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What Size Boiler Do I Need?

What boiler size do I need?

While size isn't everything, finding the answer to 'what size boiler do I need?' is up at the top of the list when replacing your boiler. It involves a bit of thinking and research into the heating and hot water needs of your home. For the record – boiler size relates to the output rating, not the actual physical size of the boiler.

In short, output rating is the power your boiler can provide to your heating system. The higher the output rating, the better suited it will be to larger properties with more radiators.

It’s not easy to define the exact boiler size you will need as each home has different requirements. However, with some basic information, our boiler size calculator will help you find a boiler capable of keeping your home warm.

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Blink boiler size

Why boiler size is so important

Your new replacement boiler needs to make your home nice and comfortable whilst keeping your energy bills as low as possible. No one likes paying more than they need to for their gas and electricity!

So, on this occasion, (boiler) size is important and we need to get it right. To do that, we’ll need to work out an ideal output rating to match the heating demands of your home.

Output rating is measured in kilowatts (kW). If you get a new boiler with an output rating that is too low then it won't be able to meet your home’s demand. In other words, you'll end up cold and miserable. On the other hand, an output rating that's too high will waste fuel, and you'll be burning money on extortionate energy bills. Not good. Yep, not to over-egg the pudding but getting boiler sizing correct is essential.

Range Rated

For most new boilers, there is the option to get them range rated. This is a technique that allows an engineer to bring down the output of the boiler to a level where it can still provide the power needed but saves on energy. This method is great for anyone who may already have a boiler that’s more powerful than their usage needs. It’s not, however, a recommended replacement for buying the right size boiler, to begin with.

Boiler size calculator

Let's get to the reason why you're here, and work out the right size for your new boiler.

There are a few things to consider to get boiler sizing right.

  • Type of boiler (system and regular boilers store domestic hot water in a cylinder making them better suited to homes with multiple bathrooms)
  • Total number of radiators
  • Number of bathrooms

Once you’ve considered all of the above, we can put the numbers into our calculator.

Type of property Bathrooms Radiators Approximate boiler size Best boiler models in this size range
Flat or small house 1 Up to 10 24-27 kW Ideal Vogue Max & Vaillant ecoTEC Plus
Medium house 1 Up to 15 28 – 34 kW Baxi 800 & Viessmann 200-W
Large house 2+ Up to 25 35kw-42kw Vaillant EcoTEC Plus & Worcester Bosch Greenstar 8000 Lifestyle

For a property with up to 10 radiators, a 24 kW combi boiler will do the job. Meanwhile, larger properties with up to 15 or 20 radiators should turn to a system or regular boiler with an output of 28-34 kW or 35-42 kW. As a general rule, allow for 1.5-2kw per radiator in your home.

For more information, we have a selection of guides to help you find the most suitable boiler for the size of your property:

Our guide will give you a decent idea of the ideal boiler size but it's always worthwhile to get a professional to provide you with an assessment and quote. This just means a qualified heating engineer visiting your home to look at your heating system.

Now that you know what size boiler you need, you can find out how much it might cost with our Boiler Installation Calculator.

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What type of boiler do I need?

As well as size, it’s important to find the right type of boiler for your home too. There are 3 different boiler types: combi, system, and regular. You can get all of these models to run on gas, oil or LPG (Liquified Petroleum Gas).

Combi boilers

Combi boilers are the most popular boiler type installed in homes across the UK. They’re cost-effective and compact, which makes them ideally suited to homes with 1 or 2 bathrooms.

They take water directly from the mains and provide central heating and hot water on demand. There are no additional cisterns (aka tanks) or cylinders, which means you save valuable space around the property. However, by not having a cylinder they are limited in how much hot water they can provide.
If you have a large property, with more than one bathroom, you might want to consider a system boiler.

System boilers

System boilers are similar to combi boilers as they also take water directly from the mains. They differ, however, as a system boiler needs an unvented hot water cylinder for storage. While this will take up some additional space, it does mean they are good for homes with a higher demand for hot water. This is because they can supply hot water to 2 or more outlets at a single time, without the water pressure dropping.

Regular boilers

Regular boilers are the older statesmen of the heating system world. Commonly found in older homes, a regular boiler needs to be installed along with a feed and expansion cistern (tank) in the loft. There also needs to be a vented hot water cylinder. Coldwater is fed to the boiler from the tank in the loft before it gets heated and then circulated to the central heating system or hot water cylinder.

Regular boilers are not recommended unless it is to replace an existing regular setup. It is considered much more beneficial to upgrade to a combi if it becomes time for a replacement.

