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Is my boiler flue blocked?

A blocked boiler flue can lead to some serious consequences for both your home and its occupants.
In this guide, we explain the dangers, the common causes and how to get the problem fixed.

Boiler flues are a vital part of your home’s heating system, as they allow the potentially dangerous combustion fumes (created by your boiler) to be safely released outside. Below, we tell you what a blockage looks like and how to get it sorted.

What happens if my boiler flue is blocked?

Every year, there are around 60 deaths caused by carbon monoxide poisoning in England and Wales, with central heating boilers posing one of the biggest risks.

So, ensuring your flue is clear from any blockages could be the difference between life and death.

The flue’s job is to allow the gases produced by your boiler, mainly water vapour and carbon dioxide, to pass safely outside. If your boiler isn’t burning gas properly, carbon monoxide can be produced.  

The way boilers work means these gases shouldn’t cause any harm. However, having a blocked flue can trap these gases in your home that can be breathed in by those living there.

What causes boiler flue blockages?

Most modern boiler flues are fixed horizontally to the side of your home, while a lot of older outlets are fixed vertically on the roof of a property. As such, these ducts are highly exposed to the outside world, meaning there are a host of things that can get clogged in there.

Everything from leaves & general debris to nests, insects and dead birds can create a blockage. The occurrence is more common in rural areas or any properties surrounded by trees, with objects such as conkers and pinecones also particularly troublesome.

How to tell if your boiler flue is blocked

It can be difficult to spot the early stages of a blockage, particularly if it is due to a slow accumulation of debris. 

However, there are some signs to look out for:

OxidizationUse a cloth to determine if there are any white stains around the connection of the flue as these can indicate a blockage.

Take a lookSome blockages will be visible from the outside, particularly in the case of garden debris. Make sure to check the exterior outlet regularly to spot small signs before they become big problems. There may also be the same white oxidization marks or a black substance on the outside of the flue.

Carbon Monoxide AlarmFitting and regularly inspecting a carbon monoxide alarm is vital to protect you and those you live with. You can buy an alarm from most DIY stores. Make sure it is approved to the latest British Standards Kitemark.

What should you do in the event of a blockage?

Turn off the appliance immediately and cut off the gas supply before opening any doors and windows to ventilate the property.

Never try to unblock the flue yourself as you won’t know how severe the problem is. It is much safer to book a boiler repair and have an engineer come and take a look at it.

If you think the blockage has been there for a long time, you should exit the property and seek medical attention.

What are the symptoms of carbon monoxide poisoning?

If the blockage has been there for a long time, you may have already been exposed to carbon monoxide

Low level symptoms include:

  • Tension headaches
  • Dizziness
  • Nausea
  • Tiredness and confusion 
  • Stomach pain 
  • Shortness of breath and difficulty breathing 
  • Difficulty concentrating 
  • Frequent emotional changes

Higher levels of exposure can lead to more severe symptoms:

  • Impaired mental state 
  • Vertigo 
  • Loss of physical coordination
  • A heart rate above 100 beats per minute 
  • Chest Pain 
  • Seizures

If you believe that you have been subjected to low levels of carbon monoxide poisoning, seek medical advice from your GP. If you are worried about high levels of exposure, go to your local A&E department straight away.

How to prevent a blockage​

The easiest way to prevent a blockage is to keep checking the flue’s exterior opening, particularly after periods of gusty wind. That way, debris won’t build up and fall deeper into the outlet.

Make sure your boiler and its flue are correctly installed by a Gas Safe Registered engineer. The positions that flues can be fixed to your home are governed by strict guidelines. They must also be fully sealed to exact standards.

Having an annual boiler service is a vital part of preventing a blockage as it will include a thorough check of your flue to determine whether it is restricted in any way and that it is operating efficiently.

For more information on how to look after your central heating system, check out our Advice Centre.