Boiler Sizes Advice & FAQs

Everything You Need to Know About Boiler Sizes

Searching for the ideal boiler size to heat your home or property? Whether you want to calculate the perfect output to power your central heating or you already know exactly what size you need, we’ve got you covered. 

We’ve made life easy by creating a complete rundown of our range of gas boilers, complete with the sizes they’re available in. From 12kW, 30kW or 40kW and every size in between, you’ve come to the right place. 

If it’s answers you’re searching for, look no further. From working out the best boiler size to heat a specific number of radiators to understanding how much power an Ideal 30kW gas boiler actually provides, our ultimate list of FAQs has all the answers you need. 

How is Boiler Size Measured?

When we talk about boiler size, it’s not hard to get a little mixed up. It’s not height, width or depth that we’re discussing, but rather its output in kilowatts (kW). The more power the boiler can generate in kWs, the bigger it is in terms of ‘size’. 

For reference, let’s take a look at our range of domestic gas boilers. Our combi models are available from 24 to 40kW; system from 15 to 32kW; and heat only from 12 to 36kW.

The size or power difference is due to the way these models operate; combi boilers generally have larger outputs in order to heat water as it’s required, while system and heat only options have smaller outputs, slowly boiling and storing hot water for future use.

For example, the smallest size the Ideal Logic+ Heat is available in is 12kW, while at the other end of the scale, our Vogue Max Combi comes in sizes up to 40kW. 

What Does kW Output Mean for a Boiler?

A kilowatt (kW) is equal to 1,000 watts and in any situation, is a unit that helps us measure power generated per second. From 12 and 18kW, to 32 and 40kW, these figures show the maximum power the boiler is capable of producing each second.

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

Does a Higher kW Boiler Cost More to Run?/How Does Boiler Size Impact Efficiency/Cost?

It makes sense that the bigger your home, the larger the size or kW boiler you will need, and the more it will cost to run. Thankfully, newer models are much more efficient than older boilers, which means that your energy bill will look much healthier. 

Using an example of a 35kW boiler such as the Logic Max Combi, this model’s size means it will use 35 kWh (kilowatt hours) of energy in an hour. That’s how energy suppliers measure the power we use in our homes, which is why the cost breakdowns on our utility bills are measured in kWh. 

What Size Boiler Do I Need for My Home/Property?

Which boiler is right for your property depends on several factors. The main things you need to consider when calculating the optimal output are the size of your home, the number of bedrooms and how many baths and showers it houses — all of these are key to kWs required. 

To help out, we’ve created a quick and easy tool to help you find your new boiler. It takes into account all of the considerations listed above and more, including whether you need a combi, system or heat only boiler. To make your search even simpler, it will suggest models that are a great fit for your needs.

It’s important to note that when you’re looking to replace your boiler and you’re planning to make renovations, you shouldn’t pick a model that’s the same size as your current one. For example, adding a conservatory or en-suite bathroom will mean you need a larger boiler for the increased space, radiators or number of taps and showers.     

What Happens If My Boiler Is the Wrong Size?

If you select a boiler that’s the wrong size, there are a number of potential issues you could face including cost and efficiency. It’s crucial to select the right size boiler for your property, to ensure your central heating system is properly equipped to provide heat and hot water.

It would be easy to assume that the larger the size of a boiler, the better it will be in any situation — the reality is very different though. If you have the opposite problem and your boiler is too small, you may end up with too little power to heat your home. 

What Happens If My Boiler Is Too Big (Oversized)?

Too much power isn’t a positive when it comes to central heating, and will most likely just mean higher utility bills. Having a boiler that’s too large could also generate more power than your central heating system can handle, leading to poor performance and reliability issues.

When your boiler is not being used for central heating or hot water, it turns itself off, which means the water used to provide us with heating and hot-running taps will cool. 

When it is called into use again, an oversized boiler will use more power than a properly sized boiler to heat the same volume of water — the amount your household needs to use.

What Happens If My Boiler Is Too Small (Undersized)?

A boiler that is too small will not be able to keep up with the demand for water and heat that your property needs. That could lead to you waking up in a chilly house and stepping into freezing cold showers in winter, which is no good for anyone.

That’s why it’s so important to properly assess the needs of your home or property to find the boiler that’s a perfect fit.  With modern boilers being so efficient, it’s now much easier to calculate what size boiler you need than in years gone by. 

How Many Radiators Can a 12kW Boiler Run?

Here at Ideal, you can find 12kW boilers in our heat only range, and 15kW in our heat only and system models. Small but powerful, we recommend that boilers between 12 and 15kW are the perfect size for heating a home with up to 10 radiators. 

How Many Radiators Can a 18kW Boiler Run?

From the Logic+ System to the Logic+ Heat, you’ll find a vast range of system and heat only boilers in an 18kW output. At this size, an 18kW model is powerful enough to run between 10 and 15 radiators, making it ideal for medium-sized properties.

How Many Radiators Can a 24kW Boiler Run?

The number of radiators a 24kW boiler can heat will depend on the type of model you require. As a guide, combi boilers between 24 and 27kW can power up to 10 radiators, while a 24kW heat only or system model can heat up to 15 radiators.