If you have any doubt about the most suitable boiler for your home and heating needs, a heating engineer will be able to recommend the right one. You can get an idea of what you need based on the size of your home and the amount of hot water you need.

What size combi boiler do I need?

Combi boilers are bigger in size than system and regular boilers as they need to produce hot water on demand for domestic use as well as heating. As they directly provide central heating and domestic hot water, they have two separate output ratings:

  • Central heating (CH) output
  • Domestic hot water (DHW) output

Regular and system boilers only have a central heating output as the domestic hot water is stored in a cylinder.

The central heating outputs of combi boilers can be split into three categories to help you find a suitable model depending on the number of radiators in your property.

Number of Radiators Recommended Combi Boiler Size (CH output)
Up to 10 24-27 kW
10-15 28-34 kW
15-20 35-42 kW

Find out more about the Best Combi Boilers.

What size system boiler do I need?

System and regular boilers do not need to be as powerful as combi boilers. This is because they incorporate a separate hot water cylinder for heating and storing domestic water.

Adding up the number of radiators around your home will help you understand a suitable system boiler output rating.

Number of Radiators Recommended System Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

Get a complete overview of system boilers in our What is a System Boiler? Pros, Cons and Costs article.

What size regular boiler do I need?

The recommended central heating output of a regular boiler is the same as for system boilers.

Number of Radiators Recommended Regular Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

Compare the best boilers with our Boiler Comparison Tool.

Boiler sizing and efficiency tips

It's good to do research when it comes to finding the right boiler, but we know it’s not everyone’s idea of a great night in. So to save you time, we’ve put together a handy list of tips to help you find the right boiler without the stress.

  • Existing boiler size
    A good place to start is to find out the size of your existing boiler. But wait! – Don't assume that's it! You may need to make a size change. Since you last replaced your boiler, your home’s water demands may have changed. You could have had children, added an extra bathroom or you may be thinking of extending it in the future. If you find your home is regularly running out of hot water, it could mean that your current boiler isn’t big enough. However, if your current boiler is more than 8 years old then it may just be a case of inefficiency rather than a lack of capacity.
  • Mains flow rate
    If you're upgrading from a regular to a combi or system boiler, the size you need will also be influenced by the flow rate/pressure of the water coming from the mains. It's important you choose a boiler big enough to heat the quantity of water passing through it. You can get an idea of your existing flow rate by taking a measure of how much water you can collect from the cold tap in 1 minute. To do this easily, you can run cold water into a measuring jug for 10 seconds to see how much, in litres, you collect. If you then multiple this by 6 you will get an idea of the litres per minute measurement. You want to try and match the flow rate you already have with your new boiler choice.

    You can also call a professional installer who will be able to measure the flow rate and temperature of your mains water for you.

    Find out more about combi flow rate here.

Boiler efficiency tips

Replacing your boiler should increase the energy efficiency of your home which can, in turn, reduce your energy bills. We've got some extra tips to help your boiler work as efficiently as possible.

  • Insulation
    A lack of insulation in your home will result in you losing lots of heat through the roof, walls, and floors. This means your poor boiler is going to have to work harder to keep you nice and cosy. Working harder means higher energy bills. To help your boiler out, consider investing in additional insulation or replacement windows.
  • Radiator size
    Not all radiators are created equal, some have higher outputs than others, measured in British Thermal Units (BTUs), and will vary in size. The bigger the surface area of the radiator the quicker it will heat a room, so each radiator should be adequate for the size of the room.
  • Smart controls
    Many of us have a smartphone and smart television so why not get smart with your heating? Most boilers are now compatible and you'll be able to adjust the temperature of your home when on the move. Smart technologies are an effective way to keep an eye on your energy use. We've compared a couple of the best smart thermostats here.
  • Annual boiler servicing
    Yes, it's a pain but trust us on this one. Having your boiler serviced every year will keep it running to the best of its ability for much longer and should save you money in the long run. Catch any issues early on to do your best to avoid a boiler on the blink.

Get professional advice on boiler sizing

To get an accurate idea of the appropriate boiler sizing for your home, we highly recommend seeking the advice of a professional heating engineer.

Comparing multiple quotes will help you to find the most suitable boiler for your home at the most competitive price. We've made finding local heating engineers easier than ever.

All you have to do is complete our simple online form – which should only take a few moments – letting us know a little about the work you need carrying out. We'll then match your needs with up to 3 companies in your area who are qualified to carry out the work. Each company will be in touch as quickly as possible to provide you with a free quote.


About the author


Adam is our resident home heating expert. His experience and advice has helped millions of customers improve the efficiency of their homes and save money.

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