What Size New Boiler do I need…?

What Size New Boiler do I need…?

Choosing the right boiler for your home is a daunting task. With so many options and types of boiler available, it can be hard to know which one is right for your home. Done right, the choice of boiler you opt for can last you for up to fifteen years and will be responsible for providing your heating and hot water needs all year round.

Depending on the heating and hot water needs of your home, the perfect boiler will vary for each household, so it’s very important that the correct boiler is fitted that matches your needs and doesn’t make you waste money on your energy bills.

There are several factors that you need to consider when answering ‘what size boiler do I need?’, but luckily we have created the below guide to help you answer just that.

Why it’s important to choose the right size boiler for your home

When we are discussing the size of a boiler, we aren’t referring to its physical dimensions. Instead, we are referring to the output in kilowatts (kW), the unit that is used to measure how much energy is output by the boiler in the form of heat. It is important to choose the right size boiler for your home because it will mean you are not wasting energy and money you don’t need.

Generally speaking, the more heat and hot water you require, the higher the output you will need, although other factors also play a part in choosing the right size.

Number of radiators

A big factor in choosing the right size boiler for your home is to take into consideration the number of radiators in your property. If you have a small home, such as a flat or an apartment, the smallest suitable boiler size will be sufficient to heat your home and provide adequate hot water. It should also help you avoid paying more than you need to for energy.

For an average three or four-bedroom house that typically has around ten radiators a small or medium boiler (for example 24-30 kW) should be sufficient. A larger house with substantially more radiators or an en-suite bathroom will generally need a larger one, typically 30-35 kW.

If you have a large house with as many as twenty radiators, a large 35-42 kW combi boiler may be needed to provide sufficient heat with the best economy or it could be better to change to a system boiler that could be sized more appropriately.

Property size, layout and demand

If you have a larger property, you will need a more powerful boiler to heat the whole building more efficiently and ensure the appropriate water pressure is maintained throughout without resulting in wasted energy.

The number of bathrooms your house has, as well as how often you run baths and showers, can also affect the size of the boiler you need. If you have a lot of people living in your home, the demand for hot water and heating will be higher – requiring a larger kW of the boiler.

Heat loss

Previously, the size of your house was often used as a factor in considering the right size boiler required. However, nowadays, a heating engineer can calculate your home’s heat loss as a better way to determine the most effective and efficient boiler size.

A range of factors are taken into account when making heat loss calculations including floor area, the number of radiators, number and type of windows, insulation and other aspects that will cause heat to be lost from your home. The calculation will then be used to determine how much heat your house will lose on the coldest day of the year, so you can purchase the most optimum size of the boiler.

Fuel Type

The type of fuel available in your property can also affect the size of the boiler you choose, as boilers for different fuels can vary in terms of efficiency.

Most UK homes use gas boilers which offer the greatest level of efficiency, whilst houses that are not connected to a mains gas supply often choose to use an oil-fired or LPG (liquefied petroleum gas) system instead.

Furthermore, some modern boilers are built ready for future energy sources, such as solar panels which should be taken into consideration when deciding the right size boiler for your home.

What type of boiler is right for your home?


Before choosing the size of your boiler, you need to determine which boiler type would be best suited to your home. There is a range of different types of boiler all of which have specific benefits and efficiencies depending on how much space you have available and what boiler system you have currently.

Below we have briefly outlined the three main types of the boiler but check out our guide for a more detailed explanation of the types of boilers you can get for your home.

Combi boilers

One of the most common types of boiler in UK homes, combination boilers, also known as combi boilers, heat hot water as and when it’s needed, so no hot water is stored. They are a combination of a water heater and a central heating boiler in one single system, and they work by staying on standby and heating water almost instantly when you require it. However, combi boilers can only support one outlet at a time, so can either be used for heating or hot water, but not both at the same time. If you have a large household that uses multiple outlets of water, this might not be the best boiler for you.

