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What Size Boiler Do I Need?


Given that your new boiler is a costly investment, but one that will serve you for more than a decade, you would want to get everything right the first time. And one of the most important things that you need to be precise about is the boiler size. By installing the correct boiler size, your home will be kept warm throughout the chilly months, and you’ll always have hot water whenever you need it.

But what is the right boiler size for your home? Unfortunately, there is no ‘one size fits all’ version when it comes to boiler size. As such, you need to put in a lot of effort and consider several factors before you can arrive at the ideal boiler size for your home.

This guide will shed light on the factors you need to consider when shopping for the right boiler size. It will also show you how to calculate the correct boiler size for your home. Lastly, it will cover some recommended boiler sizes based on the size of your property and the number of radiators in it.

But before we get to calculating the ideal boiler size for your home, let’s take a look at some basics, including what boiler size is and the consequences of installing the wrong boiler size.

What is boiler size, and how is it measured?

Contrary to how it may sound, boiler size has absolutely nothing to do with physical dimensions. Instead, boiler size refers to the output rating of the boiler measured in kilowatts (kW). Kilowatts here relates to the units of energy provided by the boiler in the form of heat. Generally speaking, the more power a boiler can produce, the bigger the size of that boiler.

 What happens if I install an undersized boiler?

Well, if you intend to knock off a few hundred pounds off your boiler budget and go for a cheaper low-powered boiler, then you are in for a ride of misfortune. One thing is for sure. The boiler will struggle to meet your home’s heating and hot water demands. As a result, you will keep running out of hot water, and your radiators will be tepid.

So, unless you want to frequently wake up to a chilly house or fancy a cold shower on a winter morning, you better not install an undersized boiler.

What happens if I install an oversized boiler?

After learning about the consequences of installing an undersized boiler, you are tempted to go for the biggest possible boiler. Well, don’t. Going for an overpowered boiler is a waste of resources and will result in low efficiency.

For instance, besides the expensive installation cost, you will also have to contend with the massive heating bill, which could be avoided by installing the right size boiler. Too much power can also exceed your heating system capacity, resulting in poor performance and poor reliability.

Another thing about oversized boilers is that they tend to ‘cycle’ on and off when in operation. This is because the overpowered boiler heats the water rapidly and shuts down after a few minutes. As the water temperature in the hot water cylinder drops, the boiler will fire up again for a short period and fires off again.

The cyclic powering on and off of your boiler will not only consume a lot of energy by burning unnecessary fuel but also hasten the wear and tear of boiler components. This will, in turn, mean thousands of pounds in operating and repair costs for you down the line.

And, of course, with a massive boiler size, especially a gas-fuelled one, your carbon footprint will be growing with each passing day. So, if you care about your surroundings, then installing an oversized boiler is definitely out of question.

What size of boiler is appropriate for my home?

Given how delicate the business of selecting the right size boiler is, it is only natural that the final decision depends on several factors. Some of these factors you should consider when shopping for the right size boiler include:

  1. The number of radiators in your home

This is undoubtedly a critical factor in determining the size of the boiler that will suit your home’s heating needs. Usually, the ideal boiler size for your home is directly proportional to the number of radiators you have.

If you live in a large house with a large number of radiators, you will need to install a large boiler to meet your home’s heating demands. Conversely, if you live in a small flat with a few radiators, settling for a small-sized boiler will ensure that your heating demands are met at an economical cost.

On average, a three or four-bedroom house will have around 10 to 12 radiators and require a small or medium-sized boiler, be it a combi or conventional. A larger house, say a five-bedroom one, will have as many as 20 radiators and will need a large combi or system boiler to provide sufficient heating.

  1. Hot water demand

When shopping for the right-sized boiler for your home, you must consider how much hot water your family uses. You will also have to decide whether a combi boiler will meet your home’s hot water demand or a conventional boiler with a hot water tank is more suitable. The type of boiler you choose will influence the size of boiler you need.

A combi boiler is designed to provide hot water on demand, and the size of the boiler is expressed in terms of how much water it can heat to a specific temperature per minute. Usually, if you need a boiler with a high flow rate, say for filling baths faster or sustaining a powerful shower, you should go for a bigger boiler.

If your home has an established hot water demand pattern or routine, then a combi boiler might not be the ideal choice of boiler. In such a situation, a system boiler fitted with a hot water tank might prove to be the better option. This is because system boilers preheat the water and store it in an insulated cylinder waiting for the next demand cycle.

As such, the hot water flow rate isn’t a factor in determining the right size of the boiler, and you can therefore install a smaller sized system boiler. On average, the output of system boilers ranges from 10 to 20 kW compared to between 24 and 40 kW for combi boilers. In addition, system boilers require less power because they heat the water slowly, unlike combi boilers that heat water instantly.

