What does kW mean when it comes to your boiler size? Re February 3, 2023

What does kW mean when it comes to your boiler size?

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What does kW mean when it comes to your boiler?

What does kW output mean for your boiler size? We answer this and more in our boiler kW explainer blog. Keep reading to learn about boiler size.

Here at WarmZilla, we appreciate that a lot of people are not necessarily going to be boiler experts when looking into purchasing a new or replacement boiler. When you do start looking into buying a new boiler, you come across a lot of jargon kW output, boiler size, flow rate, flues, extension kits – what does it all mean? The good news is that with the WarmZilla survey, you don’t need to know any of it. Just answer a few simple questions about your home and we’ll recommend the boilers most suitable for your home. It only takes 90 seconds, you don’t have to part with your personal details and you’ll get the info straight away. No hard sell and no hassle.

However, if you are the kind of person that likes to know more, we thought we would write a blog that explains the meaning of what kW is when it comes to your boiler so you can understand how your boiler and central heating system works. We want you to feel confident about your new boiler purchase and if explaining ‘what is kW’ in boiler speak gives you that confidence, then we’ll write the blog.

What does kW actually mean?

Let’s start off with the obvious, as some may say; 

  • kW stands for Kilowatts.

The k, meaning ‘kilo’ stands for ‘one thousand’, derived from the Greek word, κιλό. 

  • Watt is the term used to measure the units of power. 

Kilowatts, in simple boiler terms, refers to the power output measurements your boiler or central heating is using. Professional heating engineers use the term kilowatt to help define how much energy your boiler outputs in the form of heat. 

The general rule of kW is that the more heat and hot water you require for your home, the higher kW boiler you will need. We will discuss this in more depth throughout the article. 

However, if you are looking for more information on what size kW boiler is better suited to your home, then this blog will help you to understand the sizing of kW boiler required to heat your home efficiently.

What’s the difference between kW and kWh?

Many often get kW and kWh mistaken for being the same thing, however, it’s important to note that they are not the same and that they measure different units.

What is the difference I hear you ask?

  • kW measures the power required to run your boiler
  • kWh measures the energy needed to keep the power going

Watts measure energy in relation to time. So a boiler with a maximum output of 24kW will produce 24,000 joules of energy per second when in operation. This energy is then used to heat your home. 

So, to put it simply, while kW’s measure the power required to run the boiler, the kWh is what measures the energy being used. 

You may see that kWh is used on your energy bills, as is it used by suppliers to calculate the total cost of energy used. If your bill is higher than expected, we have written a blog on how to make your boiler more efficient. This could help you save some money on your heating bill by making your boiler more efficient.

What size kW boiler do I need to heat my home?

One thing to consider when purchasing a new or replacement boiler is to look at the hot water kW output that your home is going to require. 

Households with a higher demand for hot water, such as homes with a bath, will need a boiler with a more powerful kW size than smaller homes. Larger homes that use the shower and a bath simultaneously, may require a conventional or system boiler, which uses a tank to heat the water ready for when needed. 

Not sure of the boiler kW size you need for your home?

Take a look at the tables below:

Combi Boiler kW Size Guide

No of bedrooms and bathroomsPoorly insulated propertyModerately insulated propertyWell insulated property
1 bed + 1 bath20kW20kW18kW
2 bed + 1 bath20kW20kW18kW
2 bed + 2 bath24kW20kW20kW
3 bed + 1 bath28kW26kW24kW
3 bed + 2 bath28kW28kW24kW
4 bed + 1 bath30kW30kW28kW
4 bed + 2 bath32kW32kW30kW
5 bed + 2 bath40kW35kW30kW

System Boiler kW Size Guide

No of bedrooms and bathroomsPoorly insulated propertyModerately insulated propertyWell insulated property
1 bed8kW7kW5kW
2 bed8kW7kW6kW
3 bed10kW9kW6kW
4 bed12kW10kW7kW
5 bed15kW12kW9kW
6+ bed18kW15kW12kW

Another factor to consider when trying to find the ideal kW boiler for your home is by taking into consideration the number of radiators that the boiler has to provide hot water for. 

