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.

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 sealed system that only requires a boiler and a cylinder, whilst a regular boiler may require a 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?

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.

Looking to replace your old boiler?

If you're interested in having a sytem boiler installed, we have a range of system boilers avaialble. Talk to a local Viessmann expert in your area to find the perfect match for you.

Other factors to consider

How much space does a system boiler need?

One of the most important considerations when choosing a system boiler is whether or not you have sufficient space, this is because generally speaking a system boiler will be a little bigger than a regular boiler as it incorporates an expansion tank and pump. An additional advantage of this is that the additional components will generally be covered by the manufacturer warranty. If your house has been built with an airing cupboard, this is usually where the hot water cylinder would be designed to go. 

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. 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. Viessmann system boilers are different from other manufacturers due to the four pipe system. This allows for the easy use of weather compensation for high efficiency heating systems.

Diagram of 4 pipe system

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. 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.  Your heating engineer will be able to design the system and hot water tank to satisfy the household demand, if this is configured correctly you shouldn't run out of hot water. 

If space is an issue you can consider a storage combi boiler.

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