Installation of Condensing Boilers

There is no shortage of people and firms to fit your new boiler but try to ensure that your boiler or heating system is fitted by an installer who is registered with a Competent Person Scheme, such as the Gas Safe Register for gas installations and OFTEC for oil installations. These schemes were introduced by the government to ensure that those who self-certify their work are compliant with UK Building Regulations. We will only introduce you to registered and accredited installers.

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Most installation or upgrade work that is done on your heating system is subject to Building Regulations. These regulations vary depending on where you are in the UK, but generally all work must be notified to your installer if they are part of a self-certification (Competent Person) scheme such as the Gas Safe Register or OFTEC. If the work is not certified by one of these schemes, it must be certified by your local authority or an approved building inspector. Ensure you keep all your boiler documents – you’ll need them if you sell or rent your home.


What size boiler do I need?

As homes have very different numbers of radiators and bathrooms these days, it’s difficult to generalise about what size boiler is needed for a flat, cottage or larger house. Your installer will size your boiler according to the amount of heat and, in the case of a combi boiler, hot water you require.

Your installer will size the boiler according to how many rooms and radiators you have, as well as the number of hot water taps or devices that use hot water. The typically size range for boilers is from 12 to 40kW.


Do I choose a Combi or System condensing boiler?

There are two main types of Condensing Boiler:

  1. Combi (Combination) - These don’t need a water tank as they heat the water ‘on demand’ when it is needed, such as when you turn the tap on. This means that you don’t need to wait for your water to heat and you can save the space of a hot water tank. They are more efficient than open vent boilers, but have a lower capacity, so are generally used for smaller homes.
  2. System or Open Vent – These have a tank to store hot water and are better at providing water for larger homes with multiple bathrooms or showers, particularly if you might need to run them at the same time. You will get need an airing cupboard or another place where the hot water tank will be housed.


Installation considerations for a condensing boiler

- You’ll need to choose between wall hung and freestanding options – most condensing boilers are wall mounted as they are reasonably light.

- Condensing boilers have a shorter flue and so need to be situated on an outside wall. A second floor location is also ideal as the flue gases from a condesning boiler may travel more horizontally, obstructing outside space downstairs. The condensate that the boiler produces drains away through a condensate pipe that needs to be connected to a household waste water pipe such as a bathroom pipe.

- Planning permission may be required for a condensing boiler if your flue will extend over a metre above the highest part of the roof. You may also need permission if your home is listed or in a designated area, and if your house is in a conservation or World Heritage area your flue should not be on the side or principal elevation if it would be visible from a road.



For a gas boiler, it is important that you employ a Gas Safe registered (Previously Corgi) installer; the requirement changed in April 2009. If you are getting an oil system,  make sure your installer is OFTEC certified. We will only introduce you to accredited installers – Find an accredited installer now.

Installation Costs

For installation costs have a look at the Costs, Savings and Maintenance section.

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How often should I get my condensing boiler serviced?
It's important to have any gas or oil fired appliance like a condensing boiler serviced once a year. The flue will need checking to make sure it is clear, and a qualified installer should check that your boiler is running safely and efficiently for you. It's best to check with your manufacturer about service schedules, as some boilers may need servicing more than others.
How easy is it to install a condensing boiler?
Boilers vary in size and shape but most are designed for simplicity of installation. With space being such a premium many boilers are small enough to be fitted by one person into a kitchen cupboard, easily and with minimum disruption.
If I buy a new boiler, does it have to be a condensing boiler?
Yes. Government legislation now requires all new boilers to be the condensing type unless it would be impossible to fit one, which would probably be the case if there was no way to provide suitable drainage.
How is the Sedbuk rating calculated?
The 'seasonal efficiency of a domestic boiler in the UK' (Sedbuk) rating for boilers is calculated from the results of standard laboratory tests, together with other important factors such as boiler type and the kind of fuel used. It describes the boiler's efficiency.
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