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What Size Boiler Do I Need?

What size boiler do I need? is a question every new buyer needs to ask themselves. Many of us that aren’t industry savvy don’t realise that you’ll need a specific boiler size and type that’s determined by bedroom/bathroom count, boiler location and property size etc. Even though combi boilers are incredibly popular, in this blog post, we’re also going to be talking about open vent and system boilers.

The boiler that you choose also depends on your living situation. Are you part of a large family or do you live by yourself? How many people will be using the bathrooms at any one time? Ask yourself these questions so you know you’re getting the correct performance and highest efficiency from the boiler that you choose.

Don’t worry if you’re feeling overwhelmed – we’re here to help break it down so you no longer need to ask ‘What size boiler do I need?’

If you’re new to the world of Best Buy Combi Boilers in particular, we’ve published easy-to-understand guides on our blog which may help:

Lets begin!

The size of a boiler is known as the ‘output’. To correctly work this out, you’ll need to establish details of your current property and what you’ll require from your heating system. Ask yourself 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 it’s been undersized. If it’s been oversized, the general problem is that the gas and energy expenditure will be more. As long as it isn’t too high, you’ll get a much better flow rate.

If you’re asking Google ‘What size boiler do I need?’, then you need to find that balance.

finding the balance

What size and what type of boiler do I need?

No type or size of boiler is automatically superior to another, but choosing the right one is very important. When asking ‘what size boiler do I need?’, it’ll depend on how big your house is, the number of bathrooms, the hot water demand and the output for all the radiators that you may have.

An advantage with all modern condensing boilers is that this output is modulated between kw depending on the load, to allow superior efficiency.

What exactly is modulation?

Modulation improves the lifespan of internal components, but what does it mean? When demand for hot water changes for different times of the year, boiler output changes within it’s range to match the heating load. This is called modulation.

3 bedroom 8 radiators


Lets use an example like the one pictured above. Say you lived in a modern 3 bedroom property with 8 radiators. These radiators will have an output of around 1.5kw which if multiplied by 8, equates to an overall output of 12kw. For this scenario, we’d recommend the 29kw Viessmann 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.

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:

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 act on an ‘as-and-when’ basis and don’t require a separate hot water storage cylinder to store heated water.

Viessmann’s Combi Boiler size options include:

  • 26kw
  • 29kw
  • 30kw
  • 35kw

Ideal boiler size

Let’s use the same example as before – a modern 3 bedroom property with 8 radiators. When asking ‘What size boiler do I need’, the 29kw Viessmann Combi Boiler is once again ideal because its capacity 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. One such example, the Vitodens 111-W is a high performance wall-mounted storage boiler for family homes that has a high demand for hot water, which has also been reviewed as a Which? Best Buy. For more information, click here.

Combi Boiler Guides

detailed guide on combi boilers is available on our website if you want to learn more.

Best combi boilers 2019 will save you space and energy by heating the water on demand. If you’re asking ‘What size boiler do I need?’ For larger properties with multiple bathrooms however, you may struggle to deliver hot water demand from multiple taps.

Below are the best boilers in this category:

Viessmann Vitodens 100-W Combi 26 Gas/LPG 10 L/M 17
Worcester Bosch Greenstar 25i Combi 24 Gas/LPG 10.2 L/M 16
Vaillant ecoTEC pro Combi 24 Gas/LPG  9.6 L/M 16
Grant VortexBlue 36 Oil 20 L/M 24
Viessmann Vitodens 200-W 35 Gas  14.3 L/M 23


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.



Sealed System Boilers


which boiler is best, 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’ about system boilers, we would 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

These require a cold water tank in the loft, and a hot water cylinder elsewhere in the house.

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

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

Ideal boiler size

For our usual example, we’ll throw in another 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.




Does ‘boiler size’ or ‘output’ refer to the physical size of the boiler?

In short, no. There isn’t a connection between the boiler output and the physical size of the unit itself. Using 27kw and 35kw as examples, the 35kw doesn’t have to be physically bigger out of the two. 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


Most smaller properties such as flats or 1 bedroom houses have up to 10 standard size radiators. If we follow the pattern of each radiator equating to 1.5kw, we’d require 15kw maximum output, but with more available for a bathroom – so a 19kw model would suffice.

Always consider the output from each radiator and how this may 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. Adding this up helps determine the overall energy output you require from your new boiler.

The important thing to remember with ‘what size boiler do I need’ is to achieve a better flow rate for the larger properties, the kw will need to be higher.


What size boiler do I need? – In summary

Based on the size, age and user profile, there are many factors which have been discussed above that help you determine how to size a boiler correctly. When switching from an open vent to combi, you want to make sure that the integrity of the pipework is good. Proceed carefully if it’s buried in concrete as you’re going from a non-pressurised system to a pressurised system.

You may also need to check flow rates. Go to an unrestricted mains supply like your outside garden tap. Don’t use a mixer tap – they may restrict flow and you’ll get inaccurate readings. 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. If you’re ever unsure about flow rates, you can call our office on 02920 099898 for reliable, trustworthy advice.

There’s always more to learn with ‘What size boiler do I need?’, but speaking to an expert is the best way to get valuable information for you to help make the best decision. If you’re not confident about sizing your boiler correctly, call and speak to one of our heating experts on 02920 099898. Ultimately, the decision rests with you!



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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