Choosing the right boiler size is one of those boring adult tasks that decides whether your winter is cosy…or whether you spend January wearing two hoodies and arguing with your thermostat. 🙊
Boiler “size” means power output, measured in kilowatts (kW). And no - bigger isn’t “better”. A boiler that’s too small will struggle, and a boiler that’s too big can be like buying a Range Rover to drive to Tesco: impressive, expensive, and mostly unnecessary.
If you want the quickest answer with minimal brain activity: use our quote tool. It’ll recommend the right boiler type and kW for your home in about 60 seconds, show fixed prices, and we can often install quickly. Sorted. 🤝
Now, if you’re still here (respect), let’s do this properly.
🔑 Key Points:
The boiler size or power output is measured in kilowatts (kW).
The boiler size you require is largely determined by the number of radiators, bedrooms and bathrooms in your home.
Combi boilers are generally suitable for small and average-sized homes, while system and regular boilers (boilers with a tank) are more suitable for larger properties with mega hot water requirements 🚿
To get an accurate calculation of the boiler size that is right for your home, start here.
What type of boiler is right for your home?
First: boiler type (because size depends on it)
Before we even talk kW, you need the right kind of boiler.
Otherwise it’s like asking “what size engine do I need?” without mentioning whether you’re driving a Fiat 500 or towing a horse box.
Combi boiler:
The UK’s favourite boiler. It does heating + hot water in one unit, with hot water made on demand from the mains.
No cylinder, no loft tank. Great for small to average homes with one bathroom and normal usage.
System boiler:
Still modern, still sensible, but it uses a hot water cylinder. No loft tank required.
Ideal if your home has higher hot water demand and you want decent performance when more than one tap/shower is used.
Regular (conventional / heat-only) boiler:
Usually found in older setups. It uses a hot water cylinder and a cold water tank (often in the loft).
Still a good option for some older systems or higher demand homes, but it’s less common in new installs.
What size combi boiler do I need?
Combis need more kW because they’re trying to deliver hot water instantly.
That “instant” bit is why your combi’s kW usually looks higher than a system/regular boiler in the same house.
Here’s the rough guide people actually want:
If you’ve got 1 bathroom and up to around 10 radiators, a 24–27kW combi is usually the sweet spot.
If you’ve got 2 bathrooms and there’s a chance two people might shower in the same hour (scandalous), you’re normally in 28–34kW territory.
If you’ve got 3+ bathrooms, a combi might still work - but it depends heavily on your mains water flow rate, and many homes are better off with a cylinder setup (system/regular) to avoid the “why has my shower turned into a sad cough?” experience.
The bit everyone forgets: mains flow rate
This matters a lot for combis. You can buy a beefy kW boiler, but if your incoming water flow is weak, your shower will still feel like a fine mist applied by an apologetic fairy.
What size system or regular boiler do I need?
For system/regular boilers, the boiler’s main job is heating, because your hot water is stored in the cylinder.
So the kW requirement is typically lower than an equivalent combi setup.
As a rough rule, smaller homes with fewer radiators often sit around the 12–18kW mark.
Medium homes frequently land 18–26kW. Larger homes can be 27kW+ depending on heat loss and system design.
That said: the correct way to size this is a heat loss calc, but most people just want a reliable “am I in the right postcode?” answer - and this is it.
🎥 Prefer video? Watch our short video on choosing the right size boiler for your home below:
To get a more accurate answer to what size combi boiler your home needs, you can use our quick tool, otherwise here's what size combi boiler your home will need based on its hot water and heating demands.
Combi boiler sizing (typical UK homes)
No. of bathrooms | No. of radiators | No. of bedrooms | Recommended |
1 | 0-10 | 1-2 | 24-27kW |
2 | 10+ | 3-4 | 28-34kW |
3+ | 20+ | 4+ | 35-42kW (often consider a system boiler) |
System / regular boiler sizing (typical UK homes)
No. of bathrooms | No. of radiators | No. of bedrooms | Recommended |
1 | 0-10 | 1-2 | 12-18kW |
2-3 | 10+ | 3-4 | 18-26kW |
3+ | 20+ | 4+ | 27-40kW |
Again: these are guidelines. Your installer confirms based on heat loss and demand.
Also note: the Viessmann Vitodens 111-W is a beastly hybrid – a combi boiler with a built-in water tank too that is certainly worth exploring.
*Enquire directly with our team for quotes on this model
That's also why it became our top 10 list of the best boiler brands.
