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What Is a System Boiler?

A system boiler is a type of boiler that is commonly found in larger homes with greater demands for heating and hot water. By featuring a separate cylinder for storing water, system boilers are able to provide a constant supply of hot water throughout the home, unlike combi boilers which can only heat water on demand.

Summary of topics:

How does a system boiler work?

System boilers are completely different to combi boilers as they are not connected to the cold mains. A coil in the cylinder carries the central heating water at high temperature through the cylinder, heating it’s contents, allowing the now heated water to be transferred to multiple taps and showers at once. They can usually heat both the radiators in the home at the same time as the cylinder, ensuring no loss of comfort.

Is a system boiler the same as a regular boiler?

System boilers hold several similarities with regular boilers, but also some very important differences.

Regular boilers, also known as conventional or heat-only boilers, store hot water for later use much like system boilers. They are also found in older, larger homes as a result of this.

However, regular boilers are kept artificially under pressure with the use of a header tank in the loft (also called a feed & expansion tank), whereas system boilers use a pressurised heating circuit, topped up by mains water.

They also use an external expansion vessel to maintain the water pressure, while the expansion vessel is often internal in a system boiler. Regular boilers use more space than a system boiler and often require more complex piping, which is why system boilers are more commonly used in modern properties with multiple bathrooms.

Is a system boiler the same as a combi boiler?

System boilers have a cylinder to store hot water for later use, while combi boilers only heat water on demand. The advantage of a combi boiler over a system boiler is that it requires less space due to its compact design, which is often necessary in smaller homes such as terrace houses and flats.

The choice between a system or combi boiler is often dependent on the heating and hot water needs of a household. Combi boilers are suitable for properties with one bathroom and 1-3 bedrooms, as this home typically requires less hot water.

However, combi boilers will struggle to meet the needs of larger households as only limited amounts of water can be heated on demand. System boilers, on the other hand, can store hot water for extended periods of time and better manage the hot water requirements of numerous radiators, taps and showers.

How much do system boilers cost?

Depending on the model, size and manufacturer, a system boiler can cost between £500 and £2000.

An entry-level boiler is a good option for middle-of-the-range homes with slightly higher energy requirements than homes with 1-2 bedrooms. Premium models cost upwards of £1500 and are a great option for homes with significant heating requirements.

If you're interested in having a system boiler installed, we have a range of Which? Best Buy awarded system boilers available. Talk to a local Vaillant expert in your area to find the perfect solution for you.

Advantages of system boilers

There are numerous benefits to having a system boiler installed in the home. Some of these advantages include:

Larger supply of hot water

System boilers allow homeowners to store large quantities of hot water for later use, which can be useful when multiple showers, radiators and taps are in use at the same time.

Compatible with solar energy

Solar panels can be used in conjunction with system boilers if the correct cylinder is connected to the system. This can be a cost-effective and environmentally friendly way to heat a home.

Uses less space than a regular boiler

Unlike regular boilers, system boilers do not require an expansion tank or external expansion vessels. This makes them both easier to install and less in need of space.

Points to bear in mind

More space needed than a combi boiler

While they require less space than a regular boiler, system boilers naturally need more space than a combi boiler due to the hot water cylinder, which needs to be considered in smaller homes.

Busier times may require a little patience

Whilst your cylinder should have been sized to accommodate your usual usage (typically around 50 litres of stored hot water per person), there may be times when you have lots of visitors staying, Christmas time etc where usage is higher than normal. During these times the cylinder volume can be depleted, meaning a 30-40 minute wait until the boiler can reheat the cylinder again.


  • While combi boilers heat water on demand, system boilers store hot water for later use.
  • System boilers cost between £500 and £2000 depending on the model and manufacturer.

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