Heating Size Calculator
Enter the floor area you heat, how well the home is insulated, your climate and whether you want a combi boiler. The calculator estimates the heat loss and suggests boiler, furnace and heat pump sizes.
- Heat loss
- Boiler size
- Furnace size
A room-by-room heat loss survey by an installer gives the real figure.
Homes lose about 50 to 120 W per m² on a cold day. An average 100 m² home in a moderate climate loses about 8 kW, 27,300 BTU/h. A system boiler of about 12 kW covers it, but a combi needs about 24 kW for hot water flow.
Heating size chart
Moderate climate. Search for a home.
| Floor (m²) | Well insulated (kW) | Average (kW) | Poor (kW) |
|---|---|---|---|
| 50 | 2.5 | 4 | 6 |
| 75 | 3.75 | 6 | 9 |
| 100 | 5 | 8 | 12 |
| 125 | 6.25 | 10 | 15 |
| 150 | 7.5 | 12 | 18 |
| 200 | 10 | 16 | 24 |
| 250 | 12.5 | 20 | 30 |
| 300 | 15 | 24 | 36 |
kW.
Home size and heat
Average, moderate climate. Slide the floor area.
Heat rises from the home.
How to size heating
Home to kW. Step through it.
Area
Heated floor.
Improving insulation first lets you fit a smaller system.
Boiler, furnace or heat pump
Heating systems. Compare them.
Wet system.
Warm air.
Low carbon.
Heat pumps work best sized close to the heat loss in a well insulated home.
Before you buy
Heating checks. Tick each item.
Tick each item as you go.
Fit thermostatic valves to control each room.
What you get
Heating sizing. Each tile explains one result.
kW.
Size.
BTU/h.
kW.
Factor.
An oversized boiler cycles on and off and wastes fuel.
Frequently Asked Questions
No question matches that search.
What size boiler do I need?
About the heat loss plus ten per cent, or about twenty four kilowatts for a combi.
How much heat does a house lose?
About fifty to one hundred and twenty watts per square metre on a cold day.
What size boiler for a three bedroom house?
Often twelve to eighteen kilowatts for a system boiler, twenty four to thirty for a combi.
What size furnace do I need?
About the heat loss in BTU per hour divided by the furnace efficiency.
Can a heating system be too big?
Yes. It short cycles and wastes energy.
What size heat pump do I need?
About the heat loss on the coldest design day.
Why does a combi boiler need more kilowatts?
Heating tap water on demand needs more power than heating the home.
Warm without waste
Size the heating to your home's heat loss and it runs efficiently all winter.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.