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UK Boiler Size: Which Size Boiler Do I Need?

UK Boiler Size: Which Size Boiler Do I Need?

Every household is different, not just in terms of layout and size, but also the needs and demands of the people living in it, which is why choosing the right boiler size is such an important, but an often difficult decision to make.

It is not always the case that the bigger the boiler is, the better it will be. In fact, if you choose a boiler that is too big for your property, it will use more energy than is actually required.

This will lead to an increase in your home utility bills. On the other hand, choosing a boiler that is too small won’t heat your home properly, or provide enough hot water. The boiler is overworked and will probably break down on a regular basis.

Boiler Installation Plumber in Dartford Kent

It’s so important to choose the right sized boiler for your home.

There are so many different boilers on the market, it’s tough knowing which size boiler you should be looking for, let alone brand or type. First and foremost, I would always recommend seeking the advice of a fully qualified Gas Safe Plumber or Heating Engineer to find out what size boiler that you need in your home. If you’re looking for a rough idea of which size will suit your property best, then you’ve come to the right place! This blog post should be used as a guide only. I think it’s always good to read up before you schedule a consultation so that you have a rough idea of what the expert is talking about so that you can make an informed decision.

So read on to find out which boiler size you should be considering for your household. I’ve also included my top picks of the best system and combi boilers of each size category for 2018, so if you’re interested and would like to learn a bit more about a specific boiler, have a look at my previous blogs; System Boilers: Best System Boilers for 2018 and Combi Boilers: Best Combi Boilers for 2018.

So How Do I Find The Right Boiler Size For My Home?

To help you figure out which boiler size you should opt for, first consider what the size of your current boiler is. You can use this as a guide for when you’re choosing your next boiler, but don’t automatically assume you should stick to that size. Think about whether you’re happy with your current boiler’s overall performance; do you feel your property often wasn’t heated properly or did you often find yourself running out of hot water on a regular basis? If this is the case, the chances are your current boiler is too small for all your needs, so you should consider a larger boiler size when you are choosing the next one. Deciding whether or not a boiler is too large for your home based on performance is often difficult because the performance will be more or less the same as a boiler that is the right size, it will just cost you more to run and will probably break down more often than not.

Find a Plumber in Your Area

Google is always a good place to start your boiler research.

Depending on the boiler type, sizes vary, but there are three main categories of boiler size. At this point, it is worth mentioning that by boiler size, we aren’t necessarily talking about the physical size of the boiler, but rather the output of the boiler in Kilowatts. The higher the number of kilowatts a boiler has, the larger the supply of hot water it provides. The three main size categories for combi boilers are 24kW – 27kW, 28kW – 34kW and 35kW – 42kW, although it is important to note system boilers can offer outputs as low as 12kW. Each of these categories suit a different type of property, but choosing which size within the category you should get is determined by your own personal hot water and heating demands.

Combi boilers are currently the most popular type of boiler being fitted in the UK. Combi boilers heat water as and when you need it, and hot water takes priority over central heating. Therefore if your household demands hot water for showers etc, while the central heating is being used, the boiler focuses on heating the water for the shower, so less output will be used to heat the water for the central heating. This can mean that it may take longer for the central heating to reach the desired temperature, which isn’t a problem for most, but is definitely something worth considering when choosing your boiler size. So, the general rule here is the higher your demand for hot water, the larger the size of your boiler should be. In addition, if you have particularly large radiators for example, the output of the boiler will be used to heat up more water for your central heating, and will either lower the temperature of the hot water slightly for all hot water outlets, or take longer for the water to be heated to the target temperature – meaning you will need a larger sized boiler. If it is usual for your property to demand lots of hot water for showers and such, as well as central heating then a Gas Safe Plumber or Heating Engineer may even suggest you consider a system or conventional boiler instead. In any case, if you are unsure how your hot water demand affects the size of the boiler you will need, definitely consult an expert for advice.

But what other factors are there that you should consider when choosing the size of your boiler? Since combi boilers are fed straight from the mains you should take into account the flow rate and pressure of your mains supply. If you have a different, older type of boiler and want to change to a combi you need to make sure the size of your boiler matches the pressure of your mains. The size of the combi boiler you choose must be big enough to handle the all the water passing through it to be heated. You can check the flow rate by using a bucket with measurements. The idea is, you run water for a minute and the more water the bucket collects, the higher the flow rate is, and therefore the larger the size of the boiler should be because it is having to work harder in order to heat such a fast flow of water. Most boilers will detail the flow rate they can handle (in liters per second), so make sure you check it and compare it to your mains flow rate so that you find an appropriate boiler size. There’s no point getting a boiler which can handle a much high flow rate than the flow rate of your mains pressure, especially if it will cost you another £100, and still have the same effect.

