Combi boiler size chart
Use this as a starting point. Your engineer should confirm the size from your home's heat loss and your mains water flow, but this is what we see fitted in most UK homes.
| Home | Bathrooms | Combi size | Hot water flow |
|---|---|---|---|
| Flat or 1 to 2 bedroom house | 1 bathroom | 24kW to 26kW | About 10 litres a minute |
| 2 to 3 bedroom house | 1 bathroom | 28kW to 30kW | About 12 litres a minute |
| 3 to 4 bedroom house | 1 bathroom and an ensuite | 30kW to 35kW | 12 to 14 litres a minute |
| 4 bedroom house or larger | 2 or more, used at the same time | Consider a system boiler and cylinder | 15+ litres a minute needed |
Bigger is not better. An oversized combi costs more to buy, cycles on and off more often in mild weather, and gives you no extra hot water if your mains cannot supply it.
What the kW figure on a combi actually means
The headline number on a combi is its hot water output, not its heating output. That catches a lot of people out. Take our Ariston E-Combi ONE 24kW: it gives 26kW for hot water but 22kW for the radiators. The 30kW model gives 30kW for hot water and 28kW for heating.
Hot water kW converts straight into flow. As a rule of thumb, every 2.4kW gives about 1 litre a minute of hot water at a 35°C temperature rise (cold mains at 10°C heated to 45°C at the tap). So:
- 24kW combi: about 10 litres a minute, enough for one shower
- 30kW combi: about 12.5 litres a minute, one good shower or a shower and a basin tap
- 35kW combi: about 14 litres a minute, close to the limit of what most mains can supply
Heating demand is usually the smaller number. A typical 3 bedroom semi needs roughly 8kW to 12kW of heat on the coldest day, so almost any combi covers the radiators. The size decision is really about hot water.
Why your water mains matters more than the boiler
A combi heats water as it comes in from the mains, so it can never deliver more than your mains supplies. If your mains gives 11 litres a minute, a 35kW boiler still gives you 11 litres a minute.
Test your mains flow in two minutes
- Turn on the cold kitchen tap fully (the one fed straight from the mains).
- Time how long it takes to fill a 10 litre bucket.
- Divide 600 by the number of seconds. 50 seconds means 12 litres a minute.
Under 10 litres a minute, stick to a 24kW to 28kW combi. Paying for more output gains you nothing. Over 14 litres a minute, a 30kW to 35kW combi is worth it.
How much does a combi boiler cost?
There are two prices to budget for: the boiler and the fitting.
| Item | Typical UK price |
|---|---|
| Gas combi boiler, boiler only (Ariston range at PlumbXtra) | £594.98 to £799.99 inc VAT |
| Flue kit | Horizontal flue kit included in the price, vertical flue +£109 |
| Delivery to UK mainland (boilers) | Calculated at checkout, varies by boiler and supplier |
| Fitted, like-for-like combi swap | £1,500 to £2,500 |
| Fitted, system or back boiler converted to a combi | £2,800 to £4,500 |
| Power flush (often needed on older systems) | +£400 to £700 |
| Magnetic system filter | +£120 to £200 |
Fitted prices are national averages from the BestBuilders 2026 cost guide. Prices in the North West and North East usually sit at the lower end; London and the South East run 10% to 20% higher.
Buying the boiler yourself
You can buy the boiler from us and pay an engineer for labour only, which avoids the markup an installer adds to a supplied boiler. Before you order, check three things with your engineer: they are happy to fit a customer-supplied boiler, they agree the size and flue type, and they will register the warranty. Every gas boiler must be installed by a Gas Safe registered engineer.
How to choose a brand
Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann and Ariston all make reliable combis. Within the same size, the differences that actually matter are:
- Warranty length and conditions. Ranges run from 2 years to 10 years or more. Longer warranties usually need registration within 30 days and an annual service by a Gas Safe engineer.
- Hot water flow at the stated kW. Compare litres a minute, not just kW.
- Physical size. If it goes in a kitchen cupboard, check depth. The E-Combi ONE 24kW is 315mm deep, 70mm shallower than the 30kW.
- Your engineer's familiarity. An engineer who fits a brand every week will fit and service it faster.
Our current combi range is Ariston, chosen for price, the LPG kit included in the box, and the 8 year warranty on the Clas ONE:
Ariston E-Combi ONE 24kW22kW heating, 26kW hot water, 10.4 L/min. 315mm deep. Natural gas or LPG. 2 year guarantee.£594.98View boiler
Ariston E-Combi ONE 30kW28kW heating, 30kW hot water, 12.5 L/min. Natural gas or LPG. 2 year guarantee.£650.00View boiler
Ariston Clas ONE 3028kW heating, 12.5 L/min hot water. Natural gas or LPG. 8 year warranty.£799.99View boilerAriston or Worcester Bosch?
Worcester Bosch is the most searched-for combi brand in the UK and the Greenstar range has a deserved reputation for reliability and engineer familiarity. A Greenstar 4000 30kW typically costs £2,800 to £3,000 fitted. The honest case for Ariston is price: a Clas ONE 30 gives the same 28kW heating output and 12.5 litres a minute for around £800 boiler only, with an 8 year warranty, so the whole job often comes in several hundred pounds cheaper. If your engineer fits Worcester every week and you value that familiarity, pay the difference. If you want the same output and a long warranty for less, Ariston is the sensible choice.
LPG and electric options
LPG combis
If you are off the gas grid and use bottled or bulk LPG, you need a combi set up for LPG. All three Ariston combis above can run on LPG with the conversion kit supplied. The conversion must be done and tested by a Gas Safe engineer with LPG competence.
Electric boilers
Where gas is not an option, an electric flow boiler is the simplest swap. Note these heat the radiators only, so you need a separate hot water cylinder. Our ThermoSphere electric flow boilers come in 4.5kW, 6kW and 9kW, suited to flats and small homes. Running costs are higher than gas per unit of heat.
When a combi is the wrong choice
- Two or more bathrooms used at the same time. A combi splits its flow between taps, so both showers go weak. A system boiler with a stored cylinder handles this.
- Low mains flow or pressure. Under about 9 litres a minute, a combi will disappoint whatever its size.
- Plans for solar thermal or a heat pump. Both need a hot water cylinder. Removing yours now can make the upgrade harder later.
Common questions
Is a 24kW combi enough for a 3 bedroom house?
For heating, yes. Most 3 bedroom homes need under 12kW of heat. For hot water, a 24kW combi gives about 10 litres a minute, which suits one shower at a time. If you have an ensuite as well, go for 28kW to 30kW.
What size combi boiler do I need for 2 bathrooms?
At least 30kW to 35kW, and only if the two are rarely used at the same time and your mains flow is over 14 litres a minute. If both get used together every morning, choose a system boiler and cylinder instead.
Can I buy a boiler online and get it fitted?
Yes. Buy the boiler, flue and any extras, then book a Gas Safe engineer for labour only. Agree the model with your engineer before ordering.
How long does a combi boiler last?
Around 15 years with an annual service, according to Viessmann; 10 to 15 years is a fair planning figure. Keeping the system water clean with a magnetic filter and inhibitor makes the biggest difference.
Do I need a new flue with a new boiler?
Yes. Flues are matched to the boiler model, and BS 5440 and manufacturer instructions require a new flue when the boiler is replaced, even like for like. Budget for it with the boiler.
Not sure which size to order?
Call our heating team on 01282 429061, Monday to Friday 7:30am to 5pm and Saturday 8am to 2pm. Tell us your bedrooms, bathrooms and mains flow, and we will tell you which boiler fits.
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