Boiler Size Calculator | Advanced Heat Load & DHW

Inputs

Units

Design indoor temperature

°C

Design outdoor temperature

°C

Floor area

m²

Ceiling height

Air changes per hour (ACH)

Insulation / envelope quality HLC is a simplified heat loss coefficient per exterior area.

Domestic hot water (optional)

Mode

Boiler seasonal efficiency (%)

Safety / oversize margin (%)

Emitters

Results

Space-heating design load

0.00 kW (0 BTU/h)

Based on envelope heat loss and ventilation at design ΔT.

DHW requirement

0.00 kW

No DHW selected.

Sizing basis

Space heating

Combi covers peak DHW; cylinder adds recharge kW to space load.

Required boiler output

0.00 kW (0 BTU/h)

Includes 15% margin and 90% efficiency.

Suggested nominal sizes: —

Example Data Table

Sample scenarios with results calculated by the same method. Edit fields above and recalc to compare with these baselines.

Scenario	Area (m²)	Height (m)	ΔT (°C)	HLC (W/m²·K)	ACH	DHW mode	DHW kW	Space kW	Required kW

Formula Used

The calculator applies a steady-state design heat loss at the chosen design temperatures.

Transmission: Q_trans = A · HLC · ΔT (W)
Ventilation: Q_vent = 0.33 · ACH · V · ΔT (W), where V is volume (m³)
Space design load: Q_space = (Q_trans + Q_vent) / 1000 (kW)
Combi DHW: Q_DHW ≈ 0.07 · LPM · ΔT (kW)
Cylinder recharge: Q_DHW = 4.2 · V_L · ΔT / (60 · t) (kW)
Required output: Q_req = f(Q_space, Q_DHW, mode), then apply margin and efficiency: Q_boiler = Q_req · (1+M) / η

This is a simplified sizing aid; real designs should consider fabric specifics, infiltration testing, emitter curves, set-back strategies, and local codes.

How to Use

Select units and enter design indoor and outdoor temperatures.
Provide floor area, ceiling height, ACH, and envelope quality (or custom HLC).
Choose DHW mode. For combi, set hot water flow and temperature rise. For a cylinder, set tank volume, recharge time, and temperature rise.
Set seasonal efficiency and safety margin, then click Calculate.
Review space load, DHW requirement, and final boiler output. Use suggested sizes as a quick shortlist.
Export your results as CSV or PDF for records.

Tip: Oversizing harms efficiency and comfort; right-sizing with weather compensation and modulating controls improves performance.

FAQs

1) What design temperatures should I choose?
Use a typical indoor setpoint (e.g., 20–21°C). Choose the local winter design outdoor temperature for your climate; colder climates require a larger ΔT.

2) What is ACH and why does it matter?
Air Changes per Hour represents infiltration and ventilation. Higher ACH increases ventilation heat loss, especially at higher ΔT.

3) How does insulation affect sizing?
Better insulation lowers the heat loss coefficient (HLC), reducing transmission loss and the required boiler output.

4) How is a combi different from a cylinder system?
A combi must cover instantaneous DHW demand, so sizing is based on the larger of space load or DHW kW. Cylinder systems add recharge kW to the space load.

5) Should I include a safety margin?
A modest margin (e.g., 10–20%) helps handle uncertainties. Excessive oversizing can cause cycling and inefficiency.

6) Does emitter type change the result?
Emitter choice influences flow temperature and efficiency but not the steady-state heat loss. Use controls and weather compensation for best results.

7) Can I use this for commercial spaces?
Yes for preliminary estimates, but detailed design should consider occupancy, ventilation standards, zoning, gains, and regulatory requirements.