Also known as conventional flue boilers, these can be a back boiler, wall mounted or free standing. Open flue boilers are less efficient and in certain situations dangerous. An open flue boiler must sacrifice some efficiency in order to vent unwanted products of combustion from the house. By allowing flue gases to retain high temperature, the natural force of rising hot air creates chimney draft, which contain water vapour, carbon dioxide and oxides on nitrogen: all products of combustion. At the same time, air from the room is drawn into the burner to maintain combustion. In most cases it will be the heated air.
If there is insufficient air for the combustion process , poisonous carbon monoxide will be produced at the burner. When there is down draft in the chimney this poisonous gas will be carried into the room.
A balanced flue consists of two ducts, one in side the other. The fresh air for the combustion is taken from the outer duct and the products of combustion is taken out by the central duct to out side the house. Hence the balanced flue terminal may only be fitted on an external wall. The combustion is independent of any air supply within the room, these boilers are often called room sealed.
Balanced flue are normally less than a metre, hence they are more efficient than open flue boilers.
Most boiler now are fan assisted. They have small cross sectional flue ducts. The flue fan provides the fresh air to the burner and also removes the products of combustion from the combustion chamber. The combustion is independent of any air supply within the room. Hence these are room sealed boilers. There is no need for the flue gases to have thermal energy to drive the gases out, these boilers are more efficient than the balanced flue.
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