Flue fan assistance

Mechanically driven chimney exhaust fans have quickly become an essential part of modern day chimney and flue design, widening the parameters of what is achievable at the design stage of a new flue system as well as offering a solution to those troublesome, pre-existing flue systems/ chimneys that haven't quite worked how they should.

Chimney fans are available in a number of types and sizes, each suited to work with different appliance types, fuels and their corresponding flue systems. Their size will be dependent on the flue system resistance that is present on any given system along with the total calculated flue gas volumes that are required to move through the flue system. The 3 main types of fans are vertical discharge termination fans and horizontal discharge termination fans, both as suggested, positioned at termination level, and in-line fans that will be positioned somewhere between flue inlet and outlet position. 

Every chimney fan will need a controller to which the selection will vary dependent on the appliance and fuel type. All gas appliances will have a controller that interlinks with a solenoid valve that acts as a fail-safe to shut off the gas supply to the appliance in the event of fan failure, as well as having an over-run feature built in to ensure the complete evacuation of combustion gasses has occurred before the chimney fan will turn off. For commercial heating applications, there are also controllers available that link through a relay box to allow the connection of multiple appliances. 

To enable us to select the right fan for the project, or to decide if the flue system will even require fanned assistance, the same procedure must always be followed. At the design stage, the first step is to attain details of the proposed appliances and review their operational characteristics whilst also looking carefully through the appliance manufacturer's data sheets. Once the flue route has been determined and we have accurate dimensions to hand, we go through a calculation process that is intended to simulate the flue system, that will provide us with a result on flue system resistance/ back pressure, flue gas volumes, flue gas temperatures and flue gas velocities. We'll then make a chimney fan selection based on those results.

Our motto when designing flue systems is to 'keep it simple', but that isn't always possible when including flue systems within a complex building design that pushes the boundaries of what is required. Listed below, are just some of the benefits that chimney fans bring to the table.

• One of our most frequent requests is that the client wants large fire openings but they are unable to accommodate a flue diameter large enough to handle such high flue gas volumes under natural draught (suction). A correctly sized chimney fan will solve that problem and ensure that the flue system is under constant negative pressure and that all products of combustion are evacuated safely.

• Building designs seem to be ever increasing in complexity and our flue systems need to work with those designs. Flue systems can't always be perfectly straight, in fact, they are quite often required to be anything but straight. Anything that defers from straight and vertical will add a flue system resistance. The more bends and the more 'off' vertical the flue is, the more resistance there is present and the less likely the flue system will function under natural conditions. Chimney fans give a larger design window for complex flue systems as they will overcome that system resistance.

• Chimneys can be troublesome on occasions and sometimes that could just be down the misfortune of the chimney's location. It might be that the chimney top termination point is sheltered by a taller structure, large trees or perhaps on a property that is positioned in a countryside dip. Any of these can cause turbulence around the termination point or direct air movement in a downwards direction over the chimney or flue terminal. If it isn't possible to raise the termination point to a height that it remains unaffected by these conditions, a chimney fan can be a very welcome cure.

• The basics of chimney operation under natural conditions is that the hot air rises through the flue system, pulling the colder air towards the appliance and creating, what is known as a pressure differential. As the hot air rises through the flue system, it builds up momentum, increasing the force of the upward draught the taller it goes. This theory unfortunately means that shorter chimneys/flue systems will suffer performance issues, which is another example of where a chimney fan could be a solution.