What size boiler do I need?

A new boiler should be seen as a necessary home investment, and this is why it’s so important that you make the right choice. When it’s regularly serviced and properly maintained, a boiler should last up to around 15 years, and will be solely responsible for delivering heat and hot water to your home.

When choosing a boiler, size is really important. Too big and it might use too much energy to heat water you’re not going to use and too small, you might find yourself running out of hot water or left with a cool house that cannot heat up properly.

So what factors do you need to consider before choosing a boiler size?

How is boiler size measured?

Before we look at what size boiler you should choose, it’s important to understand how boiler size is measured.

Boilers are sized in kilowatts (kW) rather than physical dimensions. A kilowatt is a unit that is used to measure how much energy the boiler can output as heat. Generally speaking, the more heat and hot water you require, the higher the output you will need, although other factors also play a part in choosing the right size.

This means that boiler size is essentially based on the number of radiators, the number of bathrooms and the number of bedrooms. 

What size boiler should I get?

You can use our handy table below to determine which size boiler is right for your home, depending on the number of radiators, bedrooms and bathrooms.

No of radiatorsNo of bedroomsNo of bathroomsBoiler size
0-5119-18 kW
5-102224-27 kW
10-203328-34 kW
20+4+4+35-43 kW

If your home has more than four bedrooms and three bathrooms, then a combi boiler likely won’t be the best option. Instead, you may need a boiler that is better equipped to deal with high water demand, such as a system or regular boiler. 

Should I choose an oversized boiler?

In the past, homeowners may have been advised to choose a larger boiler to compensate for heat that is lost naturally, such as through the roof, walls and windows of your home. But modern advances in boiler technology means that they’re more efficient than ever, and choosing a bigger boiler than you need isn’t recommended. It could result in wasted energy and is likely to leave you paying more than you need to for your gas bill.


Calculating your heating and hot water usage

When choosing which boiler size is right for your home, it can be important to consider your heating and hot water demands, as well as other factors, such as heat loss and water pressure.

Hot water demand

Most UK households use a combi boiler, which serves both heating and hot water on demand from the mains water supply. They are practical, convenient and cost effective to run. Where there is just one bathroom, they are often quite sufficient to meet the needs of the household as far as hot water is concerned.

If, however, you have a large or growing family, or your home has more than one bathroom, a combi boiler may struggle to provide sufficient hot water. This is because the water is heated as it is used, rather than stored in a tank.

If you are likely to be using more hot water, you will certainly want to consider whether a conventional or system boiler that includes a hot water tank is the right choice.

Ultimately, which type of system you choose will have an impact on the size of boiler you need.

A combi boiler is sized to provide hot water on demand and you will see its capability expressed in terms of how much water it can heat per minute to a defined temperature rise, e.g. 12 litres per minute at 35°C rise.

The higher the flow rate you require, for filling baths more quickly or to have a more powerful shower, for example, the larger your boiler should be. This means that combis are typically oversized in order to meet heating demands, making them less efficient than other types of boilers, such as system and conventional boilers, when heating the home.

System boilers and conventional boilers preheat hot water to be stored in a water cylinder and therefore, hot water flow rate is not as critical. The heat load for an average home can vary from around 10 kW to 20 kW, which means that system boilers and conventional boilers can be sized smaller than combi boilers. They will therefore be more efficient when heating the home, but they require more space. 

Number of radiators

A key factor that will have an effect on the size of boiler you need is the number of radiators. If you have just a few, for example in a small flat, then choosing the smallest suitable boiler will offer you the greatest level of economy and ensure you avoid paying more than you need to for energy.

The average three or four-bedroom house usually has about ten radiators. For this reason, it would require a 24-30 kW combi boiler. A larger house that has more radiators and more bathrooms would need a larger 30-35 kW boiler.

If you have a large house with as many as twenty radiators, a large 35-43 kW combi boiler may be needed to provide sufficient heat with the best economy, or you would be better to change to a system boiler, which could be sized more appropriately.

Other factors to consider

Heat loss

In the past, the physical size of your house was often used as a factor in considering the right size of the boiler. Whilst this still plays a part, a much better way to choose the most effective and efficient boiler size is to have an expert calculate your home’s heat loss.

Heat loss calculations take into account a range of factors including floor area, number of radiators, number and type of windows, insulation and other aspects that will cause heat to be lost from your home.

A heating expert will then use this information to work out how much heat your house will lose on the coldest day of the year, allowing them to advise you on the optimum boiler size.

Type of fuel

The type of fuel available in your property can also affect the size of the boiler you choose, as boilers for different fuels can vary in terms of efficiency.

Most UK homes use gas boilers which offer the greatest level of efficiency, while houses that are not connected to a mains gas supply often choose to use an oil-fired or LPG (liquefied petroleum gas) system instead.

The type of fuel your boiler uses could impact the size you need, and it is recommended that you seek advice from a professional in order to make the best possible investment.

Water pressure

Although this is not usually a concern for newer houses on a mains water supply, some older homes may experience poor water pressure, and this can have an impact on the type and size of boiler you choose.

A combi boiler must be big enough to cope with the amount of water passing through it to be heated – the higher the flow rate, the bigger the boiler should be.

Whilst most manufacturers provide information on the maximum flow rate for their boilers, getting a professional to assist you could make the process easier, ensuring you choose the most economical boiler for your home and helping you to save money in the long term.