  1. Fuel type

The type of fuel available in your area will also affect the size of the boiler you need. Usually, a gas-powered boiler is more powerful or larger than an oil boiler. In the same way, an oil boiler is more powerful than an electric boiler.

These different fueled boilers also vary in efficiency, meaning for the same size boiler, one boiler can have a lower output than another. Consult a professional heating engineer to determine the correct size boiler type that will sufficiently meet your home’s hot water and heating needs.

  1. Heat loss

If your home is not well insulated, your boiler will need to produce more heat to maintain your home at the right temperature. As such, you will need to install a larger boiler than you would have otherwise required if your home was well insulated.

Heat loss can be a significant problem, especially where you live in an older house. This is the case because older homes are poorly insulated and lose more heat than most modern houses.

Of course, you can’t determine the rate of heat loss in your home on your own. As such, it is considered a better idea to consult a professional heating engineer. The heating engineer will consider factors such as floor area, number of radiators, insulation, and even the number and size of windows when calculating your property’s rate of heat loss.

Using these factors, the heating engineer can determine the correct size boiler to cater to your home’s heat loss and still provide optimum heating and hot water.

  1. Plans for renovation

If you plan to renovate your home, you have to sit down and consider whether installing the same size boiler as the older one would be suitable. For instance, if you intend to fit in an extra room or bathroom, you will need to install a more powerful boiler than the one you currently have.

  1. Water pressure

Your home’s water pressure is particularly important if you are weighing the size of the combi boiler to install. This is important because the boiler must be able to cope with the amount and pressure of water passing through it. As a general rule, the higher the mains water pressure, the bigger the boiler you need. Usually, most manufacturers indicate the maximum recommended flow rate for their boilers.

Recommended boiler size per number of radiators

As earlier said, the number of radiators in your property plays a critical role in determining the right size of boiler for your home. Here is the estimated size of combi boiler you need depending on the number of radiators you have.

  • Up to 10 radiators: 24kW-27 kW
  • 10 to 15 radiators: 28kW-34kW
  • 15 to 20 radiators: 35kW-42kW

The estimates for a heat-only or system boiler are more accurate as they involve simple calculations. To arrive at the right size of boiler you need, assign 1.5kW to every radiator in your home and add 3kW that will cater for heating the boiler itself.

In other words, to calculate the size of boiler suitable for your home, take the number of radiators in your home and multiply it by 1.5kW. Next, add 3kW to the resulting value, and voila! You have the ideal boiler size for your home.

To better understand these calculations, take a look at the table below.

Number of radiators Boiler size calculation Minimum size in kW
6 radiators 6x 1.5kW + 3kW 12kW
10 radiators 10x 1.5kW +3kW 18kW
15 radiators 15x 1.5kW + 3kW 26kW
20 radiators 20x 1.5 kW + 3kW 33kW

Boiler size per property type

Now that we’ve looked at how to estimate the right size of boiler depending on the number of radiators in your home let’s look at the estimated boiler size per property type.

What size boiler do I need for a one or 2 bedroom house?

If you live in a 1-2 bedroom flat or apartment, you probably have less than ten radiators and use one bathroom. As such, your home’s heating and hot water demand can be met by a considerably small-sized boiler. In this case, a boiler size between 24Kw and 27Kw will do the trick.

What size boiler do I need for a 3 bedroom house?

If you are shopping around for the correct boiler size for a three-bedroom house, you can settle for any boiler size between 25kW and 35kW as it would meet your heating and hot water demands without struggling. However, if you want a more precise estimate, consider installing one rated between 29 and 30kW.

What size boiler do I need for a 4 bedroom house?

A 4 bedroom house has a high demand for hot water and will require several radiators to be sufficiently heated. As such, it would need a more powerful boiler than a three or two-bedroom house. On average, a 35kW rated boiler is a good fit for a house this size.

What size boiler do I need for a 5 bedroom house?

If you live in a 5 bedroom house, you have around twenty radiators and two or more bathrooms. This means that your property needs a powerful boiler to maintain comfort and have a steady supply of domestic hot water.

A system boiler will be more appropriate in such a situation as there will be hot water stored in the hot water cylinder that can be accessed whenever needed. Of course, a powerful combi boiler, say, a 40kW  rated boiler, can also be used, but a system boiler would be more practical.


The estimates on this guide are meant to provide you with something to work with in terms of budgeting and planning. They, however, should not be used as the ultimate decision for purchasing the right size boiler for your home.

Instead, we recommend consulting a professional heating engineer who will analyse your home’s unique situation and recommend the ideal boiler size that suits your home. With the help of a heating engineer, you will be able to select the right size boiler that will sufficiently meet your heating and hot water demands on the most economical level.

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