What’s the Difference Between Combi and Heat Boilers?

Some people are often confused on the difference between a combi and heat boiler.

This is mainly because people believe that all boilers are the same, as in they serve the same function of heating water. There are certain key differences between the combi and heat-only boiler. We explain below the main differences between the combi and heat only boiler, which subsequently will help you differentiate between the two types of boilers.

Combi Boiler

Combination or “combi” boilers can be characterized as being high efficiency water heaters and a central heating boiler, hence the name, combi boiler. The combi boiler has gained in popularity over the years. In fact, they account for more than half of the total sales of domestic boilers in the UK. This is mainly due to their space-saving design and high efficiency, which results to lowered energy bills. What makes a combi boiler unique is that unlike other boilers it does not require a tank to store cold water or a cylinder for the storage of hot water. This means more space in your loft and a decreased monthly energy bill since the combi boiler can heat cold water from the mains on demand. Using a combi boiler also means that there’s no need for you to deal with noisy pumps as you are not constantly heating stored water.

Pros and Cons of a Combi Boiler


  • Access to instant hot water
  • Cost effective and energy efficient choice since you do not need to store water or heat water constantly
  • No need for cylinders or water storage tanks in the loft.


  • Takes longer to fill baths
  • Not ideal for large homes or homes with several showers
  • Since the boiler has to fire up first to heat water on demand, there’s a slight delay in water heating
  • If the boiler suddenly breaks down, you will have a situation since there will be no stored hot water.

Our list of the best combination boilers you will find it here.

Heat-Only Boiler

The Heat-only boiler can heat water inside of the exchanger. The water is then pumped and diverted to either sides of the radiator, or the coil in the cylinder to heat the water. This type of boiler usually has to get cold water from a small tank that is placed in the loft.

Pros and Cons of a Heat-Only Boiler


  • Large hot water storage tank means you get hot water whenever you need it
  • Ideal for large houses or large families that are in constant need of hot water
  • Can also include a secondary coil that heats the water with the help of a solar panel
  • Has the capability to run more than one shower at once without a change in temperature.


  • Because of the need of a hot water tank and a cylinder, the heat-only boiler takes up more space as compared to a combi boiler
  • You will need to include expensive booster pumps if you want powerful showers
  • The heat-only boiler is limited in the amount of water that can be used by the size of the cylinder.

Ending Note

These are the main differences between the heat only boiler and combi boiler. Therefore, if you require further information to help you choose a specific new boiler model or type please ask our boiler expert for a quote and full information about different boiler makes and models. Otherwise you can read about Combi or Heat only boilers on our website.

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