We are regularly asked, ‘what size boiler is suitable for my home?’. The first thing to clarify is that you mean kW size and not the physical measurements of the boiler.
You would only really be interested in the physical size of a boiler if it was located in a kitchen cupboard and you wanted to make sure your new boiler would also fit in the same space. All WarmZilla boilers have the physical dimensions in the product description to ensure your new boiler will fit.
The answer to ‘what size kW boiler do I need?’ is a bit more complicated as it depends on a few different factors – but don’t worry, we will answer those questions for you and get you up to speed on the perfect boiler size for you.
It’s worth noting that if you complete the WarmZilla Survey, then you don’t need to know what size kW boiler you need because our clever system calculates your water usage and heating requirements and supplies you with a list of boilers suitable for you. If you have any question about the survey, please contact us.
You may be thinking about adding an extension or additional bath/shower and like some advice on the additional power your boiler would need to provide adequate heating.
The average lifespan of a modern combi boiler is around 10-15 years, depending on the brand, how often you get it serviced and the demands you place on it. It’s therefore really important to ensure you get the right size boiler for your home.
How is boiler size measured?
It’s time for the exciting stuff! Boiler size is measured in kilowatts (kW). A kilowatt is a unit that defines how much energy your boiler outputs in the form of heat.
As a general rule, the more heat and hot water you require for your home, the higher the kW boiler you will need. If you’re unsure what kW output means, we have a separate blog that covers this.
Should I oversize my boiler?
In the old days where home insulation was so inefficient that much of the boiler heat was lost and boilers had an energy efficiency of 70% or less, engineers would often advise oversizing the kW of your boiler to compensate.
Now that we’re in the cosy days of efficient insulation and A-rated boilers that operate on a 90% and above energy efficiency there is no need to oversize your boiler.
The only time you would oversize your boiler was if you were considering an extension, converting a room into an en-suite, adding another shower or adding additional radiators into your property. Basically, anything that would increase the hot water or heating demand from your boiler.
There really is no need to oversize your boiler anymore, you would just be wasting energy – bad for your bank account and the environment.
Sizing for hot water
One of the considerations for the kW output of your boiler is the amount of hot water it will need to output. The hot water flow rate of a combi boiler is expressed as ‘litres per minute’ (LPM), this tells you how quickly a combi boiler can heat hot water. Litres per minute vary from 9LPM to 25LPM flow rate.
Most homes in the UK have a combi boiler installed as they offer the most convenience in terms of being able to heat your hot water and your home without the need for a separate storage tank in your loft to heat water.
Households with high water demand (running more than one bath and shower simultaneously), may require a conventional or system boiler, which uses a tank to heat water in readiness for when you want to use it.
The higher the flow rate you require to heat water and fill baths more quickly, the higher the output combi-boiler you will need. System and conventional boilers are more efficient for larger homes but they are more expensive to install and require the storage space for a separate hot water storage tank.
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Number of radiators
Another factor on the ideal kW boiler would be for your home is the number of radiators that the boiler has to provide hot water for. The more radiators the harder the boiler has to work to keep them all at your desired temperature.
A small flat with only a few radiators will only require a relatively low kW boiler (12-24 kW), whereas an average 3-4 bedroom house with around ten radiators would require a medium size combi (24-30kW). If you have a large house with as many as twenty radiators, a large combi boiler (35-42 kW) may be needed to provide sufficient heat. With a house of this size however, it would be more likely that you would have a system or conventional boiler in place.
Insulation
Insulation plays a factor in your kW boiler requirements. A poorly insulated property suffers from poor heat retention and your boiler needs to work harder to keep the house at the same temperature.
Improving your cavity/loft insulation or upgrading your windows from single to double glazed will have an impact on heat retention. You can also buy thermal leak detectors from a reasonable price on Amazon to identify areas of your home where heat is escaping.
New builds are more efficient in terms of insulation, however, new build homes by law have to have heat pumps installed rather than gas boilers. Heat pumps aren’t effective with anything but the most modern insulation and therefore aren’t suited for older homes.
