What size combi boiler do I need?

Whether it’s down to budget, property type or user profile, ‘What size combi boiler do I need?’ is an important question you need answering about your new combi boiler installation.

The size of your combi boiler is one of the first things you’ll need to discuss and agree on for a professional, high-quality installation. If you choose an installer who isn’t fully qualified or hasn’t got a lot of experience, you could be paying for it further down the line. With some common boiler problems in mind, you nay end up forking over multiple times the original payment.

So not only does it help you get a little more informed, but it can also save you a lot of money. So how do you know you’ve chosen an installer who’s reliable and professional in getting the job done? More on that further on!

Speak to a professional installer and save lots of money

Speak to a premium installer and you could be saving a lot of money.

If you’d like to know more about the different types of boiler available, why not take a look at some of our previous combi boiler blog posts:

Let’s get straight to it.

What is combi boiler size?

Wait a minute…isn’t boiler ‘type’ and boiler ‘size’ the same thing?

The simple answer is no. 

When we talk about ‘boiler size’, we’re not actually talking about the physical dimensions of the unit. Although it’s definitely still a deciding factor, boiler size actually refers to its ‘output’.

This output is measured in ‘KW’ (Kilowatts) and in most cases, will be increased if the number of kilowatts is increased as well.

Boiler size refers to power output, not physical dimensions!

What is boiler type?

‘Boiler Type’ will usually refer to one of 3 forms of boiler that are the most popular within the UK, although there are still other options. They are:

For further information on these boiler types, scroll down to the bottom of the page!

Now that we’ve got the obvious out of the way, the next problem you have to solve is choosing the right size for your boiler.

How to size a combi boiler

There are a lot of deciding factors to consider when looking at which boiler size is right for you. Questions like ‘boiler size for 3 bedroom house’ or ‘What size boiler do I need for 12 radiators’ often come up from panicked, ill-informed homeowners.

Which is where some installers may follow this up by overwhelming you with more information. Not only does this not do you any favours for avoiding stress, but you could also be more susceptible to unnecessary price increases. 

We’ve put a stop to that ourselves by only giving you the information you actually need. There are many aspects to consider when sizing a boiler including:

  • Number of radiators
  • Number of people in the property/hot water demand
  • Number of bathrooms

The table below gives you a good idea of where to start when it comes to sizing your new boiler.


Number of bathroomsNumber of radiatorsRecommended boiler size
1Up to 1029KW
1-2Up to 1529-35 (If a second bathroom is included)
2+Up to 2035


While this is a good starting point for sizing your boiler, the best way to know what’s right for you is by speaking with industry experts.

For Boilerhut, you can do this by calling us directly on 02920 998 898 or by getting a free, no-obligation quote online. All you need to do is enter your postcode and hit the green button below, the rest is easy!

What size combi boiler do I need?

Now we’ve got the difference between boiler ‘type’ and ‘size’ covered, we can move onto what you really need!

We’ve broken this section down so that you can have the no-nonsense information when it’s required.

What size combi boiler do I need for a 1 bedroom house?

What size combi boiler do I need for a 1 bedroom house?

If your property is small like a 1 bedroom flat or house with 1 bathroom, you’re not going to need a very high capacity for your boiler. As mentioned in the table above, a boiler size of about 29KW would be ideal for a property this size.

What size boiler do i need for a 2 bedroom house?

The same can apply if you’re asking this question as well. We would recommend going for a combi boiler for a property of this size. Even if you currently have a different boiler type in your 2 bedroom house and don’t want to change, combi boilers will still save you more energy and therefore, money.

How does a combi boiler save me money?

Simple: combi boilers only need to heat the water when it’s required. Other boilers like system or regular reheat and storing the water until you need it, so a lot more energy is spent keeping it hot.

Below are the best boilers in this category:


ViessmannVitodens 100-W Combi26£1,042.20 inc. VATGas/LPG
Worcester BoschGreenstar 25i Combi24£911.98 inc VATGas/LPG
VaillantecoTEC pro Combi24£976.32 inc VATGas/LPG
ViessmannVitodens 050-W29£825.60 inc VATGas/LPG


Although the boilers listed here are mains gas boilers, you can still get an LPG conversion kit. However, a premium, state-of-the-art boiler like the Viessmann Vitodens 200-W can actually do this automatically. * Please be aware that the prices featured above are for the boiler only. This does not include the cost of installation and any other inclusions.

Click here to read our informative review of the Viessmann Vitodens 200-W.

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

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

A property with 3 bedrooms will normally have one toilet and an ensuite. Because of this additional toilet, your boiler needs to have right output which in this case needs to be between 28KW and 35 KW).

Below you will find our recommendations for properties with 3 bedrooms in the UK.