Conventional Boilers

Also known as ‘Standard’, ‘Traditional’ or ‘Regular’ boilers, conventional boilers use hot and cold water storage tanks to heat your water. A conventional boiler uses fuel, such as gas or oil, to warm up the water, which is then transported to the hot water tank through the use of a pump. When your heating or water is turned on, this hot water travels to your taps and out your taps or shower. Commonly found in older, more traditional houses, conventional boilers a best for a direct replacement for your existing heating system.

System Boilers

Ideal for larger properties or homes with a higher demand for hot water, a system boiler works by taking water directly from the mains and heating it in a storage cylinder for use in your home. By using the mains water supply, system boilers eliminate the need for a cold water tank and mean you will have instant access to hot water at 2 or more outlets at a single time without the water pressure dropping.

Boiler size calculator

Once you’ve chosen the type of boiler you believe your home needs, it’s time to calculate the appropriate boiler size for your home. Below will give you an idea of the sorts of boilers you can purchase based on the three most common types of the boiler above, but if you need a more accurate calculation, we have a boiler buying guide available that will help you find the right replacement boiler or a new boiler to suit your home and central heating system requirements.

For a quick overview of boiler sizes, please see the table below:


Property Type No. of Bedrooms No. of Bathrooms No. of Radiators Recommended Boiler Size
Apartment/Flat or House with up to 2 bedrooms 1-2 1 Up to 10 24kw-27kw
Apartment/Flat or House with 2-3 bedrooms and 1 shower 2-3 1 Up to 15 28kw-34kw
House with 4+ bedrooms and 2 showers 4+ 2+ Up to 28 35kw-42kw


What size combi boiler do you need for your home?

Combination boilers are a lot bigger in size in comparison to a system or regular boiler as they need to produce hot water on demand and heating for the home. As they directly provide central heating and domestic hot water, they have two separate output ratings: Central heating (CH) output

  • Domestic hot water (DHW) output
  • Regular and system boilers only have a central heating output as the domestic hot water is stored in a cylinder.

The number of radiators in your property can help you find the most suitable size model as the central heating outputs of combi boilers can be split into three categories, each suitable for a different type of home.


Number of Radiators Recommended Combi Boiler Size (CH output)
Up to 10 24-27 kW
10-15 28-34 kW
15-20 35-42 kW

What size system boiler do you need for your home?

System boilers do not need to be as powerful as combi boilers. This is because they incorporate a separate hot water cylinder for heating and storing domestic water.

Adding up the number of radiators around your home, like with combi boilers, will help you understand a suitable system boiler output rating though.


Number of Radiators Recommended System Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 26-40 kW

What size regular boiler do you need for your home?

Like system boilers, conventional/regular boilers have the same recommended central heating output and so do not need to be as powerful as combi boilers.

Just as above, the number of radiators in your home will help you determine which is the best conventional boiler for your needs.


Number of Radiators Recommended Regular Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 26-40 kW

Boiler Size FAQs

Below, you’ll find some of the most frequently asked questions about boiler size along with our expert answers.

What happens if a boiler is oversized?

If a boiler is oversized it will ‘cycle’ on and off excessively, heating up too quickly to only run for a short period, only to turn itself off because it’s too hot. Each fire up is something close to full capacity which dumps a huge amount of heat into the system and overheats the radiators. This will end up decreasing the energy efficiency and creating more stress on the components, increasing wear and tear and shortening the lifespan of the boiler. A boiler is at its most efficient when it can run for long periods at a lower output, so this is recommended to ensure you are not paying more money in the long run.

What happens if my boiler is too small?

Having an undersized boiler may mean that it won’t have the power to heat your home properly, especially in winter, nor may it have enough hot water when more than one person in the home is trying to have a shower or run the taps. To ensure you are getting the right boiler for your home, use our handy boiler buying guide.

How many radiators can a 30kw boiler run?

A 30kW boiler will typically run 15 radiators with ease. We recommend the Worcester Greenstar 30i if you’re looking for a combination boiler.


Worcester Greenstar 30i

Worcester Greenstar 30i

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