What Size Boiler Do I Need? - How To Calculate Your Power

Rachel Francis
February 27, 2025
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Top Tips & Advice
Domestic Advice
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Replacing your boiler or upgrading your unit can be tricky if you are not sure where to start or know what size boiler you might need for the size of your home. You also want a boiler that meets all your requirements, depending on the number of radiators in your business farm, or home.

Although there are many types of boilers and heating units such as heating oil boilers, the size, and output of the boiler you need might vary. But how to know what size boiler you need, and how can you calculate your power output to make sure you are getting the right one? Throughout our blog, we will show all the different types of boilers there are, how to calculate the output you need for your boiler and what size boiler you will need.

What is the best size boiler for your home or business?

The size of your boiler will ultimately depend on how many radiators or heating units you have in your building or home. The more radiators you have, the higher the power output and input you will need. This should help you decide on what size boiler would be best for the budget and will work effectively to keep your building warm.

There are multiple factors you need to consider when deciding what size boiler you need as well. Knowing these factors will help you find the best boiler solution for your home or business. Firstly, you must decide which type of boiler would be best for you, as choosing the wrong type for your building could leave you with a costly and less efficient boiler. See all the different types of boiler below to decide which one works best for you

Different boiler types

There are 3 different types of boiler you need to be aware of, before deciding on what size boiler you will need for your building or home. They are combi boilers, system boilers and heat-only boilers. See the difference between each of these heaters to help you decide which one would be best for your building.

Combi Boilers

A combination boiler, or ‘combi’ boiler, is a highly efficient and compact heating system that has multiple functions. It provides both central heating and hot water on demand, without the need for a separate water tank or cylinder. Meaning, its heating solution all comes in one package. This makes combi boilers ideal for smaller homes or flat where space is limited. They may also work well if you have a small office space and could help you to save on heating costs.

System Boilers

A system boiler, also known as a sealed system boiler, is a type of boiler that provides both central heating and hot water for a home or building. Unlike a combi boiler, a system boiler requires a separate hot water cylinder to store hot water, but it doesn't require a cold water tank in the loft, making it more compact than traditional boilers. So if you are wanting to save space for your boiler unit, a system boiler may be best for you.

Heat-Only Boilers

A heat-only boiler, is also known as a conventional, regular, or traditional boiler. It is a type of boiler that provides heating and hot water to a home through a system involving multiple components. It is commonly used in larger homes and buildings where hot water demand is high, as it can efficiently supply hot water to several outlets simultaneously. So this may be a better solution for larger buildings and spaces.

Why is the size of a boiler important?

The size of your boiler is important, since you need to make sure you are getting the correct output for the size of your building. Picking the wrong boiler type and size can lead to multiple issues within your home or building. You could end up with an inefficient and costly boiler with regular repair costs. So, picking the correct boiler is essential, and below are some more details of the reasons why.

Avoiding undersized boilers for your needs

Undersized boilers can lead to multiple problems for your property. They may struggle to heat your building efficiently, causing rooms to stay cold and uncomfortable. The constant strain on the boiler can also lead to higher energy bills and increased wear and tear, potentially resulting in more frequent breakdowns and costly repairs. In the long run, an undersized boiler can fail to meet your heating needs efficiently and can cost you more in the long run.

Save you from wasting money on heating

By choosing the correct boiler size for your home, you can be prevented from paying high heating and repair costs. An oversized boiler may use more energy than necessary, driving up your utility bills. On the other hand, an undersized boiler will work harder to heat your home, leading to increased energy consumption and more frequent repairs due to wear and tear.

Getting the correct input & output for improved efficiency

When you get a new boiler or need to replace an old one, you want to make sure that you get the correct output and input for your home to improve efficiency. If the input is too high, the boiler may overwork, leading to wasted energy and higher utility bills. Alternatively, if the output is too low, the boiler might struggle to heat your home effectively, meaning you get lower temperatures than you need.

What size boiler do I need?

Explore the tables below to work out what size boiler you would need for each type of boiler, depending on the number of radiators and rooms, as well as your building size.

Combi boiler size calculator

See the tables below to calculate what size Combi boiler you will need, depending on the number of rooms and radiators, as well as the type of building you have. Find your correct boiler size below.

