Heat load, then a hot-water top-up
Boiler size is driven by how fast the building loses heat on the coldest day, not by floor area alone. Multiply the heated area by a heat-loss factor q (set by insulation), by the design temperature difference ΔT (set by climate), then add a top-up for hot water and a safety margin of 15 to 25%.
Insulation and climate set the numbers
| Insulation | Heat-loss q (W/m²K) | Typical building |
|---|---|---|
| Poor | 1.5 | Old, uninsulated, drafty |
| Average | 1.0 | 1980s–90s standard |
| Good | 0.70 | Modern, retrofit insulation |
| Excellent | 0.40 | New build / passive house |
Climate sets ΔT: about 25 K in a mild region, 35 K in a cold one, 45 K where winters hit −25°C. Apartments use roughly 0.8 of the house figure (heated neighbours on most sides); commercial space about 1.2.
Add for hot water, then round to a stock size
- None. Heating only — add 0 kW.
- Low. One or two people, 30 to 50 L/day — add 2 to 3 kW.
- Medium. Family of 3 to 4, one bath, 100 to 150 L/day — add 5 to 8 kW.
- High. Large household, two-plus bathrooms, 200+ L/day — add 10 kW or more.
Boilers come in stock steps (10, 15, 20, 25, 30, 35, 40 kW and up), so round the total up to the next size. A condensing boiler runs near 95% efficient, so it can be about 10 to 15% smaller than an old 80% unit for the same house. Solid-fuel boilers are sized larger — often 20 to 30 kW where gas would need 10 to 15.
Common questions
What size boiler do I need for a 100 square metre house?
It depends mostly on insulation. A poorly insulated older home needs roughly 12 to 15 kW, an average 1990s home about 8 to 12 kW, and a modern well-insulated house 5 to 8 kW. Add about 4 to 6 kW if the boiler also supplies hot water.
Is bigger safer when choosing a boiler?
No. An oversized boiler runs at low load, cycles on and off, wastes fuel and wears out faster. Aim for about 10 to 15% above your calculated heat load, not more.
Why is the old "1 kW per 10 square metres" rule unreliable?
That rule only fits average-insulation homes from the 1970s to 1990s in a moderately cold climate. A modern build may need closer to 1 kW per 20 to 30 square metres, and it ignores hot water entirely. Using it on a passive house can oversize the boiler by three to four times.
