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What Size Boiler Do I Need?

Knowing what size boiler do I need is vital for securing a successful boiler installation.

The truth is, you’ll need a specific boiler size and type to make sure you get the water demand you require. Going with an installer who understands this is very important. There are a few aspects to your property that must be considered in order to answer what size boiler do I need:



The boiler that you choose also depends on your living situation. Do you live with a large family? How many people use the bathroom at any one time? Figuring out a user profile such as this helps determine what kind of water demand you’re dealing with.

Don’t worry if you’re feeling overwhelmed. If you have any questions or any uncertainty about ‘What size boiler do I need’, the best thing to do is speak to a heating expert directly. You can call us directly on 029 2009 9898 where we will be happy to answer any queries. Alternatively, you can send us email to sales@boilerhut.co.uk or get a free, no-obligation quote by entering your postcode in the box below and hitting that green button!

If you’re unfamiliar to the heating industry and unsure of how does a boiler work, you can find further information in some of our other articles:

What does boiler size mean?

When we’re talking about ‘Boiler size’, we’re referring to the boiler’s power output, NOT its physical dimensions. This output is measured in KW (Kilowatts) and plays a large part in determining which type and model is right for you.

How do I know what size boiler do I need?

To get the right boiler size, find answers to the following questions:

  • What type of boiler do you already have? Open vent, combi or system?
  • Are you looking to change your boiler type entirely? E.g. Converting from open vent to combi?
  • Is it an old house or a more modern construction property?
  • How many bathrooms are there?
  • What’s the demand for DHW (Domestic Hot Water)

If you size a boiler incorrectly, it may take longer to heat up your home and your hot water cylinder (if you have one). If it’s been oversized, the general problem is that there’s be more gas and energy expenditure. If you have the answers for the previous questions ready, the next best thing you can do is speak directly to industry experts who can recommend a boiler size. In the case of Boilerhut, you can:

  • Call us on 029 2009 9898
  • Send us an email via sales@boilerhut.co.uk
  • Enter your postcode below and hit the green button

What type of boiler do I need?

No type or size of boiler is automatically superior to another, but choosing the correct one is still important. For all modern condensing boilers, the boiler size is ‘modulated’ in KW depending on the load, allowing superior efficiency.

What does boiler modulation mean?

Boiler modulation refers to the boiler’s process of changing its output to match the heating load. For example, if your demand for hot water ever changes (say you’re a landlord with a new tenant moving in) or you have an additional radiator installed, your boiler is capable of altering its output to match the new heating load. In doing this, it can help to improve the lifespan and overall quality of its own internal components.

3 bedroom 8 radiators

Lets use an example like the one above.

Lets say you lived in a modern 3 bedroom property with 8 radiators. Each of these radiators will have an output of around 1.5KW – if multiplied by 8, this equates to an overall output of 12KW. For this scenario, we’d recommend the Viessmann Vitodens 050-W 29KW Combi Boiler, which has a modulation range of 1 to 6. Because of this, it can operate within a range of 4.8kw and 29kw.

For a detailed review of the Vitodens 050-W 29KW Combi Boiler, please click here. Alternatively, you can watch our video review below:

Manufacturers design boilers to modulate. For a premium brand such as Viessmann for example, this is no exception. When asking ‘What size boiler do I need?’ you’ll need to be aware of how modulation plays its own role.

Let’s break it down even more and talk about the different types of boilers that are most suitable for your property

Where can I put my boiler?

When asking ‘what size boiler do I need?’, no matter how big or strong your boiler is, make sure that you can fit it into the intended location. This could be anything from an airing cupboard, to a loft or kitchen cabinet.

powerful boiler

How do radiators affect what size boiler do I need?

Most smaller properties such as flats or 1 bedroom houses have up to 10 standard size radiators. Numbers like these need to be calculated in relation to how they add up to your overall energy count. If your property is bigger and older, there’ll be a higher hot water demand as the larger radiators would need about 2k.

REMEMBER: To achieve a better flow rate for the larger properties, the KW will need to be higher.

As mentioned before, if you want to find out what size boiler do I need, the best thing you can is speak directly to an industry expert.

Just call us on 029 2009 9898, send an email via sales@boilerhut.co.uk or get a free quote by entering your postcode then clicking the green button below:

What else determines what size boiler do I need?

As mentioned, there are various aspects to your property that need to be established before answering ‘What size boiler do I need?’. One example is the type of boiler currently at your property. If you’re switching from an open vent to combi, make sure your pipework’s integrity is good enough. As well as this, proceed carefully – especially if you’re going from a non-pressurised system to a pressurised system, because the pipework itself will be buried in concrete.

You may also need to check flow rates. To do this, you need to:

  • Find an unrestricted mains supply such as an outside garden tap
  • Get a 5 litre bucket, and time how long it takes to fill up
  • If it fills up in 15 seconds, you’ve got 20L/minute flow rate – perfectly acceptable for system and combi boilers

For this process, avoid using mixer taps – they can restrict flow resulting in inaccurate readings. If you’re ever unsure about flow rates, you can call our office on 02920 099898 for reliable, trustworthy advice.

