What size boiler do you need?

The purpose of a boiler is to heat your home and provide hot water. So the larger your home and the more bathrooms you have the bigger the boiler you need. You want the boiler you buy to have enough power to heat all your rooms and supply enough hot water. If your boiler isn't powerful enough it won't be able to do this, but if you buy a boiler that's too powerful you’ll pay more for it than you needed to and might not have optimum energy efficiency. But what do we mean when we talk about boiler size? It’s actually nothing to do with the physical size of it, instead, it’s measured by the energy it produces in kW (kilowatts) - the more kWs it produces, the ‘bigger’ the boiler is. The right size boiler can also depend on its type:

• Combi boilers are mainly sized on hot water demand, as they can adapt their heating output to be energy efficient
• Heat-only boilers (system and regular) are sized on heating demand (usually the number of radiators)

Sizing the boiler correctly will give the best energy efficiency. It’s also worth remembering that it’s against building regulations to oversize a regular boiler or system boiler.

An oversized boiler isn’t a problem for modern boilers as they’re able to adjust their power output to suit the needs of the home. The exception to this is heat-only boilers. Not only will you have to pay more upfront for a larger boiler installation, you could come across other issues too including:

• It'll produce excess heat that won’t get used, meaning you’ll be paying more each month in bills.
• You’ll be using extra fuel that your home won’t use, so it can be damaging for the environment, increasing your carbon footprint.
• Instead of running consistently and continuously, your oversized boiler will run for short periods of time and use more energy than you need. 
• This ‘on and off’ nature of oversized boilers can result in it deteriorating more quickly. Parts will need to be replaced at an earlier stage and you might need to buy a whole new boiler sooner too.

To keep the boiler as efficient as possible (around 95%), it recovers heat from the waste gases produced and uses this to heat the water circulating through your radiators. This reduces the amount of heat that would otherwise be simply lost to the outside.

The waste gases are mainly water vapour and as the condensation process cools them, some of the vapour turns back into liquid, or ‘condensate’.  This is then drained away by the condensate pipe.   

What type of house do you live in?

Larger homes, with more rooms, have a greater number of radiators and showers. Especially if you have two or more floors, you’re going to need a more powerful heat-only boiler for your home. For smaller homes, such as flats and small houses, a  combi will be best suited. The only time where a smaller home would benefit from a heat-only boiler is where you’ve got several people who need hot water (baths, showers, dishwashers, washing machines) at the same time.

Before you decide, think about any plans to change your home in the near future, which might mean you need more energy. Adding an extension, for example. If that’s in your plans, it might be  worth changing your boiler type or increasing the size and power of it now.

If you have lots of people in your home wanting to use hot water at the same time, you’ll need a bigger boiler to help avoid arguments. You’ll need a system or regular boiler that can get hot water to different bathrooms at the same time.

Here, we mean separate showers, rather than ones attached to a bath. Each shower has its own water supply needs, so if you have two or more, a combi boiler won't be powerful enough for you.

The more bedrooms you have, the more radiators for your boiler to heat up at the same time. Each radiator adds around 1-2 kW to your boiler size. If you live in a 1or 2 bedroom apartment, the size of the boiler that you need tends to be from 12-28 kWs. For larger homes with 3 or4 bedrooms and a couple of bathrooms, we suggest opting for something larger, such as 18-35 kWs. For homes larger than that, with more than 2 bathrooms you would need a boiler with over 35 kWs of power.

The good news about a well-insulated home is that your boiler needs to produce less energy to keep it at the right temperature, so you don’t need to choose a large boiler. Older houses may need a more powerful one as they don’t tend to keep in heat as well as more modern builds. If you live in an older house, think about cavity wall insulation to reduce wasted heat and bring down your bills. For more information on how to save money each month, read our energy savings guide here.