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What size boiler do I need?

We’ll give you the information you need to pick the right boiler size for your home.

Choosing a boiler size.

Why is it important to choose the right size boiler?

It can be tough choosing the right boiler size for your home with so many options and types of boilers available. It depends on a number of factors including:

  • How much hot water you need.

  • Number of rooms with radiators.

  • How well insulated your home is.

  • The type of fuel in your home.

Choosing the size of your boiler doesn’t mean its physical dimensions, but the output in kilowatts (kW). The kW output is how much energy your gas boiler can produce in the form of heat.

If you choose the wrong size of boiler then you may struggle to heat your home/not have enough hot water when you need it, or waste hundreds of pounds by burning extra gas you don’t need.

What are the different types of boilers?

Combi boiler

Combi boilers.

Great for smaller homes as they can fit in a cupboard and you don’t need a hot water cylinder or cold water storage tank, so they don't take up any loft space either.

More on combi boilers.

Regular boiler

Regular boilers.

If you often use hot water in multiple places at the same time, consider a regular or condensing boiler, linked to a hot water cylinder and a cold water storage tank.

More on regular boilers.

System boiler

System boilers.

Designed for homes with more than one bathroom, system boilers give you a constant supply of hot water to multiple taps. There’s no need for the tank in the loft either.

More on system boilers.

How to calculate the boiler size you need?

How many radiators do you have?

A helpful way of deciding what boiler size you need is by counting the number of radiators in your home. As a boiler size rule of thumb, the more radiators you have, the larger gas boiler you’ll need to heat them up.

Combi boilers need more power than regular or system boilers because they heat up hot water instantly, however they only have to fire up a few minutes a day versus a system boiler which has to heat up a whole cylinder.

As an estimate, if you have a combi boiler, you’ll need the following power outputs:

  • Up to 10 radiators: 24-27kW.

  • 10-15 radiators: 28-34kW.

  • 15-20 radiators - 32-40kW.

If you have a system or regular boiler, then we estimate you’ll need:

  • Up to 10 radiators: 12-15kW .

  • 10-15 radiators: 18-24kW .

  • 15-20 radiators - 30-40kW.

How many bathrooms do you have?

Depending on the size of your home you may have more than one bathroom, and if you’ve got multiple people taking showers at the same time then you’ll need the right size boiler for your home.

It’s generally recommended that if you have multiple bathrooms you’ll need a regular or system boiler installed, as a combi boiler may not be able to produce enough hot water. If you have multiple bathrooms you’ll need a boiler size around 35-42kW, whereas for one bathroom you’ll need between 10 and 25kW.

Does boiler efficiency matter?

In short, yes. Boiler efficiency (A-G) determines how much of the energy that is used in your heating system is creating hot water and how much is wasted. If you have a condensing boiler then its energy efficiency rating will normally be A, around 90%, whereas a non-condensing boiler could be much less cost-effective.

If you have an old boiler then it’s worth checking its efficiency rating to see if you need to switch to a new boiler as you may be wasting money on a boiler that is oversized for what you need.

Alongside an efficient boiler, you may want to bleed your radiators to help make the most of your heating system.

Get more advice on boiler efficiency.

How does the size of my property affect the boiler size I need?

Yes it does. For example if you have a four-bedroom home then you’ll have more radiators than a one bedroom flat. Also, if you live in a mid-terrace house then you may need less energy to heat your home than a detached house.

Get warm and cosy and save up to £540 on your annual gas bill^.

Plus if you’re an E.ON Next customer save £150 when you buy a boiler through E.ON. T&Cs apply.

Get a new boiler

Why switch your old boiler for a Worcester Bosch boiler?

They’re fast and economical to run.

Worcester Bosch boilers are A-rated for efficiency, guaranteed to be 94% efficient, helping you to reduce your energy bills.

Smart connected thermostats.

Control your central heating and hot water with the wireless Bosch EasyControl, plus get easy-to-understand graphs on gas consumption and how efficient your heating programmes are.

Awarded Best Buy by Which?

Our new Worcester Bosch boilers build on the award-winning Greenstar range, offering smarter and more powerful condensing boilers with advanced technology to maintain water pressure.

Find out more about our Worcester Bosch Greenstar boilers.

Can I downsize my boiler if I have made energy-efficient upgrades to my property?

Yes, it’s possible to downsize your boiler if you have made energy efficient upgrades to your property. Energy efficient upgrades include:

  • Improved insulation.

  • Double-glazed windows

  • Installation of more efficient heating systems.

These changes can reduce the heat loss and overall energy demand of your property. As a result, you may not require the same size of boiler as before.

When considering downsizing your boiler, it’s important to consult with a heating engineer or a qualified professional. They can assess the energy efficiency improvements you have made and calculate the new heat requirements for your property. They will consider factors such:

  • The insulation levels.

  • The size of the property.

  • The specific heating and hot water demands.

By properly evaluating these factors, the heating engineer can determine the appropriate boiler size that matches your reduced energy needs. Downsizing your boiler can be beneficial as it can lead to:

  • Improved energy efficiency.

  • Lower energy bills.

  • Potentially lower installation and maintenance costs.

However, it's crucial to ensure that the downsizing is done correctly to avoid issues such as inadequate heating capacity or hot water supply. A professional assessment will help you make an informed decision and ensure that your heating system is appropriately sized for the upgraded energy efficiency of your property.

How does hot water demand affect boiler size?

The relationship between hot water demand and boiler size is important to consider when determining the appropriate efficient boiler for your needs. The boiler size must be able to meet the hot water requirements of your household efficiently and effectively. Here's how hot water demand relates to boiler size:

Number of bathrooms and showers:

The number of bathrooms and showers in your house significantly affects hot water demand. More bathrooms and showers generally mean a higher hot water demand.

Flow rate:

The flow rate is the amount of hot water that the boiler can deliver per minute. It is typically measured in liters per minute (l/min) or gallons per minute (gpm).

Higher hot water demand requires a boiler with a larger flow rate to ensure an adequate supply of hot water to multiple outlets simultaneously.

Storage or combi boiler:

The type of boiler you choose also influences hot water demand. If you opt for a storage (system or regular) boiler, it will heat and store hot water in a cylinder, providing a reservoir of hot water.

Combi (combination) boilers, on the other hand, heat water on demand and do not require a separate cylinder.

Combi boilers are suitable for properties with lower hot water demands, while storage boilers are preferable for larger hot water requirements.

Boiler output:

The boiler's output capacity, usually measured in kilowatts (kW), determines the heating power available for both space heating and hot water production. Hot water demand should be factored into the boiler output sizing. A portion of the boiler's output is allocated to meet the hot water demand, while the rest is utilized for space heating.

Hot water demand calculations:

We can help calculate the hot water demand for your specific requirements. We'll consider factors such as the number of occupants, usage patterns, and simultaneous hot water usage to determine the appropriate boiler size.

By understanding the relationship between hot water demand and boiler size, you can ensure that your chosen boiler can adequately supply hot water to meet your household's needs without running into issues of insufficient hot water supply or excessive energy consumption.

^An average household could save up to £540 on their annual gas bill. Savings information for boilers in the UK provided by the Energy Saving Trust, based on a detached house with a boiler rating of G.