Understanding Commercial Boiler Flues

When it comes to safety, flues are one of the most crucial elements of a boiler installation to get right. This article provides you with the basics of commercial boiler flues so you know what to look for when installing a new boiler or flue.

What is a Boiler Flue?

A boiler flue is essentially like the exhaust on a car. Your boiler burns a fuel i.e., gas, lpg, oil etc. and in doing so produces a waste product. This generally contains harmful fluids such as carbon monoxide. The flue is the pipe that takes this waste fluid out of the building so it can discharge into the atmosphere where the waste fluid won’t directly harm anyone.

Commercial Boiler Flue Regulations

It is important to note that boilers with a heat input of 70kW are considered a commercial boiler. Anything below 70kW is considered a domestic boiler. Where two or more smaller boilers located in the plantroom have a total output of more than 70kW, it is still considered a commercial installation, to which commercial boiler flue regulations apply.

When the total boiler input exceeds 366.4kW then another set of regulations apply, as the installation must comply with the clean air act. The main difference is that it is not permittable for the flue to discharge at low level.

Room Sealed or Open Flue?

A room sealed flue means that the boiler takes in air and discharges the gases from the flue. The two main ways of doing this are a concentric flue, essentially a pipe in a pipe, which takes in air on the outer part and discharges the waste fluid on the inner part. The other method is two have a two pipe system. This will usually require an adapter to the flue connection on the boiler.

An open flue does not have an air intake pipe, and so the boiler will take air from the plantroom. As a result, appropriate ventilation must be installed in the plantroom.

Dealing with Condensate Waste

Since the change in ERP regulations in 2018, all newly installed commercial gas boilers must be condensing. This means that an acidic condensation forms inside the flue. For the condensate to safely reach a drain the flue must have a minimum of a 3° rise across the entire flue run. This also means that there must be no hidden joints in the flue pipe without an inspection hatch. Find out more about condense in flues in our article on what causes boilers flues to leak.

Flue Dilution Systems

A flue dilution system is simply a system that uses a fan to dilute the flue gases with air so they are less harmful. The advantage of this is that it allows you to install larger commercial boilers with a flue discharge at a lower level. However, it is generally advised that flue dilution systems not be used with condensing boilers. The is because the fan dilution dramatically increases the effects of the dense plume discharge, meaning a large white cloud at low level. The fan can also cause condensate waste to blow out the flue discharge. This can cause extensive damage to the building material below where the flue discharges.

Commercial Flue Clearances

To prevent the flue gases blowing back into the building or damaging the building fabric, there are strict regulations on where the flue terminal can be located. Distances from openable windows, air vents, other flue terminals etc., are determined by the kW input of the boiler.

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