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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What is a condensing boiler?
A condensing boiler works on the principle of recovering as much as possible of the waste heat which normally leaves through the flue of a conventional (non-condensing) boiler as water vapour. The best high efficiency condensing boilers convert more than 95% of their fuel into heat, compared to 78% for conventional types. Condensing boilers get their name because as they recapture the heat from the water vapour leaving through the flue, this water vapour condenses back to a liquid, or condensate as it is known.
Is a combination condensing boiler more efficient than a regular condensing boiler?
A combination (Combi) condensing boiler can be more efficient than a standard condensing boiler because it heats water as you need it. Combi boilers are most suitable for smaller homes, as they can only keep up with a certain level of demand. Non-combi boilers produce hot water and then store it in a cylinder for use as and when. This is still quite an efficient process - but if the hot water is not used immediately, then inevitably some heat will be wasted. It will generally depend on the size and amount of hot water you're likely to need. You installer will be able to advise you which type of boiler best suits your needs.
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 reliable are condensing boilers?
Condensing boilers are reliable if well maintained and serviced regularly. A Which? survey in 2011 found that 46% of owners who bought a boiler after April 2005 have experienced some kind of fault. Not all of these faults required a repair. In fact the most frequent problem, a blocked drainage pipe, experienced by 11% of owners, is actually a problem caused by poor installation.
Could a condensing boiler save money on my energy bill?
Which? estimates that the average efficiency of boilers in UK homes is about 70%. This means they waste up to 30% of the heat they generate in the form of hot flue gases. This suggests that installing a condensing boiler, which has an efficiency of around 88%, can reasonably achieve savings of between £50 and £200 per year depending on the size of your house - and the saving will increase as the cost of gas increases. Complete our free assessment to find out just how much you could save.
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