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Boiler types explained - combi, heat only & system


Jo Alsop

Heating Hero

The Heating Hub

Know what type of boiler you have and what type you need with our guide to combi, system and heat-only boilers.


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 (but they can work with a vented cylinder also). 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? Try our guide on combis: Combi boiler guide

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 often works with a steel hot water cylinder but can work with vented cylinders also. 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.

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).

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 some 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 but can with copper vented 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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