Choosing the correct flue for commercial applications

BFCMA, boilers, chimney, flue, space heating

Flues are an essential part of any commercial boiler installation. Dennis Milligan of the British Flue & Chimney Manufacturers’ Association offers advice on how to select and install the correct chimney or flue as part of such an installation.

When selecting a chimney or flue as part of a commercial boiler installation it’s vital to consider several key factors before making that final decision. A mismatched product can have a significant impact on performance, compromising the safety and operational efficiency of the boiler.

In simple terms, the flue is the working part of the chimney; its job is to convey the products of combustion into the atmosphere. The chimney contains the shaft within which the flue is housed.

Chimneys and flues can be constructed from a wide range of different materials — from clay to plastic. However, stainless steel is currently the most popular choice in commercial environments.

The secret to stainless steel’s success lies in its strength and durability. Flues constructed from stainless steel can be categorised according to the application they are designed for.

• A single-wall system is designed for on multi-fuel applications as a chimney liner or connecting fuel pipe on condensing and non-condensing applications.

• A twin-wall system consists of two concentric walls with pre-installed insulation filling the space between them. This system can be used with condensing and non-condensing multi-fuel appliances, diesel and gas fuel generators and combined heat and power (CHP) systems, and appliances operating under high positive pressure at low and high temperatures.

• Flexible liners made from stainless steel can be used to reline an existing chimney and are available as single-skin versions for use with gas appliances or as a twin-skin option for use with wood and multi-fuel appliances, as well as condensing gas and oil appliances.

All these options now require a CE mark as part of the Construction Products Regulations. The CE mark clearly defines the operational and performance limits of the product and should help ensure that the correct flue is selected for its chosen application.

A label detailing the classification should be present with each flue component, and deciphering these markings should also help with the decision-making process. The chart above offers some insight into the different markings you may come across:

As a rule, the straighter the chimney the better, as bends in the flue can have an adverse effect on performance. For natural-draught appliances it is recommended that a vertical rise of 600 mm should be allowed immediately above the appliance before any change of direction. There should be easy access for cleaning, and this can be achieved with the help of swept elbows that incorporate debris collection and an inspection point.

When it comes to flue sizing it is important to match the internal diameter of the flue with the outlet on the appliance. Sizing for single-wall appliances should be carried out in accordance with BS EN13384-1:2015, whilst BS EN13384-2:2003+A1:2009 should be followed for more than one appliance.

Insulation should be consistent along the entire length of the flue, and horizontal runs should not exceed the appliance manufacturer’s instructions.

The instructions of the chimney and appliance manufacturers should always be followed when undertaking an installation.

The chimney or flue of choice will need to be supported properly, with joints positioned to allow for integrity inspection and testing. In addition, there should be test points along the length of the flue to facilitate combustion or leakage testing. The CE designation of the flue specifies the distance required between the flue and combustible material to ensure safe operation.

The chimney will need to terminate in a position that allows combustion products to disperse safely without the risk of causing a disturbance to people or property. Wall terminations need to be directed away from the building to allow flue gases to freely disperse.

BFCMA, Flue, chimney, boilers, space heating
A label detailing the classification should be present with each flue component. There is more information in the BFCMA’s downloadable ‘Commercial flue guide’ (link at end of article)

A terminal will be needed on natural-draught and fan-assisted flues that have an internal diameter of less than 170 mm.

Flue terminals should be designed to minimise resistance to the flue-gas flow, while also providing protection from rain, snow and, even, small birds. They should minimise downdraught and have the capability to allow for the discharge of combustion products in abnormal wind conditions.

The build-up of condensation in flues can also be an issue, and any commercial system should allow condensates to flow freely to a drain.

Both the Clean Air Act and Environmental Protection Act help to regulate the way in which gases are dispersed. and local authorities use chimney height to help control emissions.

By taking into account the type of fuel being burned and the maximum rated output of the appliance, the ideal chimney height can be calculated, resulting in emissions that are not damaging to the health of the general public.

Discharge velocity is another key factor in determining how readily emissions are dispersed into the atmosphere, so it is important to design or select a system with this firmly in mind.

Adjacent buildings will also need to be considered before a final decision is made on height.

When considering a chimney or flue for a commercial application the options can appear endless. However, it is worth taking into account that the European Standard BS EN1443:2003 categorises chimneys and flues into just three basic designations.

• Custom-built chimneys — constructed on site using factory-made flue liners.

• System chimneys — prefabricated chimney systems manufactured off-site.

• Connecting flue pipes — pipe specifically designed to connect an appliance to the flue or chimney.

By fully exploring individual requirements, looking closely at the surrounding environment and examining the characteristics of different systems, the ideal commercial chimney or flue can be identified.

Dennis Milligan is vice-president of the British Flue & Chimney Manufacturers Association (BFCMA).

The BFCMA has recently published a ‘Commercial flue guide’. It can be downloaded from the link below.

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