Boiler Flues Explained Re January 7, 2021

Boiler Flues Explained


Why do I need a boiler flue?

Are you in need of a new or replacement boiler, or has someone mentioned the words boiler flue to you and you’re completely flueless? (sorry).

Once you finish reading this article which explains exactly what a boiler flue is and what it does, you will be feeling like a professional engineer before you know it.

If you are looking for a more in-depth article, discussing everything you need to know about the costs involved in buying a new boiler, we already have a blog posted which may be better for you.

What is a boiler flue?

In simple terms, it’s a boiler flue pipe or duct that connects your boiler to the outside of your house, extracting the waste gases and condensation from your heating system, so they don’t get pumped around your home.

How does a boiler flue work?

You might be interested in what the pipe coming out of the side or roof of your house does. Well, if you are, you’re in the right place.

When gas is burned in the boilers combustion chamber, it creates heat. At the same time, waste gas fumes are also created. The boiler flue is designed to extract these fumes from your home, as they could become potentially dangerous.

The reason why condensing boilers are more efficient is that the waste gas created will pass through the extra heat exchanger, which soaks up the heat from the fumes, and passes it on to the cold water returning into the boiler.

boiler flue

Do I already have a flue? If so, where would it be located in my home?

If you have a combi, regular or system boiler, then yes, you will have a boiler flue already.

However, if you have a back boiler, which is a gas boiler behind your fireplace, then you won’t have an additional flue as the gases will simply exit via your chimney. Please note, back boilers are no longer replaced due to them being inefficient.

Converting to a new combi boiler could save you money on your energy bills.

To get a boiler quote online in minutes click here…….

Where is my boiler flue located?

If you take a look outside your house, you will find the boiler flue pipe exiting either the side of your house (horizontal flue) or through the roof of your home, this is called a vertical flue.

Tip on working out how old your boiler flue is:

If you have a ROUND flue, this means that your heating system is newer, probably under 15 years old.

If your boiler flue is SQUARE, this means that your system is most likely older than 15 years old.

Talking flue types, this brings us on to different shapes, types, and sizes of boiler flues.

Different flue types, shapes, and sizes

Horizontal boiler flues

Horizontal boiler flues are the most common type of flu in the UK. If your boiler flue is horizontal, it will exit straight out of the external wall, behind the boiler.

Vertical boiler flues

This is not your chimney.

Normally, vertical boiler flues are found in homes where the boiler isn’t located on an external wall. They are installed exiting the roof of your home.

Vertical boiler flues are often more expensive than a newer round flue, due to the extra parts and work that may be needed to install them. Unless your engineer is extremely tall, installing a vertical boiler flue may need scaffolding to access the area. Other complications could occur also.

Flue regulations

It is extremely important to consider the regulations surrounding the placement of a new boiler flue, because of the hazardous gases that are extracted. These are strict regulations that have been created to keep you and others safe.

The noxious gases that come from the boiler flue could cause potential health problems, and the heat of the gas could also scald either yourself, your neighbour, or even members of the public if the boiler flue is not installed following these regulations.

Below is a list of safety measurements that have been put in place to ensure that the minimum safety standards are met.

  • A flue must be certain distances from windows, air bricks, openings, corners, and the ground.
  • There should be a 2.1 m distance above ground level if facing public thoroughfare or frequently used pathway
  • 300mm below, above, or alongside an opening window, air vent, or other ventilation openings
  • 75mm below guttering, drainpipes, or soil pipes
  • 200mm below eaves (roof overhang)
  • 300mm above ground, roof or balcony level
  • 2000mm below a Velux window (for a vertical flue)
  • 300mm above roof pitch or flat roof (for a vertical flue)

The above are minimum clearances for typical domestic boilers. Please note that regulations are subject to change, and larger clearances may be necessary for larger, more powerful boilers.

Here at WarmZilla, we assure you that our Gas Safe engineers will already know all of this information so you are in safe hands!


As well as regulations around clearances, boiler flues – and any other pipe that exits a wall – are also now governed by new Part L Building Regulations from June 2022. These require improved sealing around pipes in walls, for example through the use of proprietary pipe grommets or collars, to help maintain air tightness. The new regulations aim to support the UK in meeting its Net Zero goals by increasing energy efficiency and air tightness in buildings. 

