Heating systems
How to choose a flue
or all flue systems, flexibility is key – especially if the location of a boiler (or boilers) is non-negotiable. This is most often the case in light commercial and commercial installations, where the flue run is just as important as the boiler(s) being used. It is crucial that any flue run is unbroken, to ensure combustion gases can be safely exhausted from the boiler to outside. As the vast majority of boilers now being installed are condensing models (and
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replacing non-condensing units on retrofit projects), they are compatible with many different types of flues, depending upon the location of the heating system and/or the application in hand. These flue types include: straight-through-the- wall; extended concentric or extended twin pipe; or extended non-balanced. In addition, some manufacturers also offer extended flue systems, which
provide greater flexibility in terms of boiler positioning. Such a wealth of options has resulted in boilers (such as ATAG Commercial’s own XL Series, for example) being equipped with connections for both flue gas and air inlet (parallel tubes), which allow for both open and room sealed arrangements. Looking at the types of flue system available, there are mainly three different options to consider. Firstly, an open flue system sees air drawn directly from a room, with flue gases discharged via either an existing chimney or ducting arrangement into a cascade header. The plant room requires adequate ventilation (to include the combustion air supply) in accordance with BS 5440 and BS 6644. Open flue systems typically climb upwards to the top of a building, in order to be evacuated above roof level – although some lower output commercial boilers can terminate at a lower level, depending on the flue discharge requirements. Another option is a room sealed flue system, where combustion air is ducted into the boilers via a cascade header, as well as discharged through the same system. As with open flue systems, the plant room must be ventilated according to BS 5440 and BS 6644. However, plant rooms will require less ventilation than an open flue system – plus, with reduced air flow, warmer room temperatures are possible, which can be significant in ‘tighter’ buildings. In addition, room sealed units reduce the risk of flue gases being emitted back into the plant room. Indeed, in certain applications, commercial boilers can be installed in occupied spaces if a dedicated plant room does not exist. Finally, a concentric flue system has a very compact room sealed flue, where the air supply and exhaust gases are both managed within a one-piece concentric duct system. Essentially a ‘duct within a duct’, flue gases are discharged through the inner duct, with combustion air drawn in via the outer chamber. Concentric system terminals can either be positioned horizontally, to
32 June 2019
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ATAG’s Paul Martin (left) says the flue is of critical importance in any commercial boiler system (above)
Flues are an essential part of any commercial boiler system. However, when considering which flue to use, there are several important factors to take into account. After all, using the wrong type of flue can impact significantly on a boiler’s performance, as well as affect safety and the operational efficiency of the heating system, as Paul Martin, national sales manager at ATAG Commercial, explains
exit a building through the wall, or vertically through the roof. Whatever flue system is chosen, it must adhere to the necessary UK legal
requirements, such as the Clean Air Act. This was designed to minimise the air pollution from chimneys and flues, for boilers with outputs greater than 366.4kW. The pollution levels from boilers of this size are considered dangerous to the health of people in the local area if not dispersed at a high level (above the roofline of a building). While flues can be made from a range of materials, stainless steel remains the most frequently used by commercial boiler manufacturers. After all, as flues expel combustion gases into the atmosphere, they need to be strong and durable. Indeed, all metal flue products are required to carry CE certification, as per the Construction Products Regulations; CE compliance ensures a flue will perform to the specifications of the system to which it is installed, while taking into account essential aspects such as condensate and corrosion resistance. Furthermore, the standard IGEM/UP/10 (Installation of Flued Gas Appliances in Industrial and Commercial Premises. Communication number 1774) covers topics including ventilation, flue sizing and the height and location of flue terminations, as
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