The open flue is the simplest of all flues. Because heat rises, it relies on the heat of the flue gases to create an up draught. There are two different types©

• natural draught

• forced draught.

This boiler type draws its air for combustion directly from the outside through the flue assembly used to discharge the flue products. It is inherently safer than an open-flue type, since there is no direct route for flue products to spill back into the room. Natural draught room-sealed appliances have been around for many years and there are still many thousands in existence. The basic principle is very simple – both the combustion air (fresh air in) and the products of combustion (flue gases out) are situated in the same position outside the building. 

The products of combustion are evacuated from the boiler through a duct that runs through the combustion air duct – one inside the other.©

Fan-assisted room-sealed appliances work in the same way as the natural draught flue, with the products of the combustion outlet being positioned in the same place (generally) as the combustion air intake. But, there are two distinct differences: 

• The process is aided by a fan, which ensures the positive and safe evacuation of all combustion products and any unburned gas that may escape.

• The flue terminal is circular, much smaller and can be positioned in many more places than a natural draught flue.©