What Size Gas Boiler Is Best For My Home?

Choosing the correct gas boiler size is an important decision. Getting it right can mean the difference between saving or wasting money. So, it’s time for you to choose a new boiler.  But how do you know which size you need? Back in the day, boiler installers would often recommend over-sized boilers as a way of compensating for heat lost by inefficiency.

Fortunately, modern boilers have come a long way and are far more efficient, meaning that most of us can now get away with a smaller boiler.

We’ve put together a short guide to help you choose the most appropriately sized boiler for your needs.

How Is Boiler Size Measured?

Boiler size is all about how powerful it is, i.e. its ‘output rating’ rather than its physical dimensions. The output rating is measured in kilowatts (kW) such as 23kW, 45kW etc.

Which size you need will depend on the size of your property and the needs of the household. It is essential that the boiler you select is powerful enough to meet your requirements. This is something for you to think carefully about and discuss with your engineer before making any purchasing decisions. If your boiler isn’t powerful enough, you’ll struggle to heat all of your radiators. Even worse, you could find yourself short of hot water. Conversely, choosing a boiler that is too powerful for your circumstances could result in unnecessarily high bills and a colossal waste of energy.

High output or low output boiler?

Don’t make the mistake of thinking higher output = better boiler, or that lower output = lower energy bills. It doesn’t work like that! An underpowered boiler will be working harder and won’t save energy. Be careful to get the balance right when making this important investment.

Another top tip – don’t assume that the output rating of your current boiler will be just fine for your new boiler. If your heating and hot water needs and circumstances have changed, this will need to be taken into account.

Does boiler type make any difference to the size I need?

Yes, very much so. Different types of boiler use different levels of power. Deciding which type of boiler you need is an important consideration.

Combi boilers are very popular for smaller homes, but you may be surprised to know that they actually use more power than a system or regular boiler. This is because the all-in-one unit nature of a combi has to provide heating and hot water in one place as opposed to system or regular boilers which store hot water and reduce the amount of power the boiler needs to run on.

How do I work out the correct energy output rating?

It’s fairly simple to work out the most suitable output rating (size) for your boiler. Let’s start with number of radiators. For each radiator in your home, allow for 1.5 – 2kW of water. So a home with 10 radiators would need a boiler with at least 20kW of power. 

As previously mentioned, this is just a guideline because different boiler types require different power levels. The chart below will help you to work out the recommended size range for your home.