈ 0.33·ACH·Volume(m³)·ΔT.US: Qinf (BTU/hr) ≈ 1.08·CFM·ΔT, with CFM = ACH·Volume(ft³)/60.
- Space subtotal: Qspace = (Qenv + Qinf)·(1 + distribution%).
- DHW load (if enabled):
SI: QDHW (W) ≈ (Flow L/min / 60)·1000 kg/m³·4.186 kJ/kg·K·ΔT(°C).US: QDHW (BTU/hr) ≈ 500·gpm·ΔT(°F).
- Combined sizing:
Priority DHW: Qcombined = max(Qspace, QDHW).Simultaneous: Qcombined = Qspace + QDHW.Space only: Qcombined = Qspace.
- Pickup / safety: Qreq = Qcombined·(1 + pickup%).
- Boiler input: Input ≈ Qreq / efficiency.
Conversions: 1 kW = 3412.142 BTU/hr. 1 ft = 0.3048 m. 1 ft² = 0.092903 m². 1 m³ = 35.3147 ft³.
How to Use This Calculator
- Select the unit system to match your project.
- Enter indoor and outdoor design temperatures to set ΔT.
- Provide floor area, average ceiling height, ACH, and envelope rate.
- Set distribution losses, efficiency, pickup factor, and sizing mode.
- Enable DHW and specify peak flow and temperature rise if applicable.
- Click Calculate to see loads and recommended boiler size.
- Use Download CSV or Download PDF for recordkeeping.
This tool provides engineering estimates. Verify selections against local codes, manufacturer data, and detailed heat-loss studies.
Fixture Flow and Instantaneous Hot‑Water Load Examples
Illustrative domestic hot water loads at a 35 °C (≈63 °F) rise using Q = ṁ·cp·ΔT.
| Fixture / Use | Flow (L/min) | Instantaneous Load (kW) | Instantaneous Load (BTU/hr) |
|---|---|---|---|
| Shower (efficient) | 8 | 19.5 | 66,655 |
| Shower (standard) | 12 | 29.3 | 99,983 |
| Kitchen faucet | 6 | 14.7 | 49,991 |
| Bathroom faucet | 4 | 9.8 | 33,328 |
| Washing machine (fill) | 10 | 24.4 | 83,319 |
| Bathtub fill (fast) | 15 | 36.6 | 124,978 |
Quick Boiler Size Guide by Combined Load
Map your calculated required output to a practical nominal size step.
| Calculated Required Output (kW) | Approx. BTU/hr | Suggested Nominal Size | Typical Application |
|---|---|---|---|
| ≤ 8 | ≤ 27,300 | 10 kW | Small apartment / flat |
| 8–12 | 27,300–41,000 | 15 kW | Efficient small home |
| 12–18 | 41,000–61,500 | 20 kW | Average home (mild) |
| 18–25 | 61,500–85,300 | 30 kW | Average home (cool) |
| 25–32 | 85,300–109,100 | 35 kW | Larger home / combi DHW |
| 32–45 | 109,100–153,500 | 50 kW | Large house / small building |
| 45–70 | 153,500–238,800 | 75 kW | Multi‑unit / light commercial |