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One of the most popular questions we get with all the choices available nowadays, how do we know what boilers are suitable? We are going to simplify and make sense of what you already know and need to know.

The Boiler You Already Have

By size, we are not referring to the physical dimensions of the boiler but to the heat output (in kW).  If you're just having a replacement boiler and you're happy with how warm your house is and how much hot water you get (combi boiler) then choosing the same output is perfect. However, if your boiler struggles to heat your home or water, you may need a bigger one.  The boiler model will tell you what size you have i.e. 24kw, 30kw, 34kw etc and there's usually a label found on the front or on the underside of the boiler, if you have the manufacturer's manual it's in there too.

Should I Choose An Oversized Boiler?

Whilst it sounds tempting to pay a little extra for a bigger boiler thinking it may heat your home faster or compensate for any heat loss, modern boilers these days are built so efficiently that it is no longer recommended. Heating more water than you actually need to fill your radiators or to run for your household will likely just end up costing you more than you need to pay.

It's worth bearing in mind for central heating systems that if the room feels cold while the radiator is hot all over, a bigger boiler won't make any difference as the radiator is probably undersized.

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Combi Boilers

A combi is the type of boiler with the most choice from every manufacturer. Let's keep it simple the size is mainly to do with the amount of hot water you receive from your taps and shower. The combi heats the water and sends it directly to the taps (on-demand) or to the radiators when required. The more hot water you need the bigger the output required (kW).

• 24 to 30 kW is suitable for 1 bathroom and up to 10 average-sized radiators
• 30 to 34 kW is suitable for 1 bathroom and 1 en-suite and up to 15 average-sized radiators
• 34 to 42 kW is suitable for 2 bathrooms and up to 20 average-sized radiators*

*For more than 2 bathrooms it's recommended that a system boiler and unvented cylinder is fitted. From time to time we are asked to fit a 42 kW combi for 2 bathrooms or more but a really good cold water mains pressure is needed. If every bathroom is in demand at the same time the water flow to each bathroom will suffer. The maximum amount of hot water that you can receive from a combi is around 16 litres a minute, typical mixer showers use around 6.

Standard/System Boilers

If you have a cylinder and/or a cold water tank in the loft then you will have either a standard or a system boiler and the size you will need will depend on how much hot water your house needs from the cylinder. Generally, this is how many baths and showers your property has and the boiler needs to provide for. Standard and System boilers are also known as 'heat-only' boilers as they only heat the water and send it to the cylinder, which in turn supplies the radiators, taps and showers. So what size boiler do i need.

• 15 kW is suitable for 1 bathroom and up to 10 average-sized radiators
• 18 to 24 kW is suitable for 1 bathroom and 1 en-suite and up to 15 average-sized radiators
• 24 to 30 kW is suitable for 2 bathrooms and up to 20 average-sized radiators
• If you have a larger house with 3 or more bathrooms or more than 20 radiators please get in touch as we can still help with what size boiler do i need.