What is a system boiler and how does it work?

Learn how this type of boiler works, its benefits and how it is different from combi and regular boilers.

Why are some homes more suitable for system boilers?

More and more UK homes have combi boilers, but they aren’t necessarily right for every home. If your household often uses multiple hot taps or showers at the same time, a system boiler would be more suitable. 

How are system boilers designed?

Unlike combi boilers, system boilers are connected to a separate cylinder to store hot water until it’s needed, so you get a continuous supply to multiple taps at the same time. This makes them ideal for homes with more than one bathroom. You’ll usually find hot water cylinders in the airing cupboard and they are heavily insulated to prevent the water from cooling. 

How does a system boiler work?

Unlike a regular boiler a system boiler doesn't require a feed and expansion tank to fill the system. The system fills with water directly from the mains water supply. The central heating pump is inside the boiler as can be the control valve so taking up less space in the airing cupboard. The boiler heats a coil inside the hot water cylinder to provide hot water to showers and taps. The boiler also heats the water that circulates through the radiators.

Hot water from multiple taps

Ideal for homes with high hot water demand, or more than one bathroom, and good existing water pressure. 

Good water pressure

The water comes directly from the mains instead of a water tank, so you’ll have good water pressure. 

Use solar power

Because it has a separate hot water cylinder, it can be compatible with solar panels or solar thermal if the right type of hot water cylinder is fitted.


No F&E tank needed

It doesn't need an F&E tank installed above it, so a system boiler is perfect for homes where the boiler needs to be upstairs or in the loft.

How can I be sure I’ll have enough hot water?

When you have a system boiler, the hot water available is dependent on the size of your cylinder. If all of the hot water has been used, you’ll have to wait for more to be heated. It’s easy to stop this though - you just need to make sure that your hot water cylinder is big enough for your needs. Check out our latest advice if you want to learn more about what size boiler is right for you

You should also use your boiler’s timer to make sure the cylinder is full when you need it. For example, morning showers or evening bath time. Remember that it takes up to an hour to heat a cylinder, so set it to come on a bit before you need it.