If you live in an average-sized home, see our guide on choosing the best boiler for a 3-bedroom house.
Check our new boiler costs guide to get an idea of how much it may cost you and if you're interested in paying for your boiler monthly, refer to our boilers on finance guide. Whatever type of boiler you need, Heatable provides it.
Why boiler sizing matters
If you undersize it…
Your boiler spends winter fighting for its life. Radiators lukewarm, hot water inconsistent, and you develop a weird new hobby: staring at the pressure gauge like it’s a stock chart.
If you oversize it…
You pay more for a boiler you don’t need. Modern boilers can modulate down, but oversizing can still mean inefficiency through short cycling and you’ve basically paid extra for bragging rights nobody asked for.
The goal is simple: the right boiler, the right size, for your actual life.
What size combi boiler by house type
You should be calculating boiler size by the number of bathrooms/showers (as above), or, use our online quoting tool.
However, as a rough guide, check out the below:
| Minimum kW Size | Minimum kW Size | |
What size boiler for... | 1 Bath/Shower | 2 Baths/Showers |
|
2 Bedroom House | 25kW | 30kW | |
3 Bedroom House | 25kW | 30kW | |
4 Bedroom House | 30kW | 35kW | |
5 Bedroom House | 30kW | 35kW |
Not sure of the best model? Read our article on the best combi boiler or check out some of our favourites that we have reviewed:
Have a large house with plenty of bathrooms, but have your heart set on a combi boiler?
Viessmann has managed to propel us some way into the future and made a hybrid boiler – the Viessmann 111-W.
Image of the interior of the Viessmann 111-W - showing how it has its very own internal water tank:
Patrick Garner/Heatable
The Germans have essentially gone full Jurassic Park here, controversially merging the DNA of a conventional boiler and a modern combi boiler to create their very own monster. Viessmann finds a way.
How much are boilers anyway? We’ve broken it down into an article so digestible and moreish, that it made half of our readers clinically obese. Read all about new boiler costs.
Learn all about combi boilers in our what is a combi boiler guide.
Note: Thinking about an immersion heater to heat water only instead? Read our guide to find out how they compare to combi boilers.
Wondering how much a new boiler may cost? Use our boiler installation cost calculator and if you're wondering how two of the biggest boiler brands compared, check out Worcester Bosch vs Baxi.
Got a larger home? You may also be interested in our guide on the best 40kW combi boilers.
What about converting? From a conventional to a combi…
Combi boilers are remarkably more efficient, cheaper to run and provide instant hot water without delay from your taps, shower or bath.
Your combi boiler will always be a higher rated KW than the equivalent requirement for a regular boiler (boiler with a tank), this is because they need some extra punch to deliver hot water on demand.
It’s no surprise that converting to a combi boiler is the UK’s most common New Year’s Resolution.
It can be quite an expensive job, but, at Heatable, we're offering unbeatable fixed prices on (the right size!) boiler conversions from just £2,995. *Monthly payments available 💸
Do you have a back boiler and want it replaced with a modern combi boiler? Ideal looking like an ideal choice? Get a fixed online price.
Here are some popular choices of boiler for Heatable customers who’ve changed from a boiler with a tank or back boiler to a combi:
No. of baths/showers | Boiler Size (min) | Popular model | |
1 | 20kW | Viessmann Vitodens 050-W 25kW | |
2 | 30kW | Worcester 4000 30KW | |
3 | 35kW | Viessmann Vitodens 050-W 35kW | |
3+* | 35kW* | Viessmann Vitodens 111-W 35kW |
If you hadn't already guessed, what size combi boiler you would require if you’re converting from a regular boiler is no different to if you already had a combi boiler…
You’d simply have to take into account your hot water demand as displayed above.
Want to have your boiler moved to a different location? Check out our cost of moving boiler.
Curious to know which boiler brand performed worst? Find out in our report of the worst boilers.
Next Steps For Your New Boiler Journey:
When planning to install a new boiler for your home, there are several important factors to consider. Make sure to refer to the following guides to help you make informed decisions:
To dive deeper into these topics, head over to our advice section, or check out our YouTube channel to learn more.
Go get that yummy, correctly sized boiler 🏃🏻♀️
You may need a new boiler.
You may need it fast and with affordable monthly payments.
You may even need it from a company with top-end local engineers, with 10,000 Trust Pilot reviews to back them up. Plus a Which? Trusted Trader award for good measure.
You may need a new boiler quote from us, Heatable 😘
