Boiler flues are a crucial part of your heating system. They are a way for combustion fumes that have been created by your boiler to be released outside the home. If your boiler’s flue becomes blocked, these gases have nowhere to go other than back inside your home. In some cases, this can be fatal, with the NHS stating that around 60 deaths a year are caused by carbon monoxide poisoning.

The two main gases your gas condensing boiler produces are water vapour and carbon dioxide. Nitrogen oxide can also be created when the oxygen and nitrogen in the air are burned together. If your boiler isn’t burning the gas correctly, carbon monoxide could be produced too. Boilers are made to be safe, and these gases shouldn’t harm you. However, a blocked flue can mean the gases become trapped in your home, causing you to breathe them in.

Carbon monoxide is an odourless, colourless gas which makes it almost impossible to detect. You should have at least one carbon monoxide alarm fitted in your home. This will make a sound, like a smoke alarm, when it detects the gas. If the alarm goes off, you’ll be alerted to a potential carbon monoxide problem. The gas could be entering your home due to a leaky boiler, or it could be your flue that’s the main problem.

What can cause a blocked flue?

There are many reasons your flue may have become blocked. Blockages can be caused by old birds’ nests, dead birds, leaves or general debris. If your home is surrounded by trees, leaves and other debris, such as conkers or pinecones, can fall into your flue. The more these build up, the more your flue will become blocked. This is why your flue needs to be checked regularly and any debris removed.

Flues can exit your home vertically via the roof or horizontally through a wall. It’s most common to see them coming out of a wall, and this is often a better option. Leaves and other debris are less likely to fall into a flue that is horizontal. However, horizontal flues may be more prone to birds’ nests. If your flue is positioned on the roof and you find it is often becoming blocked, you can invest in a cover that will prevent things from getting into the flue.

If you think that your flue is blocked, you should turn off the boiler completely. Don’t just turn the heating off. Instead, make sure you cut off the gas supply to the boiler entirely. If you’re unsure how to do this, then just turn the boiler off and call a Gas Safe registered engineer to come out and look at the leak for you. 

If you believe that your flue has been blocked for a long period of time, you should look out for any of the following symptoms provided by the NHS, as they could be a sign of carbon monoxide poisoning.

Short-term exposure symptoms include:

  • Dizziness
  • Feeling and being sick
  • Tiredness and confusion
  • Stomach pain
  • Shortness of breath and difficulty breathing
  • Difficulty concentrating


Long-term exposure symptoms include:

  • Mood swings and irritation
  • Personality changes (intoxication)
  • The feeling that everywhere is spinning (vertigo)
  • Breathlessness and a heart rate of more than 100 beats per minute
  • Muscle spasms or seizures
  • Loss of consciousness

How to tell if your flue is blocked

Because of the kinds of gases that exit through your flue, there are strict guidelines about where your flue can and can’t be positioned.

The flue needs to be at least:

  • 300 mm away from an opening window or air vent
  • 25 mm below guttering, drain pipes or soil pipes
  • 300 mm above ground, roof or balcony level
  • 2,000 mm below a Velux window
  • 1,200 mm from an opening, such as a door or window, into the home


The flue cannot open onto a public walkway. This is because the gas that comes out of the flue is hot, so people who are walking on a footpath could get burned.

If the flue exits through a ground floor wall, a grill or metal box is required to cover the flue. This prevents it from being damaged.

How to seal a boiler flue

It is a requirement that your boiler’s flue is completely sealed. If there are gaps around the edge of the flue, there's a chance that some of the gas could re-enter the house. The flue needs to be sealed on both sides of the wall or roof where it exits the home. This is usually done using sand cement. If you’re unsure whether your flue is fully sealed, you can ask a heating engineer to check this for you. Their basic test usually involves smoke pellets. These handy devices can be lit and held up to the seal. If there’s a gap, or air is getting in, the smoke will rise and begin to dissipate outside. This means the flue needs to be resealed.

Making sure that your flue is working correctly is critical in order to maintain the health of your family. If you believe it might be blocked, you should call a suitably qualified engineer to have a look as soon as possible. 

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