What Is FuranFlex and What Is It Used For?

If you work in heating, ventilation, or chimney refurbishment, you’ve probably heard the term FuranFlex Flue System. But what exactly is it — and why has it become one of the most trusted modern solutions for chimney and flue lining? In this article, we’ll break down what FuranFlex is, how it works, what installations it’s suitable for, and what regulations apply to its use.

What Is FuranFlex?

FuranFlex is an advanced composite flue lining system used to reline existing chimneys or ducts without the need for demolition or traditional masonry work. Developed in Hungary and now widely approved across Europe and the UK, FuranFlex uses a thermosetting resin technology that hardens in place to create a completely new, seamless flue liner inside an existing chimney or duct.

Unlike traditional stainless steel or clay liners, FuranFlex is installed as a flexible fabric tube, which is then inflated and steam-cured. Once cured, the liner becomes rigid, smooth, corrosion-resistant, and gas-tight, effectively forming a brand-new flue that conforms perfectly to the shape of the old one.

How FuranFlex Works

The FuranFlex installation process is quick and minimally invasive:

  1. Inspection and preparation: The existing chimney is surveyed and cleaned.

  2. Insertion: A deflated FuranFlex liner is lowered into the flue.

  3. Inflation: The liner is inflated using air pressure, expanding to fit the internal shape of the chimney.

  4. Steam curing: Steam is applied, triggering a chemical reaction that hardens the liner.

  5. Final testing: Once hardened, the liner is tested for air-tightness and ready for use.

This process typically takes just one day, making it ideal for retrofit or refurbishment projects where minimal disruption is key.

Types of FuranFlex

There are several FuranFlex variants designed for different applications:

  • FuranFlex RWV – Designed for gas and oil boilers, with high resistance to moisture and condensate.

  • FuranFlex Black (or Classic) – Used for solid fuel appliances such as wood-burning stoves, withstanding higher temperatures.

  • FuranFlex25 – A version specifically tailored for condensing appliances, supporting smaller diameters and higher efficiency systems.

Each type is certified for use under the relevant European and UK standards, ensuring safety and compliance.

What Is FuranFlex Used For?

FuranFlex is used across a wide range of applications, including:

  • Boiler flue relining (gas, oil, or biomass systems)

  • Chimney renovation for residential and commercial buildings

  • Apartment block duct and ventilation refurbishment

  • Industrial exhaust systems

  • Heritage or listed buildings where preserving structure is essential

Because the liner molds exactly to the existing chimney, it’s particularly valuable in irregular or narrow flues where installing traditional liners would be difficult or impossible.

FuranFlex and UK Regulations

In the UK, all flue linings must comply with Building Regulations Approved Document J, which covers combustion appliances and ventilation. FuranFlex liners meet these requirements and are HETAS and Gas Safe registered installers’ approved when