What is a boiler flue?

A boiler flue is a pipe that travels from your boiler to an external wall of your house – you’ve probably seen it puffing out steam, especially on colder days. This pipe is designed to carry waste gases and condensation from your heating system. Instead of being released into your home, they’re expelled outside.

One of the gases your boiler flue pipe will disperse outside is carbon monoxide, which is potentially very harmful. This is why it’s so important to have a boiler flue correctly installed, in line with gas safety regulations.

Here we’ll delve deeper into the different types of boiler flues and their purpose. We’ll also discuss boiler flue regulations and how to check your flue is properly maintained.

Do all boilers have flues?

There are lots of different types of boiler systems, including regular boilers, combi boilers and system boilers. If you have one of these, you will also have a boiler flue in your home.

Some properties may use an open flue boiler to create central heating. If you have one of these, you will have a boiler vent as well as a flue. Open flue appliances take the air they need for combustion from the room they are installed in, rather than sucking it in from outside. It’s important they have adequate ventilation to replenish the fresh air supply.

The only types of boiler that don’t have flues are back boilers, which are installed behind fireplaces. Instead, they use a chimney to get rid of waste gases. As a side note, back boilers are not the most efficient kind of boiler. If you have one installed, you may want to consider replacing it soon to save yourself money in the long run.

Now you know the basics, let’s take a look at the different types and shapes of boiler flues.

Different types of boiler flues

Today, most properties have horizontal flues that stick out the side of the nearest external wall to the boiler. However, some buildings, which may be less traditional in shape or don’t have a boiler installed on an external wall, may have a vertical flue instead. This pipe will stick out the roof of a property but should not be confused with a chimney.

Although they do the same job, vertical flues tend to be more expensive than horizontal flues. A gas safe registered engineer may need to cut through your roof or remove tiles to install a vertical flue and they often require extra parts to operate efficiently. When your flue needs inspecting or repairing, scaffolding may be required for safe access, which adds to the expense.

Different flue shapes

Flues also come in different shapes. Modern boilers, i.e. those under 15 years old, tend to have round flues. Older boilers usually have a square flue.

If you are looking to buy a new boiler, you may be asked what shape your boiler flue is. This information will give the boiler installation company an idea of the age and style of your current heating system. As we mentioned before, if you have a round flue, it’s likely to be a modern heating system and will already be complying with current boiler flue regulations – using a 22mm gas pipe. If you have a square flue, it’s likely to have a small 15mm gas pipe. A gas engineer would need to upgrade your flue and pipework when a new boiler is installed to meet the current boiler flue regulations.

Keep in mind that if your boiler flue pipe needs upgrading, the installation quote you receive will be more expensive. As well as new parts, it may also include any remedial work required. If you think about it, a round-shaped flue won’t take up as much space as a square flue, so you will need to have the external wall re-bricked or plastered. You’ll need to ask the installation company if this is included in the quote. Always check what you’re paying for and get quotes in writing.

Meeting boiler flue regulations

Boiler flues get rid of potentially harmful gases, like carbon monoxide, so there are strict regulations regarding their installation and upkeep.

Boiler flue regulations stipulate how a system should be positioned. A horizontal flue must be at least 30cm below a window, wall vent or door. A vertical flue must be situated no less than 100cm from an adjacent opening window. This is to ensure that waste gases cannot easily re-enter your home. If your flue faces your neighbour’s property or a public space, you will also be required to ensure it is at least 2.1 metres above ground level.

Vertical flues that pass through a property are usually concealed in a cabinet or boarded over. Boiler flue regulations require the cabinet or panels to be accessible at regular intervals, to ensure the flue can be safely inspected and if necessary, repaired.

Plume kits and extensions

If your boiler flue cannot be installed in an appropriate place, you will need to buy a plume kit to adapt the location. These kits tend to cost around £100 to £150 depending on the boiler manufacturer.

In some cases, there may be a large distance between your boiler and an external wall. This would require you to buy a boiler flue extension, which costs around £50. It’s best to speak to a registered gas engineer who will take a look at your home and advise you if this will be necessary.

Keeping your boiler flue safe

The best way to make sure your boiler flue is safe and operating efficiently is to have it checked by a Gas Safe registered engineer. As a rule of thumb, we’d suggest having your boiler system serviced every 12 months. If you have an open flue boiler, you may want it serviced a little more regularly, as debris can be easily drawn into the system and affect the combustion process over time.

If you have any concerns about the safety of your boiler or boiler flue, get in touch with a Gas Safe registered engineer as soon as possible. It’s always better to be safe than sorry!