What size boiler do I need banner image

The size of the boiler you require is one of the first things you will need to decide on before a new boiler installation can take place in your home. 

In order to select the right size boiler, there are a few questions you should answer:

The more bedrooms your property has, the higher your heating demands likely will be 

This will have an impact on the boiler size you choose as if you only have a few radiators and choose a higher kW output boiler, you could end up with higher energy bills than necessary. Whilst you should count the number of radiators in your property, you should also consider whether you are likely to install additional radiators in the future too

You should also consider the hot water demands of your household and whether multiple showers and hot taps will be being used at the same time. If they will be, this will determine not only the size of your boiler that should be installed but also the type of boiler too. For instance, if you have a large family who are often using multiple showers and hot taps at the same time then a combi boiler might not be the most suitable boiler for your needs

Is it a combi boiler, conventional boiler, or a system boiler?

E.g. by converting a conventional boiler system to a combi boiler system. If you are looking to do this, it is important to make sure your current pipework is good enough

Was it built pre-1970s or is it a modern construction? If your home is built to older building regulations, it likely won’t be as insulated as modern properties and will have greater levels of heat loss

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Check your water flow rate

Checking your water flow rate

In addition to the above, it is worth also checking your water flow rate. To do this, you need to:

  • Find an unrestricted mains supply such as an outside garden tap
  • Get a 5-litre bucket and time how long it takes to fill up 

If it fills up in 15 seconds, you’ve got a 20L/minute flow rate. This is a suitable flow rate for system and combi boilers.

What does boiler size mean?

Boiler size refers to the output of a boiler not the physical dimensions of the boiler unit. This output is measured in kW (Kilowatts) and plays a key role in determining which boiler is right for the needs of your property and household. kW is used to measure how much energy the boiler outputs as heat. As a rule of thumb, the bigger your property and the higher your hot water demands, the larger the kW output boiler you will require. 

No type or size of boiler is automatically superior to another, but choosing the correct one is still important. For all modern condensing boilers, the boiler size is ‘modulated’ in KW depending on the load, allowing superior efficiency.

What does boiler modulation mean?

Boiler modulation refers to the boiler’s process of changing its output to match the heating load. For example, if your demand for hot water ever changes (say you’re a landlord with a new tenant moving in) or you have an additional radiator installed, your boiler is capable of altering its output to match the new heating load. In doing this, it can help to improve the lifespan and overall quality of its own internal components.

Choosing the right size boiler for your property

Combi boiler sizing

The table below gives you an initial overview of where to start when it comes to sizing a combi boiler. 

NUMBER OF BATHROOMSNUMBER OF RADIATORSRECOMMENDED BOILER SIZE
1Up to 10Up to 29kW
1-2Up to 15Up to 35kW
2+Up to 2035kW+

What size boiler do I need for a 1 bedroom flat

What size boiler do I need for a 1 bedroom property?

On average, a 1 bedroom house or flat will have 6 radiators and 1 bathroom. For a property of this size, we recommend a 24-26kW combi boiler due to the low heating and hot water requirements that the property will likely have.  

Our recommended boilers for a 1 bedroom property

BRANDMODELOUTPUT (kW)FUEL
ViessmannVitodens 100-W Combi26Gas/LPG
Worcester BoschGreenstar 25i Combi24Gas/LPG
VaillantecoTEC pro Combi24Gas/LPG
Looking for a boiler for a 1 bedroom property?

What size boiler do i need for a 2 bedroom house?

What size boiler do I need for a 2 bedroom property?

On average, a 2 bedroom house or flat will have 7 radiators and 1 bathroom. For a 2 bedroom property, we recommend a 24-29kW combi boiler. If you have a 2 bedroom house with a conventional boiler or a system boiler, it may be worth converting this to a combi boiler as not only will this save you space in your property, it will also save you money on your energy bills. This is because combi boilers only heat water when it is needed unlike boiler systems with a separate hot water cylinder.

