What Size Combi Boiler Do I Need?
A combi boiler is an effective way to heat your home and provide hot water to taps, all from a single compact system. One of the most common enquiries we receive from property owners is ‘What size combi boiler do I need?’ This is because choosing the wrong size can result in expensive electricity bills or inefficient heating.
The ideal output depends on several factors, including property size, insulation levels, the number of bathrooms and how much hot water is used at peak times. In this helpful guide, we’ll go over the most important aspects to consider so you can choose the right size boiler for your needs.
What Does Combi Boiler Size Mean?
A combi boiler’s size often refers to the boiler’s input and output power, rather than its physical dimensions. The power of a combi boiler is measured in kilowatts. The kilowatt rating is what indicates how much heat the boiler can produce for space heating and hot water. While the physical size of modern combi boilers is similar across many models, the output can vary significantly. Two boilers of the same physical size can perform very differently in a home.
This is why sizing a combi boiler is so important. An incorrect output can have real consequences for both comfort and efficiency. A boiler with too small a capacity may struggle to meet heating demand during colder months and can fail to deliver enough hot water when multiple taps are in use.
Equally, an oversized boiler with a higher output than necessary can be inefficient, using more energy than required for everyday heating and hot water, which can increase running costs and place unnecessary strain on the system.
What Factors Will Determine the Right Combi Boiler Size?
There are many factors to consider when choosing the right combi boiler size. Important aspects to consider include the size of the home, the number of radiators, ceiling heights and how well the building is insulated. Larger properties or homes with poor insulation typically require a higher output to maintain comfortable indoor temperatures, particularly during colder weather. Hot water demand is also important, especially in homes where multiple outlets are used simultaneously.
Future proofing should also be considered. Planned extensions, loft conversions, additional bathrooms or an increase in household occupancy can all raise heating and hot water requirements. Selecting a boiler that can accommodate foreseeable changes helps avoid the need to upgrade.
Combi Boiler Size Guide
Many property owners ask us ‘what size boiler do I need?’ While every property should ideally be assessed, the guidance below provides a useful starting point based on typical home and combi boiler sizes, radiator numbers and hot water demand.
What size combi boiler for a 2 to 3 bed house?
A small to medium home, such as a flat or a 2 to 3 bedroom house with one bathroom and up to ten radiators, will usually suit a combi boiler in the 24 to 30 kW range.
What size boiler for a 3 bed house?
For a typical 3 bedroom house with one bathroom and up to twelve radiators, a 28 to 32 kW combi boiler is often recommended.
What size boiler for a 4 bed house?
When considering what size boiler for a 4 bed house, hot water demand becomes more critical. Properties with two bathrooms and higher simultaneous usage typically require a 30 to 35 kW combi boiler to maintain performance without pressure drops.
What size boiler for a 5 bed house?
For larger homes, those asking what size boiler for a 5 bed house should be aware that combi boilers may be stretched. Outputs of 36 to 50 kW can work in most cases, but it’s important to consult a professional, as you may benefit from alternative heating sources.
How to Work Out Your Combi Boiler Size
Boiler sizing is about making sure the boiler can produce enough heat to warm your radiators and hot water supply. Each radiator in your home needs a certain amount of heat to warm up a room. By assigning a kW value to each radiator size and adding them together, you get a general idea of what kW boiler is needed for your home’s overall needs. For example, if you have 10 averaged sized radiators, the boiler needs around 15 kW to heat the home. With a hot water tank added, the total increases to 18 kW, however a proper assessment will always give the most accurate result.
Why is Oversizing a Bad Idea?
Bigger is not always better when it comes to combi boiler sizes. An oversized boiler can heat your home, but it often does so inefficiently, using more energy than necessary for everyday heating and hot water. This can lead to higher energy bills, increased wear on components and a shorter system lifespan for your boiler.
When is a Combi Boiler Not Enough?
A combi boiler may not operate efficiently in larger properties or homes with high hot water demand. In households with multiple bathrooms, simultaneous use of showers and taps can stretch the combi boiler’s ability to deliver efficient heating. Even if a boiler’s output is high, it may not be able to reliably provide consistent flow and temperature at peak times.
Alternative systems, such as a heat only boiler, may be more suitable for efficient heating. Those asking ‘what is a heat only boiler’, it’s a traditional system that works with a hot water cylinder and storage to allow hot water to be stored and supplied to several outlets at once.
For those considering low-carbon heating options, an air source heat pump may also be worth exploring. These systems work best in well-insulated homes and are increasingly used in new builds.
What Size Combi Boilers are Available with Intergas?
24 kW Combi Boilers — suitable for smaller homes and flats with 1-2 bedrooms, light hot water use and fewer radiators. Models like the Combi Compact HRE 24/18 are ideal for such properties.
25 kW Combi Boilers — ideal for 2 bed homes, providing reliable heating and hot water for small homes.
30 kW Combi Boilers — good match for average 3 bedroom properties, balancing heating and hot water needs. In this instance the Combi Compact Eco RF 30 is recommended, as it delivers dependable heating and hot water performance.
32 kW Combi Boilers — best for larger 3 bed or smaller 4 bedroom homes with high hot water demands.
36 kW Combi Boilers — designed for larger properties with multiple bathrooms and bedrooms for greater heating and hot water capacity. The Combi Compact HRE 36/30 is a well-balanced option for larger homes with more demand for hot water and heating.
Hybrid Combi Boilers — best suited to medium and larger homes, typically 3 to 5 bedrooms. The Xtend split Hybrid Combi Boiler can help homeowners who require reliable heating but also want to lower carbon emissions.
FAQs about Combi Boiler Sizes
How do I calculate boiler size?
One way to calculate boiler size is by looking at the number and size of radiators in your home. As a simple guide, allow around 1 to 2 kW per radiator depending on size, then add extra capacity for bathrooms or hot water demand. This provides a general starting point.
Is it better to oversize or undersize a combi boiler?
Neither option is ideal. An undersized combo boiler will struggle to meet heating and hot water demands. An oversized combi boiler will at least be able to heat your home, but excessive oversizing reduces efficiency and increases running costs.
What’s the difference between a 25 kW and a 30 kW boiler?
The main difference between a 25 kW and a 30 kW boiler is output. A 30 kW boiler can provide higher heating and hot water demand, making it ideal for larger homes and higher usage. A 25 kW boiler is typically used for smaller properties.
Are there any alternatives to a combi boiler?
Yes. There are a few alternatives to combi boilers. These options are worth considering if you have a large property or a high demand for hot water. For larger homes or properties with multiple bathrooms, a heat-only boiler with a hot water cylinder can deliver more consistent hot water to several outlets simultaneously.
Property owners looking to reduce carbon emissions should consider hybrid air source heat pumps or standard air source HPs. An air source HP works best in well-insulated homes and new builds, while hybrid air source heat pumps combine a heat pump with a boiler, offering flexibility, efficiency and lower emissions.
Should I consider a hybrid heat pump instead of a combi boiler?
Consider all your options, whether that’s assessing what size combi boiler you need, deciding between heat pumps vs gas boilers or hybrid heat pumps. Before making any decision, consider your home’s heating needs, hot water demands and the overall size of your property. We recommend reading our blog posts on ‘what are hybrid heat pumps?’ and ‘how much do hybrid air source HP cost?’ so you get a clear idea.
