Flues are a crucial part of every commercial boiler installation. Selecting the correct one for a commercial boiler is very important, as getting wrong can lead to poor performance and even cause safety issues. This guide will explain what flues are, how they work and how to make sure you are making the right choice for your commercial boiler system.

What is a Commercial Boiler Flue?

A commercial boiler system is a system that heats water which is then used to warm buildings. Any boiler that has an output larger than 70kW is regarded as a commercial boiler because domestic ones are smaller than that size.

The flue is the part of the system that removes the products of combustion out of the building and into the atmosphere, essentially a pipe that is attached to the boiler and goes outside of the building. The type of flue that is chosen should depend on the application of the flue and the type of fuel that is used by the boiler. The terms flue and chimney are used interchangeably as they are essentially the same thing; the chimney includes the shaft within which the flue is housed.

What are the Types of Chimneys and Flues?

There are different systems of stainless steel flues that are categorised according to the application they are designed for:

Twin all System Chimney

These chimneys consist of two concentric stainless steel metal walls with pre-installed insulation filling the space between them. This type of chimney is available in different models for use with:

  • Condensing and non-condensing multi-fuel appliances
  • Diesel and gas fuel generators and Combined Heat and Power (CHP) systems
  • Appliances operating under high positive pressure at low and high temperatures

Single Wall Chimneys

Single wall chimneys are designed for use on multi-fuel applications as a chimney liner or connecting flue pipe on condensing and non-condensing applications.

Flexible Liner

Flexible stainless steel liners can be used to reline an existing chimney, rather than replacing it entirely. There are two types of flexible liner so it’s crucial to ensure you choose the right one:

  • Single skin liners for use with gas appliances, or:
  • Twin skin liners for use with wood, multi-fuel appliances, gas or oil appliances.

Commercial Boiler Flue Regulations

The regulations regarding both chimney and flue stipulate that they should be considered ‘Part of the Building’ and therefore need to comply with ‘The Construction Products Regulations’. The main points of these regulations are summarised below:

CE Mark. The flue you choose must have a CE mark, which defines the operational and performance limits of the product, and satisfies the legislative requirements of the EEA.

Temperature rating. The minimum temperature for continuous use of the flue. 400 degrees celsius is suitable for multi-fuel, 200 degrees celsius suitable for gas and oil.

Condensate resistance. Any commercial system should allow condensates within the flue to flow freely to a drain.

Corrosion resistance. Depending on the type of fuel used, the flue needs to be resistant from attack from products of combustion.

Liner material specification. The lining material of the flue must be able to withstand the corrosive effects of both the type of fuel used and the conditions caused by condensation.

Soot fire resistance and distance to combustibles. The flue needs to be resistant to soot fires and placed a safe distance away from materials that would burn when heated, e.g a timber frame.

As well as these regulations, the following points need to be considered when choosing a flue:

  • The chimney should be as straight as possible as bends in the flue can have an adverse effect on performance. For natural draught appliances, it is recommended that a vertical rise of 600mm should be allowed immediately above the appliance before any change in direction. Ease of access for cleaning is necessary at any change of direction in the flue, which can be achieved with the use of swept elbows that incorporate debris collection and an inspection point.
  • To conform to The Clean Air Act, the chimney needs to reach the required discharge height to render emissions harmless and not a danger to public health. This can be calculated by taking into account the type of fuel being burned and the maximum rated output of the appliance.
  • The chimney or flue chosen needs to be supported properly, with joints positioned to allow for inspection and testing. There should also be test points along the length of the flue to allow for combustion or leakage testing.


Making the wrong choice when choosing a commercial flue can lead to decreased performance of your commercial heating system or even danger to yourself or others, for example if you have the incorrect flue for the distance to combustibles. By considering the requirements of your commercial premises, looking at the surrounding physical environment and examining the characteristics of different systems, you will be able to make the correct choice of chimney or flue.

If you’re looking to install a new commercial boiler or upgrade your old one, then at Invictus Mechanical we have over 30 years experience and a proven track record for excellence. Give us a call today to find out more.