We have created a table to help you roughly gauge the size (kW) boiler you would require for your home. As previously mentioned, if you take the WarmZilla Survey, you won’t need to know this information as our system calculates the best kW boiler for your needs. You can also use our Boiler Comparison tool to compare three boiler models or brands at one time, so that you can compare the kW outputs and efficiency to help find the perfect boiler or your home.
Combi Boiler kW Size Guide
| No of bedrooms and bathrooms | Poorly insulated property | Moderately insulated property | Well insulated property |
|---|---|---|---|
| 1 bed + 1 bath | 20kW | 20kW | 18kW |
| 2 bed + 1 bath | 20kW | 20kW | 18kW |
| 2 bed + 2 bath | 24kW | 20kW | 20kW |
| 3 bed + 1 bath | 28kW | 26kW | 24kW |
| 3 bed + 2 bath | 28kW | 28kW | 24kW |
| 4 bed + 1 bath | 30kW | 30kW | 28kW |
| 4 bed + 2 bath | 32kW | 32kW | 30kW |
| 5 bed + 2 bath | 40kW | 35kW | 30kW |
System Boiler kW Size Guide
| No of bedrooms and bathrooms | Poorly insulated property | Moderately insulated property | Well insulated property |
|---|---|---|---|
| 1 bed | 8kW | 7kW | 5kW |
| 2 bed | 8kW | 7kW | 6kW |
| 3 bed | 10kW | 9kW | 6kW |
| 4 bed | 12kW | 10kW | 7kW |
| 5 bed | 15kW | 12kW | 9kW |
| 6+ bed | 18kW | 15kW | 12kW |
Regular Boiler kW Size Guide
| No of bedrooms and bathrooms | Poorly insulated property | Moderately insulated property | Well insulated property |
|---|---|---|---|
| 1 bed | 7kW | 6kW | 4kW |
| 2 bed | 8kW | 7kW | 5kW |
| 3 bed | 9kW | 8kW | 6kW |
| 4 bed | 10kW | 9kW | 7kW |
| 5 bed | 14kW | 10kW | 9kW |
| 6+ bed | 16kW | 14kW | 10kW |
Other factors
There are a few other factors which can have a bearing on the kW boiler that would be best suited for your home:
- Fuel Type
Currently, the most efficient boiler fuel type for U.K homes is gas but many homes are off the national gas grid and use LPG or oil boilers. Less efficient fuel types require a more powerful kW boiler. - Water Pressure
Different areas can have different water pressure levels and older homes can have lower water pressure than newer homes. If your home has low water pressure, then you will require a higher kW boiler with a good flow rate to ensure you don’t suffer from a drop in pressure when multiple taps are running at the same time. - Alternative Energy
If your boiler is connected to, or you plan to connect it to alternative energy sources such as solar power in the future then you won’t require as powerful a kW boiler as it won’t have to work as hard to heat your home.
Most modern boilers are built to be compatible with alternative energy sources. If you aren’t sure then check with the boiler manufacturer you’re looking at. - Maintenance
Like any piece of machinery, the better condition it’s in, the better it will perform. Giving your boiler an annual service is essential in making sure it keeps running smoothly in much the same way as a car.
We hope that this guide has given you more of an idea about what boiler size means and what size would be best for your home. There are a lot of factors involved but you can take the easy choice and take the WarmZilla Survey or use our Boiler Comparison tool, and we’ll tell you what size boiler would be the most suited for your home.
If you have any questions, as ever, please don’t hesitate to get in touch.
FAQs
Boiler size is measured in kilowatts (kW). A kilowatt is a unit that defines how much energy your boiler outputs in the form of heat.
The only time you would oversize your boiler was if you were considering an extension, converting a room into an en-suite, adding another shower or adding additional radiators into your property. Basically, anything that would increase the hot water or heating demand from your boiler.
When it comes to choosing the correct size boiler for your home, there are a few things that you’ll need to consider including;
- Total number of radiators in your home
- Total number of bedrooms
- Total number of bathrooms
- Number of people living in the property
You may also want to consider other factors like; fuel type, water pressure, alternative energy and maintenance.
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