How Many Radiators Can a 30kW Boiler Run?

For combi boilers, sizes from 28 to 32kW can keep between 10 and 15 radiators nice and toasty. On the other hand, 30kW heat only and system boilers can heat 15 to 20 radiators, making them the ideal choice for running larger properties.

How Many Radiators Can a 35kW Boiler Run?

For domestic properties, 35-40kW are some of the largest size boilers available, providing enough power to heat big family homes. You’ll find these outputs across all types of Ideal models, with our 35kW boilers capable of running and heating 15 to 20 radiators.

What Size Boiler (in kW) for a One-Bedroom Flat?

What size boiler would best suit a flat that’s home to one person? Let’s say this property features one bedroom, a shower but no bath, and less than 10 radiators. For this home we’d recommend looking at a 26kW model such as the compact Vogue Max Combi.

There isn’t a huge amount of space to heat up, and there isn’t going to be a lot of competition for using the hot water. An efficient combi boiler will provide all the heating and hot water this property needs — and it won’t take up too much precious space.

What Size Boiler (in kW) for a Three-Bedroom House?

How about finding the right size boiler for an average family-sized property? For a home containing three bedrooms, a bath and a shower, and up to 10 radiators, a good choice for this more spacious home would be a 24kw Logic Max System boiler.

With more rooms and a family occupying the property, we’re going to have plenty of space to heat and increased demand for hot water. This model also features an Ideal System Filter for increased efficiency and an easy-to-use digital display, so it’s a great choice for family homes.

What Size Boiler (in kW) for a Five-Bedroom Property?

With a larger property comes more people, more baths and showers and, of course, more washing up. For a five bedroom property, a boiler size like the 32kW Vogue Max System, compact and built with premium components, would fit the bill perfectly in this situation.

Appliances like dishwashers are more likely to be used in these homes and there will probably be a queue to use the bathroom in the morning too. That all adds up to a lot of water to heat, as well as plenty of bedrooms to be kept warm on cooler nights, which this boiler is equipped to handle. 

Is a Larger Size Boiler Always Better for Bigger Homes?

While a larger home generally means there’s more demand for heating and hot water, it doesn’t always mean a bigger boiler is required. Even for larger properties, the size of a system or heat only boiler will always be smaller than a combi model.

You may have noticed we recommended a smaller boiler size (kW) for a three-bedroom house than for a one-bedroom flat. That’s because system boilers work less frequently than combis, heating up a cylinder full of water that is distributed around the home as necessary. 

In contrast, a combi boiler instantly heats water from the mains any time that hot water is needed, meaning it’s called into work much more often.

How Can Installers Help Me Find the Right Size Boiler?

Once you have a strong idea of the size boiler you need for your property, it’s a good idea to take advantage of the expertise of a Gas Safe registered engineer. As heating experts, an installer can advise the best boiler for your individual property and needs.

To help you find the right fit for your property, they’ll take into account additional factors like the water pressure in your area, whether you need a gas or electric boiler, and how well the building is insulated. 

To find Gas Safe engineers in your area, simply enter your postcode into the Find an Installer field on the Ideal home page.

Size of Ideal Combi Boilers

Boiler	Output	Available For
Logic Combi	24kW, 30kW and 35kW	Installers, Social Housing
Logic Max Combi	24kW, 30kW and 35kW	Homeowners, Installers
Logic+ Combi	24kW, 30kW and 35kW	Homeowners, Installers
Logic Combi ESP1	24kW, 30kW and 35kW	New Builds, Social Housing
Logic Code Combi ESP1	26kW, 33kW and 38kW	New Builds, Social Housing
Vogue Max Combi	26kW, 32kW and 40kW	Homeowners, Installers
Vogue Gen2 Combi	26kW, 32kW and 40kW	Homeowners, Installers

Size of Ideal System Boilers

Boiler	Output	Available For
Logic System	15kW, 18kW, 24kW and 30kW	Installers, New Builds, Social Housing
Logic Max System	15kW, 18kW, 24kW and 30kW	Homeowners, Installers
Logic+ System	15kW, 18kW, 24kW and 30kW	Homeowners, Installers
Vogue Max System	15kW, 18kW and 26kW	Homeowners, Installers
Vogue Gen2 System	15kW, 18kW, 26kW and 32kW	Homeowners, Installers

Size of Ideal Heat Only Boilers

Boiler	Output	Available For
Logic Heat	12kW, 15kW, 18kW, 24kW and 30kW	Installers, New Builds, Social Housing
Logic Max Heat	12kW, 15kW, 18kW, 24kW and 30kW	Homeowners, Installers
Logic+ Heat	12kW, 15kW, 18kW, 24kW and 30kW	Homeowners, Installers
Mexico HE	15kW, 18kW, 24kW, 30kW and 36kW	Homeowners, Installers
Evomax 2	30kW, 40kW, 60kW, 80kW, 100kW, 120kW and 150kW	Installers

Need a larger boiler than the options listed above? Visit the Ideal Commercial Boilers site. 

To find out more about alternative central heating solutions take a look at our Guide to Heat Pumps.