System boilers are one of the most commonly used appliances in the UK. They’re often installed in new build homes that have multiple bathrooms or fitted to provide hot water for new bathroom extensions. Like conventional (or heat-only) boilers, system boilers work by powering your central heating whilst also generating hot water to be stored in a cylinder for later use.

When replacing a boiler or advising a customer, it’s important to understand the different types so that you can select one which meets the needs of the house in question. To help you get up to speed, this guide explains what system boilers are and how they work, as well as highlighting their key differences compared to combi boilers (the most common kind).


diagram of a system boiler


System boilers explained

A system boiler plays two vital roles in a home: it powers your central heating system directly and heats up water that’s stored in a tank ready to be sent to taps, baths, and showers. By contrast to conventional boilers, this type of appliance takes its water directly from the incoming mains supply without an additional tank in between.

The direct connection to the mains supply is useful in a lot of ways. For a start, you don’t need a cistern (or cold water tank). This frees up space that would be taken up in the loft were you to opt for a heat-only boiler. System boilers also offer much better water pressure as a result, so you’ll get a more consistent flow rate throughout the home.


Advantages of a system boiler

There are plenty of reasons to choose this style of boiler, particularly for larger households. We’ve summarised the main advantages of a system boiler over other types below:

  1. Easy to install hot water and central heating system components are built into the system boiler itself, so this type of appliance isn’t too difficult to install.
  2. Support multiple taps, showers, and baths the hot water tank ensures that pressure is maintained even when more than one outlet is drawing water at the same time (this isn’t the case with combination boilers).
  3. Excellent water pressure – the direct mains supply means that system boilers offer better water pressure and a more consistent flow rate than conventional boilers.
  4. Reduce wasted loft space – system boilers don’t need a cold water feed tank to supply them, freeing up loft space for storage.
  5. Cost-effective and efficient – compared to a conventional boiler, this type of boiler will save money on your utility bills.
  6. Compatible with solar thermal – system boilers can be powered using energy from solar panels, making them the ideal choice for environmentally minded homeowners.


Disadvantages of a system boiler

As you might expect, there are some downsides to system boilers:

  1. Require more room than combi boilers – system boilers are more compact than heat-only appliances, but they still need a separate hot water tank (usually kept downstairs in an airing cupboard or similar space). As a result, they take up more room than combination boilers.
  2. Hot water isn’t instant – the reliance on a hot water tank means that you need to program your boiler to heat up the water in advance.
  3. The supply of heated water is limited – with a system boiler, the size of your hot water tank dictates the amount you can use before the water needs to be reheated.
  4. Heating cylinders require good insulation – hot water is stored in a cylinder before use and can start to cool down if the tank isn’t properly insulated (that said, good insulating materials are cheap and readily available).


System boilers vs combi boilers

When installing a new boiler in a customer’s home, it’s highly likely that you’ll be choosing one of these two types of boilers. As such, it’s really important to understand the key distinctions between them. 

System boilers are different from combination boilers in a number of ways:

  • You need a separate hot water tank, which isn’t required with a combi boiler.
  • System boilers can supply hot water to multiple taps, showers, or baths simultaneously, making them suitable for larger households (unlike their combi counterparts).
  • A system boiler requires you to program it to come on in advance of when you need hot water; combination boilers, on the other hand, heat your water on demand.
  • Combi boilers make use of two independent heat exchangers for hot water and central heating, whereas only one is used inside a system boiler.
  • System boilers are compatible with power showers – combi boilers can’t support these.


How does a system boiler work?

Now that you’ve got a good understanding of the features of a system boiler, it’s time to take a closer look at how they actually work. 

System boilers receive water directly from the incoming mains supply and pass it over a heat exchanger (depending on the energy supply, this could be powered by gas, oil, or electricity). As the temperature of the heat exchanger rises, energy is transferred to the water that comes into contact with it.

Once the water is hot, the in-built pump diverts some of it to the hot water tank for later use – this will then be used to supply hot water to taps, showers, and baths around the home. The rest of the hot water travels to the radiators, powering the central heating.



This guide has outlined the key features of system boilers, comparing them with other types of boilers and explaining how they work. To find out about the other varieties of boilers, take a look at our guides to combi boilers and heat-only boilers.

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