Boiler Size Calculator

Boiler Size Calculator shows what a wet (hydronic) heating system actually needs: the minimum boiler output (kW and BTU/h), real radiator output at your chosen flow temperature, circulating flow rate and pipe size, whether the boiler will condense, and the expansion vessel required to keep the pressure relief valve shut. Metric first — kW, litres, °C and bar — with BTU/h alongside. It works fully offline and nothing you enter leaves your phone.

HOW IT WORKS

The app follows the standard hydronic sizing chain:

1. Uses your design heat loss (coldest day, room-by-room calculation) and adds a small pick-up margin (5–10%) for reheating after setback.

2. Calculates domestic hot water demand for combi boilers:
Q = (L/min ÷ 60) × 4.18 × temperature rise.
Example: 12 L/min at 35 K = 29.3 kW.

3. Boiler size is set by whichever is larger: heating or hot water. In modern insulated homes, hot water usually dominates.

4. Derates radiators using:
F = (ΔT mean ÷ ΔT rating)^1.3
Radiators rated at ΔT 50 (75/65/20°C) lose significant output at lower temperatures:

* 70/50 → ~1.12 kW (from 1.50 kW)
* 55/45 → ~0.77 kW
* 45/35 → ~0.46 kW
These match manufacturer correction tables.

5. Calculates flow rate using:
Q = m × c × ΔT
Outputs kg/s, L/min, and suitable copper pipe size.

6. Checks condensing operation. Boilers condense when return temperature is below ~55°C.

7. Sizes the expansion vessel using sealed-system principles:
Expanded volume = system volume × expansion coefficient ÷ acceptance factor.

WHAT YOU CAN DO

* Enter heat loss and get boiler size in kW and BTU/h
* See whether heating or hot water drives sizing
* Calculate hot water kW from tap flow and temperature rise
* Check real radiator output at your system temperature
* See correction factors, required rated output, and any shortfall
* Get flow rate and minimum pipe size
* Check if your boiler will condense
* Size the expansion vessel and choose the next standard size
* View detailed breakdowns and radiator output tables
* Learn the physics in clear, simple terms

BUILT FOR CLARITY

Clean brass-and-copper design with key numbers front and center. Light/dark themes, small download, no clutter, no account.

IMPORTANT

This is a simplified educational tool, not a full heating design. It does not include room-by-room distribution, pipe losses, pump sizing, cylinder reheat, or control strategy. Final design must be confirmed by a qualified heating engineer.

Gas, oil, and LPG work must be carried out by licensed professionals (e.g. Gas Safe in the UK or local equivalent). Never DIY.

PRIVACY

No personal data is collected. The app works fully offline. Ads are served by Google AdMob, which may use an advertising identifier per Google policies.

Download Boiler Size Calculator and find out what your heating system really needs — including the part most people get wrong about radiators.