Home Improvement

What Size Boiler Do I Need for My Household

Your boiler is one of the most important household appliances. Every responsible homeowner knows that in order to have an efficient heating system, they need the right type of water heater, most suitable to their household’s needs. But if you’re moving into a brand new building, how do you know what kind of boiler you need? Fear not, because Fantastic Services is here with the answers!

This article is for all of you homeowners who:

  • Are buying their very first boiler;
  • Are looking to replace the old boiler with a new one;
  • Might be considering changing the boiler type.

How is boiler size measured?

First thing’s first, when we say boiler size, we don’t mean the physical dimensions, but the output in kilowatts (kW). A kilowatt is a unit that measures how much energy in the form of heat the boiler outputs. The bigger the demand for heat and hot water, the higher the output. So, when you’re on the market, looking for your next boiler, this is the main criteria you should consider. But how do you know what the demands of your new household are?

Types of household boilers

There are three main types of modern household boilers. All of them would do a great job in heating the water you use in the shower, as well as keep your heating system going. However, if you’re searching for greater efficiency, you better read on. 

  • Combi boiler. A combi boiler (combination boiler) is a single unit, without a bulky water tank, that generates all the heating and hot water for the home. The combi boiler heats water instantaneously upon the opening of a hot tap. This is a high-efficiency boiler system available at affordable prices. The only drawback is that it doesn’t work as well when there is a demand for hot water from two taps at the same time. That’s why it’s more common in smaller properties with one or two occupants.
  • Heat-only boiler. Unlike the combi type, a heat-only boiler (also known as regular or conventional boiler) works with a hot water cylinder. This type of boiler is better-suited for large homes with a greater demand for hot water, where the occupants often use multiple taps and showers at the same time.
  • System boiler. A system boiler also works with a hot water cylinder. It directly heats the central heating system and collects hot water for later use. System boilers store water like heat-only ones, but they also take their water supply directly from the mains, like the combi variety. This makes them more compact than heat-only boilers, but still able to deliver a high volume of domestic hot water, unlike the combi kind. They’re also the most expensive.

The more baths and showers you have, the larger boiler you will need. And of course, don’t forget the number of radiators around your house. Below, you will find general advice on what boiler size you need according to these three criteria.

Not sure what size to pick? Need your boiler installed? Visit our website to check out price rates for boiler installation!

What size boiler do I need for Combi boilers

If you’ve decided to get a combi boiler for your new household, here is the size you need according to your property’s size and heating system.

Showers/bathsRadiatorsBoiler size
1-2Up to 1024-27 kW
2-310-1528-34 kW
3+15-2035-42 kW

If your household has three or more bathrooms, you’ll be pushing the boundaries of a combi boiler’s efficiency. That doesn’t mean it won’t work well, but you’ll have a much better experience if you opt for a heater with a water cylinder instead.

What size boiler do I need for heat-only boilers

If a heat-only boiler is what your household needs, make sure to pick the proper size from the table below.

Showers/bathsRadiatorsBoiler size
1-2Up to 109-18 kW
2-310-1518-26 kW
3+15-2027-40 kW

What size boiler do I need for System boilers

For system boilers, here is the size you need according to your property’s size and heating system.

Showers/bathsRadiatorsBoiler size
1-2Up to 109-18 kW
2-310-1518-26 kW
3+15-2027-40 kW

As you can see, the output from a heat-only or system boiler is generally the same. They also don’t need a higher output size like combi boilers thanks to their hot water cylinders. If you’re sure your household needs a heater with a water cylinder, it’s mostly a question of price. For more information, check out our article on how to choose a new boiler.

What size hot water cylinder do I need?

If you’ve settled on either a system or a heat-only boiler, know that the cylinder size is just as important as the boiler size. Again, when we say size, we don’t mean physical dimensions. In this case, we refer to water capacity. This would depend on the number of occupants of the property and the number of bathrooms or showers.

Hot water cylinders can be vented or unvented. Vented cylinders take water directly from the mains. This requires them to be installed alongside a system boiler. Unvented cylinders receive water from a cold water tank which means you should install them with a heat-only boiler.

The cylinders can also be heated directly, or indirectly. Indirect heating means the boiler itself heats the water. Direct heating means water is warmed up by an immersion heater. These heaters are placed within the cylinder and can work with the help of solar panels for lower energy costs. Here is a general guide on choosing the right hot water cylinder.

1175/120 litres120/150 litres
21150 litres180 litres
32180 litres210 litres
43210/250 litres250/300 litres

Are you in need of a professional boiler repairman?

Are your boiler and heating system healthy? Sometimes, you might not need a new boiler, but you just have to take care of an issue that prevents it from working properly. Always make sure a reliable boiler technician checks your boiler before taking a big decision like looking for a new one. If you’re not sure about what’s wrong with your current water heater, give a call to the professional team of plumbers at Fantastic Services. They will check the problem and advise whether it’s time to retire your appliance. If this is not the case for you, that’s even better, but why wait for your boiler to fall short of your expectations, you can subscribe to one of our boiler cover or central heating plans and have a technician any time you need him at no extra cost. Our plans include repair costs, annual check-up and much more.

Is your boiler giving you trouble?

Reach out to a reliable boiler technician now!

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  • We're certified:


  • The size of a boiler is not about its physical dimensions, but its heat output in kilowatts;
  • Household boilers come in three types – combination, heat-only and system boilers;
  • Combi boilers don’t require a hot water cylinder, unlike heat-only and system boilers;
  • The size of the boiler your household needs depends on your property’s water consumption and the number of radiators in your heating system;
  • When choosing a boiler with a hot water cylinder, you also have to consider its water capacity.


Have you ever been in the market for a new boiler? Tell us about your criteria in the comments and how satisfied you are with the purchase!

Image source: Aldeca Productions/shutterstock.com

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