One of the most obvious and most important factors to keep in mind when choosing the right boiler size is the type of property you have. The larger the property, the larger boiler size you will need, that goes without saying. However, you must also remember the property type isn’t just defined by the size of the property, but also the number of bathrooms it has as well as the number of radiators. To help you find your boiler size category based on your property type I have made the following table:

Boiler Size Guide:

Type of Property Number of Bedrooms Number of Radiators Recommended Boiler Size
Flat or House with 1 – 2 Bedroom(s) 1 Maximum 10 24kW – 27kW
Flat or House with 2 – 3 Bedrooms 2 10 – 15 28kW – 34kW
House with 4 or More Bedrooms 2 15 – 20 35kW – 42W

Here are some useful links to popular boiler manufacturers which help to give you a guideline in terms of understanding what size boiler you’ll probably need for your home.

Boiler Size Calculators

Worcester Bosch Combi Boiler Size Calculator
Glow-Worm Boiler Size Calculator
Vaillant Boiler Size Calculator
Biasi Boiler Size Calculator
Baxi Boiler Size Calculator

Boiler Recommendations for Your Type of Property:

Type of Property Combi Boilers System Boilers
Flat or House with 1 – 2 Bedroom(s) Worcester Bosch Greenstar 25i Combi Boiler
Ideal Logic+ 24 Combi Boiler
Worcester Bosch Greenstar 15i System Boiler
Worcester Bosch Greenstar 12i System Boiler
Flat or House with 2 – 3 Bedrooms Worcester Bosch Greenstar 34CDi Combi Boiler
Worcester Bosch Greenstar 32CDi Compact Boiler
Worcester Bosch Greenstar 18i System Boiler
Worcester Bosch Greenstar 24i System Boiler
House with 4 or More Bedrooms Worcester Bosch Greenstar 38CDi Boiler
Worcester Bosch Greenstar 42CDi Classic Combi Boiler
Worcester Bosch Greenstar 30cdi System Boiler

You’re probably wondering why I’m only recommending Worcester Bosch boilers? Am I biased in any way or are they paying me to write these recommendations? I wish! I’ve been in the plumbing & heating business for over 15 years now and I’ve learned a thing or two about boilers. Yes, there are many good manufacturers out there, but I’ve stuck to recommending the Worcester Bosch brand because they are so reliable. We’re able to offer guarantees of up to 10 years on all of our Worcester Bosch installations because we are Worcester Bosch Gold Accredited Installers.

7,8,9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

7,8,9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

So there you go, how to figure out which boiler size is best for your home. It may seem like a lot of information to take in, but once you have considered the main factors, you will be one step closer to making sure you find the perfect boiler size type for your property.

Remember if you’re still unsure, that’s not a problem, all you need to do is schedule a consultation with a Gas Safe Plumber or Heating Engineer for that extra bit of expertise – I would recommend you do this whether you know what size you want or not; having a professional’s opinion is a fairly easy way to see if you’re making the right decision.

I consider the boiler to be the brains of the house so with something as important as choosing the right one, I wouldn’t leave anything to chance, so don’t cut any corners or rush your decision, because ultimately boilers aren’t cheap and are a long-term investment. The last thing you want is to be stuck with an expensive, yet unsuitable boiler for your home, so take your time when making your choice. I’d also like to mention once more that if you want to know a bit more about the boilers showed above then read my previous blogs about them, which go into much more detail about their features, pros, cons etc.

If you liked this blog post I would appreciate if you could share this on Facebook or Twitter.

I would like to stress once again that it’s always important to get any gas related advice or installation work done from a fully qualified gas safe registered plumber. These types of systems can be very dangerous if they’re not installed properly so it’s worth checking the plumber’s reviews and qualifications.

If you live in South East London or Kent and need some advice in terms of what boiler system to choose for your home then feel free to get in touch. We’d love to hear from you.

Richard Costello

Richard Costello, from Dartford in Kent, is the owner of MultiPlumb and has been involved in the plumbing industry installing bathrooms and boiler systems for over 15 years. If you have any questions or suggestions for future blog posts feel to get in touch at

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3 replies
  1. Sarah
    Sarah says:

    It’s certainly worth considering different size boilers in relation to your home – whilst many homeowners believe that the bigger the boiler the better (just as with fridges, room sizes etc.), the practicalities could actually reveal that there are better options out there – if you have a small family, a small house etc. it can be a better option to get a small, efficient boiler.

  2. james thomas
    james thomas says:

    hey, thanks for sharing this info as an experienced service engineer in one of the best Combi boiler company most of the people don’t know about this the way your explaining is good keep going


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