Combi boiler size based on rooms & radiators

Use the table below to calculate what size combi boiler you need for your building based on the number of rooms/radiators:

Property Type Number Of Rooms Number Of Bathrooms Number Of Radiators Size Of Combi Boilers
2-Bed House/Apartment 1-2 1 0-10 24-27 KW
3-Bed House/Apartment 3-4 2-3 10-20 28-34 KW
4+ Bed House/Apartment 4+ 3+ 20+ 23-43 KW

System boiler size calculator

To determine the size of the System boiler you need, refer to the tables below. These tables will help you calculate the appropriate boiler size based on the number of rooms and radiators in your home, as well as the type of building you have. Find the right boiler size for your needs below.

System boiler size based on rooms, radiators, and property type

Use the table below to determine the right size system boiler for your building based on the number of rooms and radiators.

Property Type Number Of Rooms Number Of Bathrooms Number Of Radiators Size Of System Boiler
2-Bed House/Apartment 1-2 1 0-10 9-18 KW
3-Bed House/Apartment 3-4 2-3 10-20 18-28 KW
4+ Bed House/Apartment 4+ 3+ 20+ 27-40 KW

Heat-only boiler size calculator

Use the tables below to determine the size of the heat-only boiler you need. These tables will guide you in selecting the correct boiler size by considering the number of rooms and radiators in your home, along with the type of building you have. Find the suitable boiler size for your needs below.

Heat-only boiler size based on rooms, radiators & property type

Use the table below to find the right size heat-only boiler for your building based on the number of rooms and radiators.

*table

Property Type Number Of Rooms Number Of Bathrooms Number Of Radiators Size Of Heat Only Boilers
2-Bed House/Apartment 1-2 1 0-10 12-15 KW
3-Bed House/Apartment 3-4 2-3 10-20 18-24 KW
4+ Bed House/Apartment 4+ 3+ 20+ 30-40 KW

By utilising our boiler size calculator, you can ensure that you always get the right size boiler for you. If you would like to know more about how you can service your boiler and other boiler related blogs, see the pages below. Alternatively, if you need your boiler servicing at your farm home or business, contact us today

FAQs

How many radiators will a 30kW boiler run?

A 30kW boiler can typically run between 15 and 20 radiators, depending on factors such as the size and efficiency of the radiators, the insulation quality of the building, and the overall heating requirements of the space.

To estimate the number of radiators accurately, you should consider the heat output of each radiator (measured in BTUs or watts), the size of the rooms, and the specific heating needs of the building. As a general guide, each kW of boiler capacity can support approximately 1,500 to 2,000 BTUs of radiator output.

Should I get a 25kW or 30kW boiler?

Choosing between a 25kW and a 30kW boiler depends on your home's size and hot water needs. A 25kW boiler is typically suitable for smaller homes with 1–2 bathrooms and up to 15 radiators, while a 30kW boiler is better for larger homes with 2–3 bathrooms and 15–20 radiators. If you have high hot water demand or plan future expansions, a 30kW boiler might be more appropriate. It's best to consult a heating engineer for personalized advice.

How many kW boilers do I need for a 4-bed house?

For a typical 4-bedroom house, you would generally need a boiler with a capacity of 24kW to 30kW. The exact size depends on factors such as the number of bathrooms, the level of insulation, and your hot water demand. If the house has multiple bathrooms or higher hot water needs, opting for a 30kW boiler might be more suitable. Consulting with a heating engineer can help determine the ideal boiler size for your specific requirements.

What happens if a boiler is oversized?

If a boiler is oversized, it can lead to multiple issues. Some of the issues include: 

  • Inefficiency
  • Increased Wear and Tear
  • Inconsistent Heating
  • Increased Costs

It's important to choose the right boiler size for max performance and efficiency in your building or home.

Does a bigger boiler increase water pressure?

No, a bigger boiler does not increase water pressure. The boiler's size or capacity is related to its ability to heat water, not to create or increase water pressure. Water pressure in a home is determined by the incoming mains supply and any pressure-boosting systems in place, such as a pressure pump. If you have low water pressure, a bigger boiler won’t resolve the issue. You might need to look into pressure-boosting solutions or check for issues in your plumbing system.

Get oil boiler services from Rix

At Rix, we provide full oil boiler servicing for homes, businesses, and farms in Yorkshire & Lincolnshire. Our fully qualified OFTEC engineers will be on-site to ensure that your boiler is maintained and safe. Keeping your boiler serviced and maintained is key to keep heat pumping through your building and ensure you don’t need to replace your boiler at a large cost. Explore our oil boiler services for homes, businesses, and farms.

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