The vast majority of UK households rely on a combi boiler for their heating and hot water, and homeowners are often encouraged to choose this option when replacing a boiler. However, in some situations a system boiler may be a much better alternative.

In this article, we look at what a system boiler is, give an overview of how they work and look at some of the key reasons you may wish to consider having one installed.

Why Choose a System Boiler?

The system boiler explained

What’s the difference between a system boiler and a combi boiler?

A combi or ‘combination’ boiler is responsible for providing both your heating and hot water within a single, compact unit. They are often a good choice for flats and family homes, especially where space limitations mean that it is not possible to accommodate a header tank or separate hot water cylinder.

There are, however, limitations to combi boilers. In particular, they are not well suited to homes that have a high demand for hot water as they are only designed to support one outlet at a time. Because combi boilers typically switch the burner from heating to hot water when there is demand, heating performance can also be impacted in high demand households.

A system boiler, in contrast, features a separate cylinder for storing hot water. Although the major heating and hot water system components are built into the boiler, the hot water is stored in the cylinder. This means that a constant supply of hot water can be provided to a large number of taps and showers at the same time. This makes it the ideal choice for a home with multiple bathrooms.

Is a system boiler the same as a regular boiler?

Because of the separate hot water storage cylinder, it is easy to mistake a system boiler and a regular boiler (also commonly known as a conventional or traditional boiler). There are, however, some fundamental differences. A system boiler is a closed system that only requires a boiler and a cylinder, whilst a regular boiler requires header tanks in the loft to feed the hot water cylinder and to maintain the water level in the radiators. 


For this reason, system boilers are often the preferred choice unless there are specific reasons why a regular boiler may be more suitable, such as in areas with low water pressure or homes with older radiator systems that would be unsuitable for the higher pressure of a system boiler.

How does a system boiler work?

In a system boiler, cold water enters the boiler directly from the mains supply. The water is heated via a heat exchanger, which transfers energy from the gas jets to the water. Once heated, the water is pumped to the hot water cylinder, where it is stored until required. Because there is a large reservoir of hot water always available, a steady supply can be maintained even if multiple taps or showers are in use.

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Because a system boiler is a sealed system, it also benefits from condensing technology. This means that energy is recycled from the exhaust gasses to preheat the cold water, making the system even more energy efficient and helping to reduce your home’s CO2 footprint and reduce your environmental impact.

Other factors to consider

How much space does a system boiler need?

One of the most important considerations when considering a system boiler is whether or not you have sufficient space. If your house has been built with an airing cupboard, this can be the ideal place to house the hot water cylinder - but be aware that you will lose the use of this space as a storage space. 

Remember, you’ll also need to account for the additional space required by the cladding. As system boilers do not require header tanks to be fitted in the attic, they are suitable for homes with no loft space or where a loft conversion has been done or is planned.

How efficient are system boilers?

With condensing technology, the boiler itself is extremely efficient, with many models achieving up to 90% efficiency in optimum conditions. A very important consideration with a system boiler is the potential for heat loss from the hot water cylinder. To combat this, it is important that your cylinder is properly insulated for maximum efficiency. Because all taps and showers will receive near instant hot water at the same pressure, a system boiler can also help optimise your home’s water consumption.

Can a system boiler run out of hot water?

The only limitation to the amount of hot water a system boiler can supply is the capacity of the hot water cylinder. If all the hot water is used, you’ll have to wait for more to be heated. This is usually on a set timer. In order to guarantee sufficient water, it is recommended that you consider how much water is likely to be used at any given time for things like showers, baths and taps when sizing your cylinder. Be aware, however, that in many cases the size of cylinder you choose will ultimately be dictated by the space available. 

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