What size boiler do I need for a 3-bedroom house?

28-34 kW for a combi and 18-26 kW for a system or regular boiler with one or two bathrooms (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes); with three bathrooms the row moves to 35-42 kW combi or 27-40 kW system. A 3-bedroom house of 90 m² with average insulation loses about 9.00 kW, well under the combi range: the gap is the hot water.

What size boiler do I need for a 4-bedroom house?

The same row as a 3-bedroom house: 28-34 kW combi or 18-26 kW system with one or two bathrooms, 35-42 kW combi or 27-40 kW system with three. The sources put 4+ bedrooms in the top row only together with three or more bathrooms and 20+ radiators, and note that a system boiler is often the better fit at that size.

What size combi boiler for a 2-bedroom flat?

24-27 kW with one bathroom (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes); a second bathroom or en-suite moves it to 28-34 kW, because the tables treat a second bathroom as the next row. A system boiler with a cylinder would be 12-18 kW for the same flat.

Is a 24 kW combi enough, or do I need a 30 kW?

The manufacturer's guide puts a 24 kW combi at up to 10 radiators and a 28 kW at 10 to 15 (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves); the 24-27 kW row is for one or two bedrooms with one bathroom. With two bathrooms, three or more bedrooms or more than 10 radiators the tables move to 28-34 kW, which is where a 30 kW combi sits. The kW that matters for a combi is the hot water flow, so the bathrooms decide before the radiators do.

How do I know what size boiler I need for sure?

Count the bedrooms and bathrooms for the row, check the heat loss from your floor area and insulation, and count the radiators; the calculator gives all three with their sources. Then a Gas Safe registered installer does a room-by-room heat loss calculation and checks the hot water demand (bathrooms, mains flow rate) before the boiler is ordered; that is the figure the boiler is bought on, not the table.

What size boiler do I need? (UK combi and system kW by bedrooms and bathrooms)

A three-bedroom house with one bathroom needs 28-34 kW from a combi boiler or 18-26 kW from a system boiler on the UK installers' tables; the tables run from 24-27 kW for a one or two-bedroom home with one bathroom to 35-42 kW for five or more bedrooms or three bathrooms. Bedrooms and bathrooms pick the row because a combi is sized for hot water on demand, not just heating. Below: the combi and system kW for every bedrooms and bathrooms pair, how many radiators each combi output serves, and the heat loss in kW that sits under the boiler figure.

Open the boiler size calculator

1. What size combi boiler by bedrooms and bathrooms?

The installers' table (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes) has three rows: 24-27 kW for one or two bedrooms with one bathroom, 28-34 kW for three or four bedrooms or two bathrooms, 35-42 kW for five or more bedrooms or three or more bathrooms. The calculator reads the row from bedrooms and bathrooms and takes the higher of the two; the grid below is every pair.

Table 1: combi boiler kW by bedrooms and bathrooms, from the UK installers' tables
Bedrooms	1 bathroom	2 bathrooms	3 bathrooms
1	24-27 kW	28-34 kW	35-42 kW
2	24-27 kW	28-34 kW	35-42 kW
3	28-34 kW	28-34 kW	35-42 kW
4	28-34 kW	28-34 kW	35-42 kW
5	35-42 kW	35-42 kW	35-42 kW

Bathrooms move the row as much as bedrooms do: a two-bedroom home with two bathrooms is in the same 28-34 kW row as a four-bedroom home with one. The boiler size calculator shows the row beside the whole-house heat loss and one room's radiator.

2. What size system or regular boiler?

A system or regular boiler heats a hot water cylinder rather than water on demand, so its rows are lower (Heatable, What Size Boiler Do I Need?, system / regular boiler sizing for typical UK homes): 12-18 kW, 18-26 kW and 27-40 kW for the same three rows.

Table 2: system or regular boiler kW by bedrooms and bathrooms, from the UK installers' tables
Bedrooms	1 bathroom	2 bathrooms	3 bathrooms
1	12-18 kW	18-26 kW	27-40 kW
2	12-18 kW	18-26 kW	27-40 kW
3	18-26 kW	18-26 kW	27-40 kW
4	18-26 kW	18-26 kW	27-40 kW
5	27-40 kW	27-40 kW	27-40 kW

For the three-bedroom, one-bathroom starting house that is 18-26 kW against 28-34 kW for a combi. The sources add that at the top row a system boiler is often worth considering instead of a combi.

