Boiler size calculator
The heating output of a boiler should cover the heat loss of the house on cold days. The calculator estimates it from the floor area and the insulation level.
Your measurements
Result
- Heating power needed
- 9.6 kW
Choose a boiler with 10–15% reserve over the calculated demand for very cold days.
How the calculation works
- Power needed (kW) = heated area × demand per m² ÷ 1,000: about 100 W/m² uninsulated, 80 W/m² with standard insulation and 60 W/m² with high-performance insulation.
Worked example
For 120 m² with standard insulation: 120 × 80 = 9,600 W, so about 9.6 kW of heating.
What the calculator assumes
- The values include a small reserve over the heat pump calculation, for very cold days and for heating up quickly after a pause.
- Ceilings are taken as 2.5–2.7 m.
When to adjust the result
- Combi boilers with instantaneous hot water are often 24 kW or more because of the hot water, not the heating — so the modulation range matters: a boiler that turns down to 3–5 kW doesn't cycle constantly.
- In large houses or homes with several bathrooms, a stored hot water cylinder is often more comfortable than instantaneous hot water.
Avoiding an oversized boiler
Size the heating side on the building's calculated heat loss, not on the old boiler's rating — old boilers were often oversized, and a much bigger boiler than needed cycles on and off and runs less efficiently. For combi boilers, the hot-water flow rate usually decides the size, not space heating.
Condensing boilers are most efficient with low return temperatures, so balance the radiators and lower the flow temperature as far as comfort allows. A modulation range that goes low enough to match mild days matters as much as the maximum output.
Frequently asked questions
What size boiler do I need?
For heating alone, roughly the heated area times 60–100 W/m². A combi's size is often set by the hot water demand instead.
Why is my combi boiler 24 kW for a small flat?
Instant hot water needs high power; for heating the boiler modulates down, so a low minimum output is what matters.
Combi or system boiler with a cylinder?
A combi suits smaller homes with one bathroom in use at a time; a cylinder suits homes with several bathrooms or high hot-water demand.
What flow temperature should I set?
As low as keeps the rooms comfortable — often 50–60°C on most days — so the boiler condenses.