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Interaction of mechanical extract ventilation
and open-flued combustion appliances


Open-flued appliances take their combustion air from the room or space in which they are installed and so contribute to the extract ventilation when in operation. They can also be arranged to provide adequate extract ventilation when not firing. For instance no additional extract ventilation would be necessary to satisfy the requirement if:

The solid fuel open-flued appliance is a primary source of heating, cooking or hot water production; or

The open-flued appliance has a flue with a free area at least equivalent to a 125mm diameter duct and the appliance's combustion air inlet and dilution air inlet are permanently open, i.e. there is a path with no control dampers which could block the flow or the ventilation path can be left open when the appliance is not in use

Mechanical extract ventilation can cause the spillage of flue gases from open-flued combustion appliances in dwellings whether or not the fan(s) or extract air terminals and combustion appliances are located in the same room. Such spillage of flue gases is dangerous and in dwellings where it is proposed to install open-flued appliances and mechanical extract ventilation the appliance needs to be able to operate safely whether or not the fan is running. For example with:

a. gas appliances, where the appliance and the fan are located in the kitchen, the maximum recommended extract rate is 20 litres/second. A spillage test as described in BS 5440 Part 1, Clause Should be carried out whether or not the appliance and the fan are in the same room. Where a fan causes an appliance in a different room to spill, the extract rate may be reduced to cure the problem. (Further advice is contained in BRE Information Paper 21/92).

b. oil-fired appliances, installed in compliance with Technical Information Note T1/112, which can be obtained from:
Oil Firing Technical Association for the Petroleum Industry (OFTEC),
Century House, 100 High Street, Banstead, Surrey, SM7 2NN.
For further advise contact OFTEC.

c. Solid fuel appliances, mechanical extract ventilation should not be provided in the same room. For further advice contact
HETAS (Heating Equipment Testing and Approval Scheme),
P O Box 37, Bishops Cleeve , Gloucestershire, GL52 4TB

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