Flues

All boilers require a flue to remove their products of combustion. In the past traditional chimney flues placed severe restrictions on where a gas appliance could be used. Today those restrictions no longer apply since modern boilers, with far more flexible flue systems allow greater siting options.

Open Flue (Natural Draught)
With a boiler having this type of flue, air for combustion is taken from the room in which the boiler is located. Flue products are removed by natural draught vertically to atmosphere, through a suitable terminal. The room must have a route, for combustion air, direct from outside.

Open Flue Boiler
If an extract fan is located in the same room as the boiler, then manufacturer's advice should be sought, since additional ventilation will probably be required to ensure that the boiler flue products do not 'spill' into the room.

All new open flue boilers must now be fitted with a safety device which will turn the boiler off in the event of flue products spilling back into the room because of blockage or for other reasons.
Open Flue (Fanned)

Recently, open flue boilers have been introduced incorporating a small 'flue fan' to give assistance in clearing flue products. Exactly the same conditions apply to this type as with natural draught, and it must not be assumed that the requirements for running the flues are any different. The introduction of a fan does NOT turn this boiler into a 'fanned flue type.

Room Sealed (Natural Draught)
This boiler draws it's air for combustion directly from outside through the same rectangular flue assembly used to discharge the flue products. This boiler is inherently safer than an 'open flue' type, since there is no direct route for flue products to spill back into the room.

This type of boiler can only be fitted to an outside wall and the siting of the terminals in relation to doors and windows is expected to become more difficult with new regulations soon to be introduced.

Room Sealed (Fanned Flue)
With the use of a fan, the flue products are discharged directly to outside through the central core of a circular, concentric flue, around which air is drawn in and passed to the combustion chamber of the boiler.

Fanned flue boilers are generally more efficient than other types, since less heat is lost from the small circular flue, whilst at the same time allowing the option of extended flue runs if required.

Room Sealed Fan boiler
Some fanned flue boilers are available with a 'twin tube' arrangement, which provides separate connections for the flue pipe and combustion air pipe. Such an arrangement allows considerably longer flue runs than would otherwise be possible.

Twin Flue Arrangement
Some earlier fanned flue boilers were able to adopt a 'Vertex' flue arrangement, in which the flue products are discharged vertically through the central core of the concentric flue assembly to discharge through a terminal in the roof, whilst air for combustion is drawn in from the ventilated roof space.