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System boilers vs combi boilers: what’s the difference?

Joe Minihane
Written by  Joe Minihane
Kim Staples
Reviewed by  Kim Staples
5 min read
Updated: 05 Sep 2023

System boilers and combi boilers differ in one crucial area: while system boilers require a dedicated cylinder to supply hot water, combi (or combination) boilers offer hot water on demand without the need for any separate equipment. There are, though, a few other differences. Read our guide to system boilers vs combi boilers to find out more.

What is a system boiler?

A system boiler has the power to take cold water directly from the mains supply and use gas in the boiler to heat that water, before transferring it to a dedicated hot water cylinder. From there, that water can be used for showers, baths, washing up, or whatever else you need it for.

System boilers require lots of space, as you’ll need room to install a large cylinder for all that hot water. As such, they tend to be found in larger properties where space is not at a premium.

They offer better water pressure compared with combi boilers and offer quicker hot water than regular boilers, which require a cold water tank for their water supply. If you have a large household or want to fill a bath quickly, they are usually the way to go.


What is a combi boiler?

A combi boiler is exactly what it says it is - a combination system. It’s a type of condensing boiler that can handle both heating and hot water in one smaller piece of equipment. As well as heating the home, it can instantly heat hot water, so you can take a shower without having to make sure the tank is filled beforehand.

They are more energy efficient and save you money too, as they only heat the hot water that you need. However, they do not offer the same regular water pressure as system boilers, meaning that pressure drops off when a tap or home appliance is running at the same time as a shower, for example.

Combi boiler vs system boiler

System boiler:

  • Offers excellent hot water supply

  • Pulls cold water from the mains supply

  • Requires a hot water tank, therefore takes up more space than a combi boiler

Combi boiler:

  • Energy efficient

  • Save money

  • Doesn’t need a cylinder as boiler heats hot water when you need it

Do system boilers need a water tank?

Yes, system boilers need a hot water tank. This is because, unlike combi boilers, they don’t use the boiler itself to supply hot water. Instead, they heat cold water and then send it in heated form to be stored in a dedicated tank, ready to be used when needed.

Such boilers are better suited to larger homes, where water pressure is often more important. If there are lots of people at home, system boilers allow for water to run at higher pressure when required simultaneously in different bathrooms or the kitchen.

Is a system boiler better than a combi boiler?

Whether a system boiler is better than a combi boiler really comes down to the size of your home and your needs.

A combi boiler tends to be best for small and medium homes. That’s because they don't take up a lot of space and offer maximum efficiency by only heating the hot water you need.

System boilers are a better option for larger homes with lots of residents. Not only do such homes tend to have room for the requisite hot water cylinder, they also tend to need the more sustained water pressure that system boilers can provide.

Choosing a boiler for your home

Choosing a new boiler for your home is a big consideration, especially as they are expensive items.

All new boilers must now have an A efficiency rating, whether they are combi, system or regular boilers. More often than not a combi boiler is the best option for those keen to save space and boost efficiency while minimising bills.

If you have the space, you may want to consider a greener option, such as a heat pump. Government grants can help with such systems.

It’s also good to weigh up whether you want to move your boiler to a new location and, if so, what additional work, such as installing new pipes, that this may require.