Is a bigger combi always better?

No. It costs more, and if your mains supply can't deliver more water, you won't get more hot water. Size it to what you use at once and what the mains can supply.

What size combi for a 3-bedroom house?

Usually 28–30kW with one bathroom, or 30–35kW with a bathroom and an en-suite used one at a time — if the mains flow can support it.

Can a combi run two showers at once?

Only a large one on a strong mains supply, and even then each shower gets about half. For two showers at the same time, a system boiler with an unvented cylinder is the better choice.

Does a 35kW combi use more gas than a 24kW?

Not for the same job. It burns gas faster while a tap is running, but heats the same water; on heating, both modulate down to what the house needs.

What Size Combi Boiler Do I Need? kW by Home Size

The short answer

Size a combi for hot water, not heating: its kW sets how many litres a minute it can heat. At a typical 35°C rise, 24kW gives about 10 litres a minute, 30kW about 12 and 35kW about 14. As a rough guide, 24–28kW suits a flat or small house with one bathroom, and 30–35kW a three- or four-bedroom home with a bathroom and en-suite used one at a time. For two showers at once, look at a system boiler.

What does a combi's kW mean?

A combi's headline kW is its hot water output: how fast it can heat cold mains water. Heating water takes 4.186 kJ per litre per degree, so the flow it can manage is:

Mains water arrives at about 10°C in winter and a shower runs at about 40–45°C, a rise of around 35°C. That gives the flows in the table below — less in winter when the water is colder, more in summer.

Live from our shelvesGas combi boilers on our shelves by output: hot water flow at a 35°C temperature rise, how many we stock, brands, and the lowest price inc VAT

Output	Hot water	Boilers	Brands	From
24 kW	about 9.8 litres/min	5	Biasi, Ideal, Vaillant, Worcester Bosch	£647.16
25 kW	about 10.2 litres/min	7	Biasi, Worcester Bosch, Vaillant	£768.24
26 kW	about 10.6 litres/min	2	Ideal, Vaillant	£1,476.86
28 kW	about 11.5 litres/min	3	Navien, Vaillant	£671.24
30 kW	about 12.3 litres/min	13	Biasi, Ideal, Worcester Bosch, Vaillant	£748.84
32 kW	about 13.1 litres/min	3	Vaillant, Navien	£690.64
35 kW	about 14.3 litres/min	5	Biasi, Ideal, Vaillant	£793.92
36 kW	about 14.7 litres/min	1	Worcester Bosch	£1,785.56
37 kW	about 15.2 litres/min	2	Navien	£786.84
40 kW	about 16.4 litres/min	4	Ideal, Vaillant, Worcester Bosch	£1,741.46
41 kW	about 16.8 litres/min	2	Navien	£862.33
45 kW	about 18.4 litres/min	1	Worcester Bosch	£2,383.36

How much hot water do you use at once?

Typical flows (they vary with the fitting and pressure)

Outlet	Typical flow
Mixer shower	8–12 litres/min
Rainfall or power shower head	12–15+ litres/min
Kitchen or basin tap	6–8 litres/min
Bath filling	As fast as the combi allows

So a 24kW combi runs one good shower; a 35kW one runs a shower with a tap open elsewhere. Two showers at once need more than most combis can heat — that's when a system boiler and cylinder earns its place.

Your mains supply is the real limit

A combi can't deliver more water than comes into the house. Check the flow: open the kitchen cold tap fully, time how long it takes to fill a bucket of known size, and work out litres per minute. If the mains gives 12 litres a minute, a combi that could heat 16 will still only give you 12.

Pressure matters too — check the minimum working pressure in the boiler's manual. Old 15mm or lead supply pipes often limit the flow; upgrading the supply to 25mm MDPE can transform a combi's performance. See low water pressure: taps and showers.

A rough guide by home size

Combi sizes by home — a starting point for your heating engineer

Home	Bathrooms	Combi size
Flat, 1–2 bedrooms	One	24–28kW
House, 2–3 bedrooms	One	28–30kW
House, 3–4 bedrooms	Bathroom and en-suite, used one at a time	30–35kW
Larger home, or two showers at once	Two or more	System boiler and cylinder, or a 40kW+ combi with strong mains

Heating is rarely the deciding factor: most homes need far less than a combi's hot water output to heat the radiators, and many combis are set to a lower heating output than their hot water rating. A heat loss calculation by your engineer confirms it.

Before you buy

A combi must be installed by a Gas Safe registered engineer.
Bigger combis need a bigger gas supply — the engineer checks the gas pipe from the meter can carry it.
In hard-water London, fit scale protection on the cold supply to the combi; a magnetic filter and inhibitor protect the heating side.
Plan the flue route and where the condensate pipe will drain.

Questions

Is a bigger combi always better?: No. It costs more, and if your mains supply can't deliver more water, you won't get more hot water. Size it to what you use at once and what the mains can supply.
What size combi for a 3-bedroom house?: Usually 28–30kW with one bathroom, or 30–35kW with a bathroom and an en-suite used one at a time — if the mains flow can support it.
Can a combi run two showers at once?: Only a large one on a strong mains supply, and even then each shower gets about half. For two showers at the same time, a system boiler with an unvented cylinder is the better choice.
Does a 35kW combi use more gas than a 24kW?: Not for the same job. It burns gas faster while a tap is running, but heats the same water; on heating, both modulate down to what the house needs.

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What Size Combi Boiler Do I Need? kW Explained