Installing a new boiler is a major decision for you as a homeowner and as a long term commitment, care should be taken to consider both your current and future needs. There are several aspects to think about including how much hot water you and your family can expect to use, the size of your property and the types of fuel supplies available, both now and in the future.

You will also want to consider other factors such as energy efficiency and options for renewable energy use. To help you decide which boiler is right for you, we’ve prepared this at a glance guide.

Choosing the right size of boiler

Hot water demand

One of the first considerations when deciding which boiler you need is how much hot water will you use or are likely to use in the future. For most households with a single bath and shower, a combi-boiler of between 24-30 kW is generally recommended. Where there is an additional en-suite bathroom, it may be advisable to choose a larger 30-35 kW boiler.

If, however, you have multiple bathrooms that are likely to be in use at the same time, a system boiler that stores hot water in a storage cylinder may be more suitable. 

Also consider if you have a young family as your needs will likely increase as children grow. A system boiler pre-heats and stores the hot water in a cylinder with the added benefit that its output can be sized on your homes heating demand, which is often much lower; therefore, a combi boiler of 30 kW can often be replaced by a system boiler of 19 kW. Your installer will be able to advise.

Size of property

As well as providing your hot water, your boiler will probably be the main source of heating for your property. This means that it is essential to choose a boiler that is large enough to provide sufficient heat output to supply your entire property. Domestic boiler size is measured in terms of heat output, which typically ranges from 5 to 35 kW. 

There is no economy in choosing a boiler capable of supplying fifteen radiators for a flat that has just a few. Bear in mind, however, that the size of your property alone is not the most effective way of identifying the correct boiler size - it is far better to calculate heat loss.

Heat loss

Heat loss for your property is a more advanced calculation that takes into account not just the floor area and number of radiators, but also the insulation and other factors which will affect the effectiveness of the heating system. An expert is able to calculate how much heat your house will lose on the coldest day of the year and use this information to calculate the optimum boiler size. 

Should I oversize

In the past, it was common practice to choose an oversized boiler that was as much as 30 % bigger than actually required to compensate for heat loss. However, with advances in technology this is no longer always necessary and will result in wasted energy and leave you paying more than you need to for your gas bill. 

As combi boilers are sized for their hot water performance, you should be aware that this means they are typically oversized when it comes to your home’s heating demand. 

Choosing the right type of boiler

Available Space

The amount of space you have available within your property will be one of the factors which affects the type of boiler you choose. Most smaller properties have very limited storage space, which may make it impossible to house a separate hot water tank. It is for this reason that combi-boilers tend to be the most popular choice in the UK. 

If you are considering upgrading an existing boiler to a larger model, also consider whether there is sufficient space for this to be accomodated in the existing location.

Existing system

It is important to think about whether or not you wish to keep the same system as that already existing in your home. Usually it is best to retain the same system from the perspective of cost and practicality, however there may be occasions when a change to another system may be better. This is more likely to be the case for older homes as newer builds will usually have the optimum system in place. Bear in mind that different systems will also require different amounts of space and may not always be suitable.

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Hot water

As mentioned in the previous section, if there is likely to be a demand for simultaneous hot water then a system boiler which includes a hot water cylinder may be more suitable than a combi-boiler, however this will depend on whether there is sufficient space available.

Conventional boilers

Conventional boilers use a storage tank, which holds gallons of water, to supply hot water to your home. Once the stored hot water runs out, you will need to wait for this to be refilled before it can be used again, meaning that you need to consider when hot water is required.

With a conventional boiler, more space is required within your home to accommodate it as the hot water cylinder will need to be connected to cold water storage tank in the loft.


Combination boilers, commonly known as combi boilers are the most popular boiler choice in the UK and work by providing on-demand hot water directly from the mains without having to be connected to a storage tank. As a combi boiler provides hot water at mains pressure, it allows you to use a shower without the need for an additional shower pump.

All of the components are contained within one unit and there is no need for a separate hot water tank, making them ideal for properties with limited space. For those homes with a little more space, there are a few boilers known as 'storage combis', that work like a combi boiler but have a smaller cylinder as part of the boiler. This means they can provide enough hot water for a bath but much quicker than a standard combi boiler.

System boilers

System boilers work by directly supplying hot water to the radiators in your home and a hot water storage cylinder. The majority of the major components of the heating system, such as the pump and expansion vessel, are contained within a single unit, making installation and servicing simpler.

System boilers are an ideal choice for properties with more than one bathroom or those who require more hot water than a combi boiler can provide.

Choosing the right fuel type

The types of fuel available for your location along with your attitude towards green energy sources will have an impact on your choice of heating system and boiler. In the past, it was often advised that when an existing system requires replacement, it is best to install the same type of boiler, however with the options now available this is not necessarily the case.

Mains gas

The most common fuel type in the UK is mains gas, with the majority of UK homes already connected. With a gas boiler there is a constant supply and as far as non-renewable energy is concerned, it is considered to be the cleanest type of fuel.


Oil is the most common alternative for properties that are not connected to the gas network and is currently used by an estimated 4.3 million UK households. Unlike gas combi-boilers, oil boilers tend to be non-condensing and floor standing, therefore may require more space. They also tend to offer a lower hot water flow rate in comparison.

Biomass and alternative energy

Finally, consider biomass and other forms of alternative energy. if you have storage space and are able to burn fuels that require a flue, you may wish to consider a biomass system. A more eco-friendly, alternative choice, these boilers are powered by a sustainable fuel source. 

Typically fuelled by wood pellets, chips or logs, they can help to protect the environment as well as saving on your energy bills. Other alternative approaches to energy for your boiler that are seeing increasing popularity are heat pumps and fuel cell technology which are increasingly compatible with modern boilers.

Future energy sources

You may also wish to consider whether your boiler is designed with future energy sources in mind. Some advanced gas boilers are now made to be compatible with solar energy, whilst oil systems can be equipped to be partially run on bio-oil mixes, helping to make them future proof and more ecological.

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