What is a flue?
A flue is a duct for smoke and waste gases produced by a fire, a gas heater, or other fuel-burning installations. A flue is a pipe from your boiler to the outside of your house, ensuring waste gases and condensation are kept out of your property. A blocked or damaged flue puts a property’s inhabitants at risk of carbon monoxide poisoning.
It’s most likely your boiler flue will exit at the side of your house, through the closest wall to your boiler. You might have noticed steam coming from it when the air outside is cold. This is commonly referred to as a horizontal flue. This is the most common type of flue and found in the vast majority of installations throughout the UK.
It is also possible to have a vertical flue which exits through the roof of your property. The majority of modern boilers will have a round flue (pipe), and older boilers may have a square flue. Not to be confused with a chimney, vertical flues can be found in properties where the boiler isn’t located on an external wall. Rather than put the flue straight out the side, they exit from a roof space. You may see the steam coming out of the roof above your boiler.
Vertical flues are more expensive than standard, horizontal flues due to the extra parts and work required to install them. There can sometimes be issues with access, and scaffolding may be required in some circumstances.