Gas Boilers11 July 20230

Types of Gas Boilers Explained – Combi, Heat Only & System Boilers

There are three main types of gas boilers you can install in your home: combi boilers, heat-only (also called regular or conventional boilers) and system boilers. The type of boiler most suitable for your home depends largely on your hot water demands and the pressure of your mains water supply. Below we’ll explain the different types of gas boilers available for your central heating system. Always use a gas safe registered company to install your gas boiler

Combination Boilers (AKA combi boilers)

A gas combi boiler gives you instant hot water and heat by burning gas, a process known as combustion, to heat the water at source. The hot water is heated from a single, compact unit that is usually hung on a wall and also eliminates the need for a separate hot water storage tank. It is also the most energy efficient way to heat your home and water, and with instant hot water access, it removes any precious time wasted waiting for the hot water to heat up.

When considering a combi boiler, you also need to think about how many water supplies will be used at the same time, for example would you likely run 2 showers at the same time. When using two or more water outlets at the same time, water flow rates are reduced as a combi boiler can only heat the water which is fed to it from the mains. Low water pressure in your area could also be unsuitable for a combi boiler.

Heat Only Boilers (AKA Regular or Conventional Boilers)

A Heat Only boiler consists of 3 components: the boiler itself, a separate hot water tank cylinder, usually placed in an airing cupboard, and a cold-water storage tank, usually found in the loft. The cold-water tank supplies water to the boiler using gravity, hence why it’s usually placed in the loft. The boiler then supplies hot water to both the central heating system, ie the radiators, and the hot water storage tank so they can be used simultaneously.

When using a heat only boiler, the flow rate is greater which is useful for larger households with greater water demands or areas with lower water pressure.

System Boilers

A system boiler is very similar to a Heat only boiler in that the boiler heats the central heating system and a separate water storage tank or cylinder is required, but it differs when it comes to the cold-water supply which is supplied to the boiler from the mains, eliminating the need for a cold-water storage tank in the attic. This is called an unvented cylinder where all the components such as the pump and expansion vessel, are internalised.

Great for space saving as there’s no extra component needed in the attic, but the hot water flow rate depends on the main supply so the water pressure in your area also needs to be considered.



Things to consider when looking for a new boiler

  1. ALWAYS check the engineer installing your new boiler is registered on the Gas-Safe Register
  2. If swapping from a combi boiler to a system boiler, consider where you will place the water cylinder
  3. How many water outlets does your household use at any one time? Is there more than one shower running in the morning?


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