Combi Boiler Size Guide

Looking for a new Combi boiler but unsure what size you need? You’re in the right place. Our Combi boiler size guide is designed to provide you with all the information and considerations you need to make to choose the right model for your home. After all, any Combi will provide heating and instant hot water. But several factors need to be accounted for when choosing the size, from the number of radiators in your home to the amount of bathrooms you have.

Combi boiler sizes explained

The size of a combi boiler is actually related to its output in kilowatts (kW) rather than its physical height, depth or width. The higher the kW rating of the boiler, the more power it has - and typically, the larger and heavier it will be. At Ideal Heating, we provide Combi models with power ratings from 24kW to 40kW to suit a range of different properties. 

What does kW output mean for a Combi boiler?

A kilowatt is equal to 1,000 watts and is a unit that helps measure power generated per second. For example, when looking at a 35kW boiler, ‘35’ demonstrates the maximum power this boiler is capable of producing each second.

When we’re thinking about Combi boilers, we can break it down quite simply: a 35kW Combi uses 35,000 watts (35 kW) of power per second to heat the water needed to keep a home warm.

The higher the kW figure, the more powerful the boiler. But, that doesn’t mean you need to choose the model with the highest kW rating. It’s more important to choose a Combi boiler size that suits your household’s needs to:

• Ensure your central heating is running efficiently 

• Keep your heating system as cost-effective as possible

• Avoid wasting energy unnecessarily

Common questions about Combi boiler size

Now you’ve got more context about what’s meant by ‘size’ when it comes to Combis, we’ve answered some of the most common questions we get from homeowners, like you, about choosing the right one.

What size Combi boiler do I need for my home?

Choosing the right size of Combi boiler for your property depends on several factors:

• Property type (e.g. detached, terraced, flat)

• Number of bedrooms

• Number of radiators

• Amount of baths and showers

• Fuel source at your property (natural gas or LPG (liquid petroleum gas, also known as ‘bottled’ gas))

To make it easy for you to get an idea of the best type and size of boiler for your home, check out our boiler finder. It asks you a series of quick questions to understand your property and household’s needs, then uses this information to suggest the most appropriate models for you. (The tool accounts for Combi boilers as well as others, like System and Heat Only boilers.)

If I extend my home, do I need a bigger boiler?

It’s important to note that if you’re looking to replace your Combi boiler as part of a wider extension or renovation project, you probably shouldn’t pick a model that’s the same size as your current one. For example, if you’re adding a conservatory or en-suite bathroom to your property, you’re likely to need a larger (more powerful) boiler to supply heating and hot water to the bigger space and higher number of radiators, taps, or showers.

How many radiators can different sized Combi boilers run?

We’ve wrapped up the number of radiators that Combis with a power rating of 24kW and over can run in the table below.

What happens if my Combi boiler is the wrong size?

If you select a Combi boiler that’s the wrong size, you may run into some issues in your home. For example: A Combi boiler that is too big could increase the cost and efficiency of your boiler, whereas if you select a combi boiler that is too small, you may end up with too little power to heat your home.

It’s important to select the right size boiler for your property to ensure it can efficiently provide the heat and hot water you need.

What is the physical size of a Combi boiler?

As a Combi boiler has no need for a separate cylinder or water tank, it will be very compact in size. This means that a combi boiler can be the ideal solution for smaller sized homes that are more limited on space than others.

How to choose a Combi boiler sized based on the number of bedrooms

Like we’ve set out above, using the number of radiators in your property is one way of working out the ideal power rating of your new Combi. But many homeowners find it easier to use the number of bedrooms or property type. Our support team gets lots of questions on this, from “what size combi boiler I choose for a one bedroom flat?” to “what kW rating does a boiler need to be for a five bed house?”. We’ve made the table below to give you a quick and easy way of understanding the amount of kWs your boiler needs to be to keep your property warm.

To find out more about what makes one boiler more suited to a certain type of property than another, take a look at our article on the difference between combi, heat and system boilers.

Find your ideal Combi boiler size

You should now have a better understanding of how Combi boiler size affects your home’s central heating. Use our boiler finder to find the model that’s most suitable for your property and household, or get in touch with our support team for more information and guidance.

You’ll also find lots more useful articles (like this one) over on our tips and advice page, like:

• Five advantages of a Combi boiler 

• How to find the best Combi boiler for your home 

• Are Combi boilers best for showers? 

• Combi boiler vs Condensing boiler