Our recommended boilers for a 2 bedroom property

BRANDMODELOUTPUT (kW)FUEL
AristonClas ONE Gas Combi Boiler24Gas
NavienLCB700 Blue Flame Oil28Oil
ViessmannVitodens 050-W Combi29Gas/LPG
Looking for a boiler for a 2 bedroom property?

What size boiler do I need for a 3-4 bedroom house?

What size boiler do I need for a 3-4 bedroom property?

A typical 3 bedroom property will have 8 radiators if it only has 1 bathroom. For a 3 bedroom property with 8 radiators and 1 bathroom, we recommend a 29-31kW boiler. 

Our recommended boilers for a 3-4 bedroom property

BRANDMODELOUTPUT (kW)FUEL
Worcester BoschGreenstar Highflow 440CDi Combi Boiler29Gas/LPG
ViessmannVitodens 050-W Combi29Gas/LPG
AristonClas ONE Gas Combi Boiler31Gas
ViessmannVitodens 100-W System Boiler19Gas/LPG

What size boiler do I need for a 3-4 bedroom property with multiple bathrooms?

If your 3 bedroom property has multiple bathrooms, additional rooms such as a utility room,  and more than 8 radiators, we recommend a higher capacity boiler with up to a 39kW output, in order to achieve the sufficient flow rates that homes with higher hot water demands require.

If you need something with higher flow rates, it will be worth looking at storage combi options.

Our recommended boilers for a 3-4 bedroom property with multiple bathrooms

BRANDMODELOUTPUT (kW)FUEL
ViessmannVitodens 111-W Storage Combi Boiler32Gas/LPG
ViessmannVitodens 050-W Combi Boiler35Gas/LPG
ViessmannVitodens 100-W Combi Boiler35Gas/LPG
VaillantecoTEC plus 938 Storage Combi Boiler38.7Gas/LPG
Viessmann Vitodens 100-W System Boiler19Gas/LPG
Looking for a boiler for a 3-4 bedroom property?

What size boiler do I need for a 4 bedroom house?

What size boiler do I need for a 4+ bedroom property?

If you have a large house with as many as 20 radiators, we recommend that you select a boiler with an output of 35-42kW, if choosing a combi boiler. However, if you have a property of this size, you are likely to have higher heating and hot water demands, for this reason, we would recommend choosing a system boiler instead.

Our recommended boilers for a 4+ bedroom property

BRANDMODELOUTPUT (kW)FUEL
ViessmannViessmann Vitodens 200-W35Gas/LPG
GrantVortexBlue36Oil
AristonClas ONE Gas Combi Boiler38Gas
Worcester BoschGreenstar 800040Gas
VaillantecoTEC plus 624 System Boiler24Gas
Viessmann Vitodens 100-W System Boiler25Gas/LPG
Looking for a boiler for a 4+ bedroom property?

The issues with choosing the wrong size boiler

Choosing the right size boiler will ensure that your home is warm enough and that you have sufficient levels of hot water for the demands of your household, without spending more on your energy bills than you have to. 

In the past, homeowners were advised to ‘oversize’ the boiler that would be installed in their home. That was to choose a higher kW output boiler to compensate for heat loss. However, this is no longer recommended. 

By choosing the wrong size boiler, you risk wasting energy and paying more on your energy bills than is necessary. It may also mean that it takes longer to heat up your home and your hot water cylinder if using a conventional boiler or a system boiler.

Helping you to choose the right size boiler

Don’t worry if you are still unsure about the right size boiler for the heating and hot water needs in your home, we are here to help you. At Boilerhut, our team comprises heating engineers with over 40 years of experience. Using the expertise gained over the decades, we can assist you by matching you with the exact boiler you require for your heating and hot water needs. What’s more, we have implemented this knowledge into our online boiler quote tool – simply answer a few short questions and our intelligent system will quote you on the exact size boiler for your requirements.

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