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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: February 23, 2024. Updated: March 14, 2024

What Is A Flue? Boiler Flues Explained

Worcester E9 Fault Code

A boiler flue is essential for channelling exhaust gases outside of your home. It ensures your home is safe from harmful combustion gases. Flues are required across all boiler types to meet legal regulations.

Flues can be positioned vertically, out of a roof, or horizontally, out of a wall. Vertical flues will be more expensive than horizontal flues, as they must go through a roof and be water-tight.

Vertical flue - What is a flue?

Understanding Boiler Flues

  • Necessity and Regulations: A boiler cannot safely or legally operate without a flue, making it a critical component of your boiler.
  • Types and Orientations: New flues are generally round and can exit horizontally or vertically. The shape may indicate the age of your system, with round flues suggesting a more modern setup.
  • Installation Considerations: The flue’s orientation impacts installation complexity and cost. Horizontal flues are standard and straightforward. Vertical flues need extra work, such as scaffolding, to ensure safe access and installation.
  • Cost Implications: The length of the flue can affect installation cost. Vertical flues and those needing extensions or plume kits for safe gas expulsion near windows and doors may incur higher costs.


A boiler’s flue is vital to ensuring the safe operation of your heating system by directing harmful exhaust gases outside.

Understanding the type, orientation, and installation requirements of your boiler’s flue is useful, especially when considering a new boiler or maintaining your current system.

Proper installation and compliance with Gas Safe regulations are non-negotiable for the safety and efficiency of your home heating.

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