JOIN US
Top boiler installer? Get listed for free

Boiler Flues: What they are, how they work and what type you'll need

Heating Advice

Haven't got a clue when it comes to your boiler's flue? We've put together some quick and easy tips, which'll help you get to grips.

If you’ve researched ‘boiler flue’, it’s likely because you aren’t sure what one is or where it is – if you have one. Either that, or your boiler is feeling run down with a sore throat and you weren’t certain on the spelling of flu.

It’s handy to know what type of flue you have if you’re looking for an online boiler replacement quote or discussing it with a boiler installer over the phone or during a home survey, they’re often understandably confused with chimneys so have a quick read through the below for some clues on flues.

What is a boiler flue?

By definition, it’s a duct or pipe for the expulsion of exhaust gases from a heating system or generator.

In English – It’s a pipe from your boiler to the outside of your house so the waste gases and condensation from your heating system aren’t being pumped into your home – pretty important.

Preoccupied scientists of the world are yet to invent a recyclable, environmentally friendly boiler juice as of yet which leaves most UK homes still fuelling their boilers by way of mains gas supply, with some rural properties using oil boilers and LPG burners– three fuels you wouldn’t want to be inhaling remnants of if you lived in a flue-less world.

Do you have a boiler flue already?

If you have a boiler, whether that be a combi boiler, regular boiler or system boiler – then yes, you will have a boiler flue. Unless your boiler installation was undertaken by The Super Mario Brothers.

Note the exception of back boilers. If you have a back boiler (a gas boiler behind your fireplace) then you will not have an additional flue – the waste gases will simply exit via your chimney. Back boilers are no longer replaced due to their inefficiency – consider a conversion to a new combi boiler to huge potential savings on your energy bills.

Do you need one?

Based on the above description, you’ve probably already deduced that you do need a boiler flue, regardless of what type of boiler you have. To say your heating system wouldn’t meet Gas Safe regulations without one, would be akin to suggesting you might not necessarily need a parachute for sky diving.

The flue isn’t anything you need to worry about though, any Gas Safe heating engineer will have included the flue and any necessary work required for it in their boiler installation quote.

Where is your boiler flue?

It’s most likely your boiler flue will be exiting the side of your house, through the wall which is nearest to your boiler. You’ll probably have noticed steam coming from it and your neighbours – especially when the air outside is colder. This is commonly referred to as a horizontal flue.

In some cases, where building types and shapes differ, you may have a vertical flue which exits through the roof of your property. Most modern boilers, under 15 years old will have a round flue (pipe), with boilers older than that often having a square flue.

Knowing your boiler's flue type is key to getting an accurate quote

If you’re looking to get an online boiler replacement quote or are discussing one over the phone with a heating engineer, prior to a boiler installation, then they’ll very likely want to know the shape and exit point of your existing flue. Fortunately, these questions are really easy to answer.

Flue shape – it’s going to be either a square or round flue. You can take quick look by locating your flue (look for the pipe exiting your external wall or ceiling closest to your boiler). Generally speaking, they’ll want to know this because it can help give them an idea of the age and style of your current heating system.

  • Round flue – If it’s a round flue, it’s likely to be relatively modern and already receiving gas via current regulation standard 22mm gas pipes.  
  • Square flue – A square flue might indicate an older system, potentially without regulation 22mm gas pipes (likely 15mm) which would require them to upgrade the pipework in order to meet legal gas safety requirements.

A good Gas Safe installer would want to make you aware of this as early as possible. A square flue may also require some additional brick or plaster work where the flue exits the wall if replaced with a round flue.

...Bored yet?

Compare Boiler Quotes

Which type of fuel heats your home?

Cost implications

The costs implications involved in the variables of flue types aren’t to be worried about. If you have a round, horizontal flue (exiting out of the side wall of your house) then there shouldn’t be any cost implications at all.

Vertical flues (exiting via your roof) and square flues may incur additional charges, however.

For a vertical flue, there's access issues to worry about and scaffolding may be required so that the installation can be completed safely. Most (if not all) vertical flues will exit through a roof; this could mean removal of your roof tiles or cutting through your flat roof. If the jobs requires extra work, the installer will want extra payment to cover their labour. Vertical flue kits are generally more expensive than their horizontal counterparts, too.

Square flues may also require some additional brick or plaster work from the installer to patch up where the replacement has been implemented. Again, this is unlikely to greatly affect your quote and shouldn’t increase by much more than £60 on average. If, like many new boiler buyers, you’re looking to pay for your new boiler on finance – then factors such as these will have very little effect on your monthly repayments.

If you're looking to install a boiler near a window opening, wall vent, or door (within 30cm), you'll also need to have a plume kit. They extend the boilers standard flue away from your openings, meaning the harmful gas won't enter your home. Typically, a standard plume kit might come as an additional cost to you in the region of £100 - £200 depending on the boiler manufacturer - are flue kidding me?! The distance of the flue exit from the boiler could also require extension kits in order for it to reach, averaging around £50 per extension kit – although it’s unlikely you’d require this, let alone more than one.

You should note that not all boiler installers will aim to patch up your plaster and brick work back up to match its original aesthetics and you should be sure to ask beforehand. You may be required to employ an external plasterer and/or decorator.

When will this article end?

Very shortly, but you can read more boiler advice posts on our Heating Hub.

For professional advice regarding your boiler flue or any aspect of your heating system, you should always consult a professional Gas Safe heating engineer and ideally have them carry out a free home survey. You can search our directory for Gas Safe installers or get online quotes for a new boiler in a matter of clicks.

Read More

Buying Boilers,
Better Beyond Belief.

  • Instant prices from the UK's best installers
  • Compare deals without the pushy salesman
  • No stress, trouble, or obligation

Related Advice

Here's some more posts which we think you'll love

Boiler Brands
  • Made with by Heatable ©