What do I need to know about boiler flues?

Having some knowledge about boiler flues and how they work will be useful when discussing a boiler replacement with your engineer. This is because a boiler flue is an essential part of any heating system that burns fuel and therefore produces exhaust gases.

In this article we discuss how flueing works, why it is important and what the basic components are called. This will make it easier to understand where the boiler flue will be positioned and why.

Also called a boiler outlet pipe or duct, the main purpose of a boiler flue is to carry away harmful waste gases produced by the boiler and safely release them away from your home. These waste gases include carbon monoxide which can easily go undetected due to lack of colour, scent or taste. It is important to be aware of possible symptoms which include: headaches, dizziness, nausea, vomiting, tiredness, stomach pain, breathing difficulties and a fast, irregular heart rate. This is why it is important to have an audible carbon monoxide alarm in your home and to have your boiler system regularly serviced.

Whether your existing flue is an outlet for your system or regular boiler or if it is serving as a combi boiler exhaust, you will easily be able to find and recognise it on the outside of your home. It will be a white or black pipe either protruding horizontally out of a wall or vertically from the roof. When the boiler is in operation you will see steam leaving the flue which is the emission of condensation and waste gases.

You might be wondering if there is a difference between a horizontal and a vertical boiler flue but they both do the same job. In the UK, horizontal flues are generally more common than vertical boiler flues as boilers are usually installed on the internal side of an external wall. In the event that the boiler is not on an external wall, or if it is fitted in a loft, then a vertical boiler flue will be needed. In this scenario, the boiler outlet pipe will go up and out of the roof, much like a chimney.

You might also need to have a gas boiler flue terminal guard installed. This is fitted over the boiler outlet pipe to prevent debris getting into it. They are relatively inexpensive depending on the size and shape you need. In some cases, a gas boiler outlet guard is necessary to meet regulation requirements eg. where a terminal is sited below two metres from the ground, or when sited at floor level, as the terminal must be protected by a flue terminal guard. In these situations, consideration should also be given to preventing nuisance plume and fumes in areas such as access routes, passageways, patios, balconies.