Viessmann Vitodens 111-W Storage Combi Boiler26£1,516 inc. VATGas/LPG
ViessmannVitodens 100-W Combi Boiler35£1,200 inc. VATGas/LPG
Worcester BoschGreenstar Highflow 440CDi Floor Standing Combi Boiler29£2,109 inc. VATGas/LPG
VaillantecoTEC plus 938 Storage Combi Boiler38.7£1,571 inc. VATGas/LPG
Viessmann Vitodens 200-W Combi with Weather Compensation (98% Efficiency)30£1,515 inc. VATGas/LPG
ViessmannVitodens 050-W Combi Boiler35£870 inc. VATGas/LPG


Much like the previous results, the boilers featured in the table above can be used for Gas or LPG. However, the Viessmann Vitodens 200-W is the exception to the rule. This is because it’s able to convert to different fuel types automatically.* Please be aware that the prices featured above are for the boiler only. This does not include the cost of installation and any other inclusions.

With a higher demand for water, an energy efficient regular boiler (otherwise known as open vent) will also be a good choice.

However, the best way to get answers at this point will be speak directly to an industry expert.

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

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

If you increase the size of the property, the demand for hot water and heating will go up as well. Because of this, the boiler you choose will need to be more powerful to cater for the demand.

If you’re going for a 4 bedroom property, a combi boiler like the Viessmann Vitodens 050 35kw combi boiler may be sufficient, but only if it’s able to meet the demand you have for hot water. If it’s required over several taps simulatenously, it won’t be enough. No heating expert would recommend this as an option, simply because the boiler won’t be able to cope with the increased water demand. However, even if a small capacity combi boiler is no longer an option, other combi boiler options can still serve a purpose. That way, you’ll still have an answer to ‘What size combi boiler do I need?’

The important thing to remember when asking ‘What size combi boiler do I need?’, is that even if you have to go for a combi boiler with a larger output, you must have the relevant incoming water pressure.

Below you can find another table showcasing some of our favourite boilers suited for a 4 bedroom house:


ViessmannVitodens 111-W Storage Combi Boiler35£1,743 inc. VATGas/LPG
ViessmannVitodens 222-F Floor Standing Storage Combi Boiler35£2,850 inc. VATGas/LPG
ViessmannVitodens 242-F Floor Standing Storage Combi Boiler (Solar Integration)26£4,748 inc VATGas/LPG


To cater for this higher demand, you may notice that some storage combi boilers have also snuck their way into the table. This is because storage combi boilers are an excellent alternative option for larger properties. For details on the storage combi and other types of boiler available, please scroll down.* Please be aware that the prices featured above are for the boiler only. This does not include the cost of installation and any other inclusions.

Types of Boiler

Combi Boiler

what size combi boiler, boiler size

For a more detailed guide on Combi Boilers, please read the following article.

Combi boilers have been designed to combine all components for central heating and DHW Domestic Hot Water) in one single, compact unit. This is slightly different to regular/open vent or sealed system boilers, as they don’t require a cold water storage tank or a hot water cylinder to operate. It just keeps everything in one place to deliver hot water as soon as it’s needed. 

While combi boilers are undoubtedly the most popular type currently available in the UK, they’re somewhat limited to middle to small sized homes. As a result, they may struggle to supply water if the demand from multiple taps is too much.

All boilers produced in the last 12-15 years will be condensing boilers, regardless of the type.

Storage Combi Boiler

A storage combi boiler combines the elements of combi and system boilers to offer an alternative but effective solution.

Storage combi boilers feature a hot water cylinder inside the unit itself. Not only do they save well on space, but because of this they’re able to supply water across several taps, baths or showers they’ll have no problem delivering a high demand for water.

In summary, storage combi boilers are perfect for homeowners who don’t have a lot of available space but also have a high demand for hot water.

Regular/Conventional/Open Vent Boiler

what size boiler do I need, open vent, regular

Unlike combi boilers, regular boilers usually have a cold water storage tank which takes its water supply from the mains in the attic. As well as this, there may also be a hot water cylinder to house the water once it’s been heated. The hot water is kept in this state until it’s ready to be used by you, the resident. 

Open vent boilers are usually more suited for larger properties with an pre-existing open vent boiler system.

Sealed System Boilers

which boiler is best, system boilers

Sealed system boilers feature a separate hot water cylinder for storing water. Because of this, there’s a constant supply readily available whenever you need it. This makes system boilers another ideal choice for larger properties due to their capability of supplying readily hot water across a large number of taps and showers simultaneously.

System boilers  are sometimes confused with regular boilers, in that they’re a closed system that only requires a hot water cylinder. A regular boiler also includes the cold water storage tank that’s kept in the attic.

Need more information?

Have you found what you’re looking for? If you need any more information, you can get in touch with Boilerhut directly on 02920 099 898 or get a FREE, no-obligation quote by filling in your postcode then hitting the green button below.

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