Condensing boilers
explained.
Every new gas boiler fitted in the UK is a condensing boiler. Here is how they work, what the condensate pipe does, and what changes if you are replacing an older boiler.
How a condensing boiler works
A boiler burns gas to heat water. In an older boiler, hot gases go straight out of the flue. A condensing boiler passes them through a larger heat exchanger first, cooling them until the water vapour in them condenses. That recovers heat that would otherwise be wasted, so more of the gas you pay for heats your home.
Condensing boilers have been required for most gas boiler installations in England and Wales since 2005.
The condensate pipe
The condensing process makes a small amount of slightly acidic water. It drains through a plastic pipe, usually white or grey, from the bottom of the boiler to a drain, soil pipe or soakaway.
If you are replacing an older, non-condensing boiler, a new condensate pipe has to be added. Our online boiler price asks about this and prices it at the survey.
Frozen condensate pipe
In very cold weather, a condensate pipe that runs outside can freeze. The boiler then locks out and often shows a fault code (for example F.28 or F.29 on a Vaillant). You may hear gurgling from the boiler.
You can usually thaw it by pouring warm (not boiling) water along the outside section, then resetting the boiler. If it keeps freezing, the pipe can be rerouted or insulated.
Smell gas? Leave the property and call the National Gas Emergency Service on 0800 111 999.
Getting the most from it
- A condensing boiler works most efficiently when the water returning from the radiators is cooler – a lower flow temperature can help
- Weather compensation or load compensation controls can adjust the temperature for you
- A magnetic filter and clean system water protect the heat exchanger
- Have it serviced every year to keep it efficient and keep the warranty