3. How many radiators can a 24, 28 or 32 kW combi run?

The manufacturer's guide groups combi outputs by radiator count (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). It is a rule of thumb: a radiator count says nothing about the size of each radiator, which the radiator BTU calculator gives per room.

Table 3: radiators served per combi boiler output, manufacturer's guide
Combi output	Radiators	Table row
24 kW	up to 10	24-27 kW
28 kW	10 to 15	28-34 kW
32 kW	15 to 20	28-34 kW

4. Heat loss in kW against the boiler kW

The heating side alone is floor area x a W/m² figure for the insulation level (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area): poor 130 W/m², average 100 W/m², good 80 W/m². The full m² grid is in the BTU per square foot guide; the point here is the gap.

A 90 m² three-bedroom house loses 7.20 kW to 11.70 kW depending on insulation, and the tables give it 28-34 kW for a combi or 18-26 kW for a system boiler. Even a 200 m² house with poor insulation loses 26.00 kW, still below the 28-34 kW combi row. So the heat loss tells you whether the heating side is comfortably covered; the bedrooms and bathrooms tell you which boiler to look at; the installer's heat loss calculation and the hot water demand decide.

Frequently asked questions

What size boiler do I need for a 3-bedroom house?: 28-34 kW for a combi and 18-26 kW for a system or regular boiler with one or two bathrooms (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes); with three bathrooms the row moves to 35-42 kW combi or 27-40 kW system. A 3-bedroom house of 90 m² with average insulation loses about 9.00 kW, well under the combi range: the gap is the hot water.
What size boiler do I need for a 4-bedroom house?: The same row as a 3-bedroom house: 28-34 kW combi or 18-26 kW system with one or two bathrooms, 35-42 kW combi or 27-40 kW system with three. The sources put 4+ bedrooms in the top row only together with three or more bathrooms and 20+ radiators, and note that a system boiler is often the better fit at that size.
What size combi boiler for a 2-bedroom flat?: 24-27 kW with one bathroom (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes); a second bathroom or en-suite moves it to 28-34 kW, because the tables treat a second bathroom as the next row. A system boiler with a cylinder would be 12-18 kW for the same flat.
Is a 24 kW combi enough, or do I need a 30 kW?: The manufacturer's guide puts a 24 kW combi at up to 10 radiators and a 28 kW at 10 to 15 (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves); the 24-27 kW row is for one or two bedrooms with one bathroom. With two bathrooms, three or more bedrooms or more than 10 radiators the tables move to 28-34 kW, which is where a 30 kW combi sits. The kW that matters for a combi is the hot water flow, so the bathrooms decide before the radiators do.
Why is the combi kW so much higher than my heat loss?: Because the combi heats hot water on demand and is sized for that. A 90 m² house loses 11.70 kW, 9.00 kW or 7.20 kW at 130 W/m², 100 W/m² and 80 W/m² (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area), against 28-34 kW for a combi. A system or regular boiler with a cylinder is sized closer to the heating load, which is why its rows are lower.
How do I know what size boiler I need for sure?: Count the bedrooms and bathrooms for the row, check the heat loss from your floor area and insulation, and count the radiators; the calculator gives all three with their sources. Then a Gas Safe registered installer does a room-by-room heat loss calculation and checks the hot water demand (bathrooms, mains flow rate) before the boiler is ordered; that is the figure the boiler is bought on, not the table.

How many BTU do I need for a bedroom? (12x12 and other room sizes)
A 12 x 12 bedroom is 5,000 BTU/h on the ENERGY STAR chart. Eight bedroom sizes with the sun, people and ceiling adjustments, m² to kW, and the unit class for each.
What size heat pump do I need for 2,000 sq ft? (tons by climate zone)
Heating BTU/h and heat pump tons for 2,000 sq ft in every US climate zone and insulation level, the half-ton class by home size and zone, and the same rule in m² and kW.
How many BTU do I need per square foot? (room size chart)
The ENERGY STAR room size to BTU chart with its adjustments, heating BTU and tons by home size and climate zone, and m² to kW at three insulation levels.

Last updated 2026-09-21. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.