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Boiler types explained - combi, heat only & system



What type of boiler do I need?

There are three main types of boiler: combi, heat only and system. Heat only boilers (aka conventional or regular) work with a cylinder in the airing cupboard. System boilers are often found in modern homes with an ‘unvented’ hot water cylinder. Combi boilers produce instant hot water. We help you identify which one you have and consider the advantages and disadvantages of each. You may be thinking about a new boiler. This guide can help you decide whether to stick with your existing boiler type or install a different boiler type.

Summary of Boiler Types

What is a combi boiler?

A combi boiler is a single unit that generates all the heating and hot water for the home. There are no hot water tanks. How much is a combi boiler? Read our guide to Combi Boiler Prices

What is a heat only (regular) boiler?

A heat only boiler only provides heating. It works with a hot water cylinder. How much is a heat only boiler? Read our guide to Heat Only Boiler Prices

What is a system boiler?

A system boiler has all the same components as a combi boiler without the hot water production. It works with a steel hot water cylinder. How much is a system boiler? Read our guide to: System Boiler Prices


Identify your current boiler, what are the pros and cons?

How does a combi boiler work?

A combination boiler heats water via an integral heat exchanger directly from the cold mains. It provides instantaneous hot water.

This is different from a traditional system which stores hot water in a tank in advance of use. A combination boiler does not need a hot water cylinder or tanks in the loft. Everything is done through a single, usually wall-hung, unit.

For more on combi boilers, including finding the best value boiler for your home, try our Combi Boiler Guide. Need helping sizing your combi boiler? Use our other brilliant guide: What size boiler do I need?

Advantages of a combi boiler

  • Creates space when removing old hot water tanks
  • Lower maintenance costs - the integral heating and hot water functions are covered by the boiler manufacturer's warranty/guarantee
  • No long wait times for hot water

Disadvantages of a combi boiler

  • They do not work well in properties with poor flow rates and/or poor incoming water pressure
  • Hot water flow rates are reduced when providing hot water to two or more outlets simultaneously, although this is really only a problem in houses with two or more showers.
  • High-performance combi boilers - that provide 12.5 litres per minute of hot water or more - may not be effective on 15mm pipework supplies and can be oversized for the heating - for more on the appropriateness of combi boilers see our Guide to Boiler Sizing
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How does a heat only boiler work?

A heat only boiler (also referred to as a 'regular' or 'conventional' boiler) provides heating directly to the radiators and works with a cylinder to provide hot water. They often work on what is called an 'open-vented' heating system, i.e. there is a Feed and Expansion tank in the loft, but they can work on a 'sealed' system too. (To find out whether you have an open vented system and the pros and cons, read our Guide to Open Vented and Sealed Systems).

For a comparison of heat only boiler models and help finding the best heat only boiler based on price, warranty and efficiency, visit our brilliant Heat Only Boiler Guide.

What is a back boiler? A back boiler is a type of heat only boiler that is situated in a chimney breast with a fire on the front. It is currently not possible to replace a back boiler in the same location. For those with an existing back boiler that would like to replace it with a new boiler then you will need to opt for a wall hung heat only boiler elsewhere in the house or consider a combi boiler. For more help read our dedicated guide: Back Boilers: Advice on Replacement, Repairs and Servicing

    Advantages of a heat only boiler

    • Work well with old radiators. Older radiator systems that are put under the high water pressure delivered by system or combi boilers may leak.
    • Low replacement cost as heat only boilers are the cheapest type of boiler to buy

    Disadvantages of a heat only boiler

    • Only the boiler is covered by the manufacturer's guarantee/warranty. This leaves lots of other components on the system (e.g. hot water tank, pumps and valves) that are subject to replacement and breakdown

    How does a system boiler work?

    Like regular boilers, system boilers work with a hot water cylinder. Unlike regular boilers, the system is not open-vented, i.e. there are no tanks in the loft, and all of the components that would normally be outside a heat only boiler (pumps, valves etc) are integrated into the boiler. These are a modern version of the regular boiler and are often installed along side unvented hot water cylinders.

    For a comparison of system boiler models and help finding the best system boiler based on price, warranty length and efficiency, read our dedicated System Boiler Guide.

    Advantages of a system boiler

    • Good for properties that have a high hot water demand, i.e. lots of showers, as the unvented tank can deliver good hot water flow rates to multiple outlets simultaneously
    • Good cover from manufacturer warranty/guarantee as there more of system's components are integral to the boiler

    Disadvantages of a system boiler

    • Good hot water flow rates depends upon good incoming mains water pressure, if the pressure is poor then internal flow rates will be poor
    • They are require more space than a combi boiler. If you are swapping a combi boiler for a system boiler you will need to find an appropriate location of the hot water cylinder, usually on the first floor

    Stick or change boiler type: Our Top Tips

    1. If you have old radiators or pipework buried in screed - stick with an open vented, heat only boiler
    2. If you and your family have lots of showers at the same time – consider changing to a system boiler with unvented hot water tank (if your water pressure is good)
    3. On a tight budget - replace your existing boiler with the same type of boiler in the same location
    4. Want low maintenance costs - install a combi boiler with a 10 year warranty/guarantee from the manufacturer (providing your water pressure is good).
    5. Extending your property and adding bathrooms - consider a system boiler with unvented hot water tank (if the water pressure is good)


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    Heating Hero

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