I need a new boiler – what are my options?

Combi Boiler

best combi boilers 2019, what is a combi boiler

A ‘combi boiler’ – short for combination – combines heating and hot water into one single unit. They perform on an ‘as-and-when’ basis without the need for a separate hot water storage cylinder.

Viessmann’s Combi Boiler size options include:

  • 26kw
  • 29kw
  • 30kw
  • 35kw

What combi boiler size do I need for 3 bedroom property

Let’s use the same example of a modern 3 bedroom property with 8 radiators. When asking ‘What size boiler do I need’, the 29kw version of the Viessmann Vitodens 050-W Combi Boiler is once again ideal because the boiler size covers the 8 radiators, as well as 1 bathroom. If it included an en-suite, a higher capacity boiler like the 35kw version might be a better option to achieve the right flow rates.

If you need something with even higher flow rates than this, then it’s worth looking at storage combi options. The Vitodens 111-W is a high performance wall-mounted storage boiler for family homes that has a high demand for hot water. It’s also been reviewed as a Which? Best Buy. For more information storage combi boilers, click here.

What combi boiler size do I need for larger property?

If you’re asking ‘What size boiler do I need?’ for larger properties with multiple bathrooms, you may struggle to deliver hot water demand from multiple taps.

Below are the best boilers in this category:


Viessmann Vitodens 100-W Combi26Gas/LPG10 L/M17
Worcester Bosch Greenstar 25i Combi24Gas/LPG10.2 L/M16
Vaillant ecoTEC pro Combi24Gas/LPG 9.6 L/M16
Grant VortexBlue36Oil20 L/M24
Viessmann Vitodens 200-W35Gas 14.3 L/M23

There are also high end, premium boilers like the Viessmann Vitodens 200-W that can automatically adjust to any fuel type, mains gas or LPG.

Combi Boiler Guides

Want to know more about what is a combi boiler is and how a combi boiler works? Read our detailed guide on combi boilers.

Sealed System Boilers

which boiler is best, system boilers

For sealed system boilers, the heating and hot water components are built into the boiler. It needs a pressurized unvented cylinder for storing hot water. Unlike a conventional boiler, you don’t need a cold water tank in your loft or attic.

Viessmann’s domestic range of system boiler sizes include:

  • 19kw
  • 26kw
  • 30kw
  • 35kw

Ideal boiler size

Once again we’ll use the same example – a modern 3 bedroom property with 8 radiators. If we’re asking ‘What size boiler do I need’ in relation to system boilers, we’d recommend a 19kw Viessmann system boiler. As the smallest available from Viessmann, it’s modulation range is 1-6. This means that it can cover between 3.1kw to 19kw, with plenty of unused energy to go towards the hot water cylinder.

Open Vent/Conventional/Regular Boilers

which boiler is best 2019, conventional boilers

Regular boilers are also known as ‘Open Vent’,’Conventional’ or ‘Standard’ boilers. Their main feature is that they need a cold water tank and a hot water cylinder, usually kept in the attic and airing cupboard respectively.

Viessmann’s domestic range of open vent boiler sizes include:

  • 13kw
  • 16kw
  • 19kw
  • 26kw
  • 35kw

For a detailed guide on open vent boilers, please click here.

Ideal boiler size

Lets go with the same example of 3 bedroom property, except this time we’ll include an additional bathroom. We’ve already established that the heating circuit is 12kw, so for open vent boilers we would recommend the Viessmann Vitodens 100-W Open Vent 16kw. These only modulate 1-3 so it’s important to get closer to the size of the heating circuit and allow around 3kw for your hot water cylinder. In your search for ‘What size boiler do I need?’, always make sure that you’ve gone for something that allows extra energy for the cylinder!

*All modulations ranges in this article apply only to Viessmann. Check other manufacturers for modulation ranges.

*Example is based on a modern house at 1.5kw per radiator. Older houses tend to have larger radiators so you may be looking at 2-2.5kw per radiator across the board.

What size boiler do I need? – In summary

There’s always more to learn with ‘What size boiler do I need?’, but if you’re not confident about sizing your boiler correctly speak to an expert. All you need to do is call 029 2009 9898 and we’ll be happy to answer any questions you may have any discuss any particular requirements for your new boiler.

If you’re to contact us via email, you can also send any questions over to sales@boilerhut.co.uk where we can then proceed with your enquiry.

However, if you’re after information on a new boiler immediately, you can get a free, no-obligation quote directly from our website by entering your postcode in the box below, then clicking the green button.

the decision rests with you


2 replies
  1. Eli Richardson
    Eli Richardson says:

    Thanks for stating to ensure something that allows extra energy for the boiler. My uncle just moved to a new house and needs to install a cylinder. I’ll pas him the info so he can get the right size for his house.

    • Bryn Lloyd
      Bryn Lloyd says:

      Hi Eli,

      Thank you very much for the comment!

      Please bear in mind that we only specialise in complete and full installations. If you’re looking to have a specific component such as a hot water cylinder fitted, we recommended seeking help from a smaller, local engineer.

      Kind Regards,
      The Boilerhut Team


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