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What is a system boiler and how does one work?

System boilers provide central heating and a constant supply of hot water using a separate storage cylinder. This type of boiler is ideal if you have a large home with multiple bathrooms that need hot water at the same time. If you choose a system boiler, you’ll need space for the hot water storage tank, which is often kept in an airing cupboard.

What is a system boiler?

A system boiler powers your central heating directly and also provides hot water that’s kept in a separate storage cylinder. These compact appliances are capable of heating a home whilst producing enough hot water to supply several showers and taps at the same time, making them a popular choice for families and bigger households.

System boilers vs combi boilers

With a system boiler, a storage cylinder is used to provide a constant supply of hot water. This isn’t the case with an all-in-one combi boiler, which heats up water on demand as and when you need it (rather than storing it for later use).

Although system boilers take up space in an airing cupboard because they need a separate tank, they’re more suitable for houses that have a high demand for hot water. Combi boilers can struggle to supply several outlets with hot water at once – and because they switch between providing heating and hot water, they can offer less effective central heating for larger households.

System boilers vs regular boilers

Although system boilers and regular boilers both use a hot water storage cylinder, a system boiler takes up less room in your house because it doesn’t need an additional feed tank in the loft. For this reason, it’s easier to install and repair a system boiler.

Both types offer similar capabilities in terms of providing heating and hot water. We would only recommend choosing a regular boiler in cases where the water pressure (or flow rate) in your home is low or you have old radiators that can’t cope with the pressure generated by a system boiler.

How does a system boiler work?

System boilers heat your home and provide a hot water supply via the following process:

1. Cold water enters the system boiler directly from the mains water supply.

2. The water passes over a heat exchanger inside the unit, which transfers heat to it.

3. Once the water has been heated to the temperature you’ve set on your heating control, it is transported around the home to your radiators using a central heating pump.

4. In the same way, the water in your cylinder is heated to reach 60-65°C where it sits until it’s used.

5. When you turn on a shower or hot tap, the hot water in the cylinder is pushed out by mains water pressure to the outlet.

Unlike a combi boiler, a system boiler is able to heat your radiators and pump hot water to several showers and taps simultaneously.

Pros and cons of system boilers

If you have a large and busy household, a system boiler could be perfect for you. That said, it’s worthwhile checking out the pros and cons before you choose which type of boiler to go for.

What are the advantages of a system boiler?

There are a few reasons why you might want to pick a system boiler:

  • Easy installation and repair – without the need for a cold water feed tank in the loft, a system boiler is much easier to install and repair than a regular (heat-only boiler).
  • Supplying hot water and heating simultaneously – whilst a combi boiler can either provide hot water to a single outlet or central heating at any one point in time, a system boiler can supply several outlets and provide heating simultaneously.
  • Cost and energy efficiency – compared to a conventional boiler, a system boiler is more energy-efficient, meaning you’ll save money on your heating bills.
  • Strong water pressure – by taking water straight from the mains rather than from a feed tank, system boilers can offer better water pressure than regular boilers.
  • Saving space in your loft – a system boiler doesn’t need a feed tank in the loft, saving space.

Considerations before deciding on a system boiler

In order to check a system boiler is the right fit for you, consider the following points:

  • They take up more space than a combi boiler – as system boilers need a separate hot water storage tank in an airing cupboard, they take up more room in your house than an all-in-one combi boiler.
  • Limited hot water supply – once the hot water in your storage tank has been used up, you’ll need to heat up some more water using your system boiler. The larger your storage cylinder, the more hot water you’ll have available before it runs out.
  • Water needs heating in advance – a system boiler relies on the storage tank to supply hot water, so you’ll need to set your boiler to heat up water in advance. This takes longer with a system boiler than with a combi boiler, which heats water almost instantly.

What size system boiler do I need?

It’s important to choose a system boiler that’s a suitable size for your household and its requirements.

Smaller one- or two-bedroom houses with fewer radiators will only tend to need a small system boiler, so a 12kW or 15kW model would be fine. Bigger three- or four-bed properties with between 10 and 20 radiators will require an 18kW or 25kW model. You’ll need a 30kW+ system boiler for houses that are larger than this.

We offer a complete range of system boilers in different sizes, from 12kW up to 30kW.

How much is a system boiler?

When you get a quote for a new system boiler, this will typically include the cost of the appliance and the installation work.

The total cost of installing a new system boiler will depend on your existing boiler type. Replacing an old system boiler with a new one is much cheaper than a conversion or new installation, particularly if your current hot water storage tank is still in good working order.

  • Replacing an old system boiler with a new one will typically cost around £2,000 (provided that you don’t need a new hot water storage tank).
  • Installing a new system boiler and storage cylinder in a property that currently has no boiler will often cost more than replacing like for like due to the additional parts and labour that are required.
  • Converting a traditional back boiler into a system boiler requires more installation work and will is therefore often the most expensive installation, often cost over £3500.

These aren’t exact figures. The actual cost of installing your new system boiler will also be affected by things like which model you go for, with larger boilers costing more.

The location of your house can also play a part in determining the cost – installation prices are typically higher in the south of the UK than elsewhere.

How long should a system boiler last?

The average system boiler will last for 10-15 years.

An annual boiler service will ensure that your system boiler is still in good working order and identify any potential issues before they become a problem, which helps to make your new appliance last longer.

Similarly, you should book a boiler repair with a Gas Safe-registered specialist as soon as you spot any indications that your system boiler isn’t working properly. Signs of malfunction could include impaired heating and hot water supply or unusual sounds/smells.

Dependable, high-efficiency system boilers from Glow-worm

You can rely on a Glow-worm gas boiler to provide heating and hot water when you need it. Take a look at our system boilers for more information about the various models we manufacture.

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Interested to learn more about the other types of boiler? You may want to take a look at these posts: