Boiler Size Calculator (kW for heating plus hot water, combi or system)

Understanding the boiler size calculator — concept and principle

This calculator estimates the boiler output needed in kW and BTU/h to heat a home and provide hot water, for a combi boiler that heats tap water on demand or a system boiler that reheats a stored cylinder. Homeowners use it to check installer recommendations.

Larger of heat or hot water: with the default inputs the result is 24.4 kW (combi set by hot water).

How it is calculated

Space heating = design heat loss × (1 + margin %)

Combi hot water: P (kW) = flow (L/min) ÷ 60 × 4.186 × (T_hot − T_cold); boiler size = larger of heating and hot water

System boiler: P = cylinder litres × 4.186 × (T_store − T_cold) ÷ (reheat minutes × 60); boiler size = heating + hot water

1 kW = 3,412.14 BTU/h

The principle

Heating water takes energy equal to mass × specific heat × temperature rise; water's specific heat is about 4.186 kJ per kg per °C, and 1 L weighs about 1 kg. A combi boiler must heat running water instantly, which needs far more power than most homes need for radiators, and it pauses heating while doing so. A system boiler stores hot water, so its hot water demand is spread over the reheat time but adds to heating.

Worked example

Default inputs: heat loss 8 kW, margin 10%, combi, flow 10 L/min, cold 10 °C, hot 45 °C.

Space heating = 8 × 1.10 = 8.8 kW.
Hot water = 10 ÷ 60 × 4.186 × (45 − 10) = 24.4 kW.
Combi size = larger value = 24.4 kW, set by hot water.
In BTU/h = 24.42 × 3,412.14 = 83,319.

Each point is the calculator re-run with only “Combi: hot-water temperature at the tap (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Combi: hot-water temperature at the tap (°C)	Boiler output needed (kW)
11.2	8.8
22.5	8.8
33.8	16.6
45	24.4
56.2	32.3
67.5	40.1
90	55.8

How to read the result

For combis the hot water flow usually sets the size; a higher flow or colder mains water in winter needs a bigger boiler. For system boilers, a longer reheat time allows a smaller boiler. Oversizing a boiler for heating can make it cycle on and off, reducing efficiency.

Common mistakes

Guessing heat loss instead of using a room-by-room calculation.
Using summer mains water temperature.
Expecting a combi to supply two showers at once at full flow.

Keep in mind: This is an estimate, not engineering advice. Boiler selection and installation must be done by a qualified, registered heating engineer following local regulations.

Frequently asked questions

What size combi boiler do I need?

It is usually set by the hot water flow you want; about 24–35 kW is common for homes.

Why is hot water so demanding?

Heating running water from 10 °C to 45 °C at 10 L/min needs over 24 kW.

How do I find my heat loss?

A heating engineer can calculate it room by room using a standard method such as EN 12831.

Sources: OpenStax, University Physics Volume 2, 2016, §1.4 (specific heat of water 4,186 J/(kg·°C)) · EN 12831-1:2017, Energy performance of buildings — Method for calculation of the design heat load

Checked 9 Nov 2026. Sensible heat Q = m·c·ΔT with c = 4,186 J/(kg·°C) for liquid water: OpenStax, University Physics Volume 2 (2016), §1.4, Table 1.3; design heat loss as in the Heat Loss Calculator (EN 12831-1:2017 method); 1 W = 3.412 14 BTU/h (NIST SP 811).

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