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What is a heat only boiler?

Summary of topics:

A heat only boiler, also known as a regular, conventional or open vent boiler, is a system involving two tanks, a hot water cylinder and the boiler itself. Heat only boilers are typically found in larger properties as they can better meet the demand for hot water across multiple taps and shower heads.

Below we explain how heat only boilers work, how they differ from other systems and how to decide if a heat only boiler is right for your home.

How do heat only boilers work?

Heat only boilers work by heating water and distributing it throughout a home using a system with various controls, including motorised valves, pumps and tanks. A cold water tank stored in the loft will take water from the mains and feed it into a hot water cylinder.

The boiler will then heat the hot water cylinder, at which point the hot water is supplied to taps and showerheads when needed. A pump will also circulate heated water to radiators when the heating is turned on.

Homes with a heat only boiler will also have a feed and expansion tank to maintain a consistent level of water in the heating system. Water is sometimes lost from the system through leaks or evaporation, which is why the feed and expansion tank will replace water in the system to maintain a constant level of pressure.

What is the difference between a heat only and system boiler?

Unlike system boilers, heat only boilers maintain pressure using a feed and expansion tank, whereas system boilers use a pressurised heating circuit topped up by mains water. Heat only boilers utilise a feed and expansion tank to maintain water pressure whereas the expansion vessel is usually internal in a system boiler. System boilers require less space within the property than regular (heat only) boilers and require less piping due to the need for tanks and other elements of an open vented system. However, both systems store hot water for later use, as opposed to heating it on demand like combi boilers.

What is the difference between a heat only and combi boiler?

As mentioned above, combi boilers heat water instantly on demand as opposed to storing it for later use. Combi boilers do not require a cold water tank, an external hot water cylinder or a feed and expansion tank, which means they take up much less space in a home. This makes a combi boiler a much better option in properties where storage space is limited.

The advantage of a heat only boiler is that it can supply water to multiple taps and showerheads more easily than a combi boiler. The hot water tank can store large quantities of hot water and deliver it instantly, making it the ideal choice for larger homes with a larger hot water requirement.

What size heat only boiler do I need?

The size of regular (heat only) boiler you will need will depend on the size of the property and its level of insulation. You should always speak to a boiler expert before purchasing a new boiler, but below is a general guideline of typical boiler sizes.

Once you have looked through the table, you may be interested in looking at our range of Which? Best Buy regular boilers to give you an idea of what would be suitable solution for your home.

Number of bedroomsLevel of insulationRecommended size
More than 4Poor16kW
More than 4Some insulation14kW
3-4 bedroomsModerate insulation10kW
2-3 bedroomsGood insulation9kW
2-3 bedroomsExcellent insulation7kW-8kW

Summary

  • A heat only boiler heats a home using a cold water tank, a hot water cylinder and a feed and expansion tank.
  • Heat only boilers are typically found in larger homes because they are better able to supply hot water to multiple radiators, taps and showers at the same time.
  • Heat only boilers differ from system boilers in several ways, including the way that they maintain pressure using a feed and expansion tank.
  • Heat only boilers are more suitable than combi boilers for larger homes with greater demands for hot water, while combi boilers are better suited for smaller homes.
  • The size of heat only boiler you will need will depend on the size of your home and its level of insulation.

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