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Last updated: 20th February, 2024


What Size Combi Boiler Do I Need?

What Size Combi Boiler Do I Need?

Find out whether the size of your combi boiler will make any difference to its capabilities.

How is a boiler sized?

Contrary to popular belief, size does make a difference...... When it comes to your boiler.

When we talk about boiler size, we’re often referring to the output in kilowatts (kW) as opposed to the physical dimensions of the boiler itself.

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Kilowatts are the measurement of energy that is output by the boiler, in the form of heat. We know it can be confusing when it comes to buying a new boiler, which is why we've not only provided our best combi boilers of 2022, but will also explain how to determine which size boiler you need in the article below.

Generally the bigger the home the bigger kW combi boiler you will need.

You can always try our boiler kw size tool which askes a few questions about your home to give you an approximate result to the kw needed for your home.

Which size boiler is right for my home?

Boiler Sizes

The right kW boiler for your home depends on a number of factors, including your demand for hot water, the number of bedrooms, bathrooms and radiators. In general, the higher your demand for heat and hot water, the higher the output you will need.


Small single flat home

If you live alone or have a small family and live in a flat or bungalow with one to two bedroom(s), one bathroom and up to ten radiators then a boiler with an output between 24-29kW is ideal for your home.

Average sized family

If you have an average sized family and live in a medium terrace, bungalow or semi detached house with three to four bedrooms, one to two bathrooms and up to fifteen radiators, a boiler with an output between 30-35kW is ideal for your home.

Larger family home

If you have a large family with a high demand for hot water and live in a large semi-detached or detached house with over four bedrooms, over two bathrooms and up to twenty radiators, a boiler with an output of 35-43kW is generally ideal for your home, or you may require a much larger system boiler if your family needs lots of hot water at any given time.

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Count your radiators

One way of identifying the right boiler size for you and your home is by counting the number of radiators you have. The more radiators you have, the bigger the boiler you’ll need due to the higher amount of power that is required to heat the entire home. Generally, you can expect the following number of radiators to require the following power outputs:

  • Up to 10 radiators: 24–27 kW

  • 10 to 15 radiators: 28–34 kW

  • 5 to 20 radiators: 32–40 kW

  • Up to 10 radiators: 12–15 kW

  • 10 to 15 radiators: 18–24 kW

  • 15 to 20 radiators: 30–40 kW

Other factors

Another factor that plays a part in deciding what size of boiler you require is how well insulated your home is. If you don’t have cavity wall insulation, loft insulation, double/triple glazed windows and decent sealing around doors and windows, then you could be losing a lot of heat from your home and therefore require a higher output if you are not intending to make those changes.

The type of fuel used to heat your home can also affect the efficiency of your boiler. Gas boilers offer the greatest levels of efficiency and are the most common type of boiler in the UK, whilst oil or LFG (liquified petroleum gas) are less popular and less efficient.

Another major factor when it comes to deciding which boiler is best for you is the price. None of us want to overspend! For more information on the cost of a new boiler, read our advice to ensure you are as informed as possible.

Should I oversize to be on the safe side?

Unfortunately, a boiler is not like a school jumper that you can grow into. Oversizing on your boiler could lead to wasted energy and is likely to make you pay more than you need for your fuel bills.

Leave it to the experts

To find out which boiler size is best for you, get a bespoke, fixed boiler quote here with installation available as soon as the next day. If you'd prefer to speak to one of our friendly team over the phone please don't hesitate to give us a call on 0333 305 6880.

Whichever boiler size you choose, keep in mind that its efficiency and operation will be at their best if your boiler is serviced annually by one of iHeat’s Gas Safe registered engineers.

What Size Combi Boiler Do I Need? Understanding the Ideal Boiler Size for Your Home

Choosing the right size combi boiler is essential for efficient heating and hot water supply in your home. The ideal boiler size, measured in kilowatts (kW), varies based on your home's size, hot water demands, and other factors like insulation quality and usage patterns. Here, we will explore suitable boiler sizes for different properties, featuring models from top brands like Vaillant, Viessmann, Alpha, Ideal, and Worcester Bosch.

Small Single Flat Home

In smaller homes, such as flats or bungalows with one to two bedrooms, one bathroom, and up to ten radiators, a combi boiler with an output of 24-29kW is usually sufficient.

Vaillant ecoTEC Pro

The ecoTEC Pro 24kW and 28kW models are ideal for small spaces, offering compact design and high efficiency. Their user-friendly controls and quiet operation make them a top choice for smaller homes.

Alpha E-Tec

The Alpha E-Tec 25kW and 28CkW models are excellent for budget-conscious homeowners, providing reliable and efficient heating.

Ideal Logic+ Combi

With the Logic+ kW24 and kW30, you get a blend of performance and affordability. These models are popular due to their ease of use and straightforward installation.

These boilers are designed for smaller spaces, ensuring efficient heating and hot water without the extra cost and energy usage associated with larger units.

Average Sized Family Home

Medium-sized properties, like terraced, bungalow, or semi-detached houses with three to four bedrooms, one to two bathrooms, and up to fifteen radiators, typically need a boiler with an output of 30-35kW.

Vaillant ecoTEC Plus

The ecoTEC Plus 30 and 35 models are well-suited for average-sized homes, offering excellent efficiency and quiet operation.

Viessmann Vitodens 050-W

Known for their compact size and efficient performance, the Vitodens 050-W 30 kW and 35 kW models are ideal for medium-sized homes.

Worcester Bosch Greenstar i

The Greenstar 30i and 35i models are reliable and efficient, making them a great choice for an average family home.

These boilers can handle the increased demands for hot water and heating in medium-sized homes, maintaining comfort and efficiency.

Larger Family Home

Larger homes, such as large semi-detached or detached houses with over four bedrooms, more than two bathrooms, and up to twenty radiators, generally require a boiler with an output of 35-43kW.

Viessmann Vitodens 100-W

The Vitodens 100-W 35 kW, with its high efficiency and wide modulation range, is a strong contender for larger homes.

Worcester Bosch Greenstar 8000 Style

Suitable for larger properties, the Greenstar 8000 Style in the 35-40 kW range combines high performance with a modern design.

Ideal Vogue Max Combi

The Vogue Max Combi in 40 kW is excellent for homes with extensive hot water needs, featuring a premium build and advanced technology.

For homes with exceptionally high hot water demand, a system boiler with a separate hot water cylinder might be more suitable, providing a consistent supply of hot water for multiple outlets.

Clarifying Technical Terms

  • Kilowatts (kW): This unit measures the energy output of the boiler. A higher kW rating means the boiler can heat more radiators and provide more hot water.

  • Modulation Range: This refers to the boiler's ability to adjust its output based on demand, enhancing efficiency.

Smart Home Compatibility and Environmental Impact

Many modern boilers, including some models from Viessmann and Worcester Bosch, are compatible with smart home systems, offering improved control and energy management. Additionally, these boilers are designed with environmental considerations in mind, featuring reduced emissions and energy-saving features.

Common Misconceptions and Maintenance Tips

A common misconception is that a larger boiler always provides better heating. However, an oversized boiler can lead to wasted energy and increased costs. Regular maintenance, such as annual servicing and checking for any visible issues, can keep your boiler running efficiently.

Considerations for Unusual Layouts or High Demand

In homes with unique layouts or high hot water demand, system boilers with separate hot water cylinders can offer a more efficient solution. They provide a steady hot water supply, especially beneficial when multiple outlets are used simultaneously.

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Frequently Asked Questions

How Do I Know What Size Combi Boiler I Need?

The size of the combi boiler you need depends on various factors such as the size of your home, the number of bathrooms, and the number of radiators. Generally, larger homes with more radiators and bathrooms require boilers with higher kW output for efficient heating and hot water supply.

How Many Radiators Can a 30kW Combi Boiler Run?

A 30kW combi boiler is typically suitable for medium-sized homes and can efficiently run up to 15 radiators. This capacity makes it ideal for properties with three to four bedrooms and a couple of bathrooms.

Is a 24kW Combi Boiler Big Enough?

A 24kW combi boiler is often sufficient for smaller homes, such as flats or bungalows, with one to two bedrooms, one bathroom, and up to ten radiators. It's designed to meet the heating and hot water demands of smaller spaces effectively.

Should I Oversize My Combi Boiler?

Oversizing a combi boiler is not generally recommended as it can lead to energy inefficiency and increased costs. It's important to choose a boiler size that matches your home's heating and hot water requirements.

How Do I Know if My Boiler is Too Big?

Signs that your boiler may be too big include frequent on-off cycling, which can lead to inefficiency and increased wear and tear. A properly sized boiler will run efficiently without frequently turning on and off.

What is the Problem with Oversizing a Boiler?

Oversizing a boiler can result in wasted energy, higher operational costs, and increased wear on the boiler components due to frequent cycling. It can also lead to uneven heating and less comfort in your home.

Will an Oversized Boiler Cost More to Run?

Yes, an oversized boiler can cost more to run. It tends to cycle on and off more frequently, which is less efficient and consumes more energy, leading to higher heating bills.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.