How hard your boiler has to work to reach the desired temperature will depend on the number of radiators you have in your home.

A small flat with only a few radiators will only require a relatively low kW boiler (12-24 kW), whereas an average 3-4 bedroom house with around ten radiators would require a medium size combi (24-30kW). If you have a large house with as many as twenty radiators, a large combi boiler (35-42 kW) may be needed to provide sufficient heat. 

With a house of this size, however, you may want to consider a system boiler or a conventional (regular) boiler instead.

Does a higher kW boiler cost more to run in your home?

Luckily, the answer is no. 

All modern boilers are now fully modulating, which means that this is no longer an issue when purchasing a new or replacement boiler.

Whereas older boilers used to cut on and off on a cycle, which resulted in wasting gas, modern boilers will fire up on a high flame, as the water or radiators reach the optimum temperature, the size of the flame will reduce, this is called modulating. The flame then stays at a simmering level, which doesn’t waste gas. 

Although you may worry that a higher kW boiler may cost more, they don’t. The boiler does indeed need to work a little harder to heat your home faster, but as mentioned, once they’ve hit the right temperature, they go into simmer mode.

How can I cut the cost of my energy bills?

It is important once purchasing a new or replacement boiler that you get regular servicing to make sure your boiler is working as efficiently as possible. The easier it is for your boiler to reach your desired temperature, the fewer kWh are used, which will reduce your energy bills. 

If you don’t get your annual servicing or any maintenance required, your boiler could end up needing to work harder to heat your home, costing more. Avoiding servicing and maintenance could also result in debris build-up in your radiators, which will eventually make your central heating system less efficient, again, increasing your energy bills. 

If your boiler is more than 15 years old, look into replacing it with an A-rated condensing boiler.

Here are some more ways of reducing your energy bills:


Installing a new boiler in your home will improve your heating system, and could potentially save you around £300 a year. This will also cut your home’s carbon dioxide (CO2) emissions, helping save the planet (your good deed for the year, completed).

Bleed your radiators

As well as improving the efficiency of your radiators, this minor maintenance task will lead to cheaper bills too! How do you know when you need to bleed your radiators? Turn your heating on, and see if any of the radiators take a while to heat up. If you have a cold spot at the top of your radiator, this is when you need to bleed them. 

Want more information on how to bleed your radiators, we’ve got you covered. Click here for our ‘How to fix a cold radiator guide.

Top up the pressure

A pressure drop will hinder the efficiency of your boiler. But it’s actually really easy to correct. Have a look at your gauge on your boiler. Ideally, it should read around 1.5 bar if the pressure is healthy. If it doesn’t, this is when it needs correcting. 

Want some more information on this, guess what? We also have this covered! Click here for more information on why your boiler may be losing pressure and how to fix it. 

Use a Powerflush

Here at WarmZila, a Powerflush is deemed the last resort. This is because a Powerflush can cause damage to old pipework by loosening joints due to the high amount of pressure being pushed through the system, which could result in leaks in your home. Which nobody wants. 

However, if you have tried bleeding your radiators, and they’re still not getting warm, you may have a build-up of ‘sludge’ (a mixture of debris and rust) which has collected in the pipes and caused it to become increasingly difficult for the hot water to be distributed around your system. This is when a Powerflush is suggested. 

Your Gas Safe heating engineer will be able to perform a Poweflush on your boiler and heating system, removing all the rust and debris. We’ve written a blog on different system flushes that may help you.

Insulate your pipes

Another way of helping reduce your energy bills and make your heating system more efficient is by insulating your pipes. All you need is some easy-fit, foam tube with can wrap around your pipes, keeping the heat in and water hotter for longer. You can find this foam at many DIY home stores. 

Buy a new boiler

If you have tried every trick in the book and you’re still not achieving the results you want, it may be time to get yourself a new boiler. 

As mentioned above, it is important to consider the rating and efficiency level of your boiler to ensure it will meet the demands of your home. You want to be on the lookout for A-rated boilers for the best efficiency levels.

1 Comment
  • June 18, 2023, 10:29 pm

    Therе’ѕ dеfinately a lot to lеarn about this topic. I love all of the points you’ve made.

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