FlueSnug is suitable for all 100mm boiler flues, ​​and is the simplest and neatest solution to meet Part L guidelines. The product can be installed in less than five seconds and snuggly fits the 152mm core-drilled hole in the wall through which the boiler pipe and flue exits. The main benefits are:

  • It replaces the need to mortar around flue
  • It allows installers to quickly access the flue for any service or maintenance requirements, without the need to remove existing brickwork, mortar or sealant
  • For internal and external use
  • Provides a much neater, cleaner, more professional finish
  • It is the only all-in-one solution that complies with the new Building Regulations
  • Endorsed by leading boiler manufacturers, and boiler installation experts, like WarmZilla

How to know if your boiler flue is blocked

Spotting a blocked boiler flue can be difficult, especially if it’s due to a build-up of debris. However, here are some signs to look out for so that you can inform your Gas Safe Engineer. 

  • Oxidization
    Use a cloth to determine if there are any white stains around the connection of the flue. White stains often indicate a blockage in your flue. You might also find a black substance on the outside of your flue, this also indicates a blockage.  
  • Have a good look
    Very often you can that your boiler flue is blocked from the outside. Make sure you check the exterior outlets regularly, this will prevent any small problems from getting bigger.  
  • Carbon Monoxide alarm 
    Getting a carbon monoxide alarm is vital when it comes to protecting you and those who live in your home. If there is a blockage in your boiler flue, this may cause carbon monoxide poisoning. It is important to note that carbon monoxide is odourless and you can’t see it, so getting an alarm is essential for keeping you safe. If you are unsure about the dangers of Carbon Monoxide, head over to our blog that covers this so that you are all clued up!

In the event of a boiler flue blockage, it is important to turn off all appliances immediately and cut off the gas supply. Make sure you open all doors and windows to help ventilate the property. 

Never try and unblock the flue yourself because you never know how severe the problem can be. Call your Gas Safe Engineer as soon as you can and book a boiler repair. You may want to consider seeking medical attention if the blockage has existed for a while.

Checking and maintaining your boiler flue

Checking your boiler flue is not essential, however, occasional maintenance is good. With the ever-changing, very unreliable weather in the UK, you may find that your boiler flue has leaves and dirt inside, which is important to remove.

Annual servicing on your boiler should cover this. Your engineer will double-check the condition of your boiler flue to ensure it’s working properly and efficiently. That’s if you go with WarmZilla, of course.

How to know if I need a new boiler flue?

If you have a square-shaped boiler flue, this means that your current heating system is likely to be older than 15 years old and probably has a 15mm gas pipe. This would need to be replaced with a 22mm gas pipe to meet the legal gas safety requirements.

The good news is, if you already have a horizontal, round flue, you are likely to see no difference in cost if needed.

Side note: if your gas pipe needs replacing, this could result in some additional plastering or work needing to be done to the external walls of your home where the previous flue exited the building. This could cause some increase in the costs.

All of these important aspects of getting a new boiler flue or replacement boiler flue will be discussed throughout the process with us here at WarmZilla.

If you’re thinking about getting a replacement boiler then get a boiler quote in minutes with WarmZilla (no boiler knowledge required)

How much does a new or replacement boiler flue cost?

With WarmZilla, the price of your boiler flue is included in your boiler quote.

Once you begin the survey online as a part of your boiler quote, based on your answer about the flue, Warmzilla will either tell you that you require a vertical or horizontal flue.

As mentioned, horizontal flues are easier to install, whereas vertical flues are more difficult to install, as it’s through the roof.

Your boiler is legally required to have a flue, so the cost will always be incorporated into the final price.

What is a plume kit and will I need one?

It is often used when your boiler flue is too close to a door or window, or located too close to a neighbour’s house. This could potentially expose harmful gases resulting in harm for those living next to you. There should be a distance of at least 600mm between the border and the end of the flue. But, don’t stress, your engineer will know all of this!

As ever, the cost of the plume kit is included in your WarmZilla fixed price boiler quote. It will automatically be included in your order if the answers to the survey indicate that it’s necessary.

A plume kit will usually cost between £100 – £150.

Is installation included in the cost?

Gas Safe installation is included in your fixed price boiler quote from WarmZilla.

However, the cost may vary based on the answers given in the quotation survey.

A combi/combi swap (4 – 8 hours) will be far less labour than a back boiler conversion (1.5 – 2 days) and this will be reflected in the price.

It’s also worth bearing in mind that if you were getting a quote from a national energy company, they have much higher overheads to cover than a nifty little whippet of an online company like WarmZilla. Our lower overheads are passed on to the customer, giving you unbeatable prices on new boiler

Hopefully, this gives you some clarification on what a boiler flue is and what it does.

Do you feel like a professional engineer yet? If not, don’t worry, we’re always here on live chat or you can email us at and we’ll be happy to answer any questions you may have.

Remember, to get a quick and easy quote for a new or replacement boiler, head over to our main page and begin your search.

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