Fl _ Section 1 — Domestic buildings

Interaction of mechanical extract
ventilation and open-flued
combustion appliances

1.8 _ 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 advice 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), P0 Box 37, Bishop’s Cleeve, Gloucestershire, GL52 4TB.

Alternative approaches  
1.9 _ As an alternative to paragraphs 1.2 to 1.7 the requirement will be satisfied by following the relevant recommendations of:

a. _ BS 5925: .1991 Code of practice for ventilation principles and designing for natural ventilation. The relevant clauses are 4.4, 4.5, 4.6.1, 4.6.2, 5.1, 6.1, 6.2, 7.2, 7.3, 12 and 13; or

b. _ BS 5720: 1979 Code of practice for mechanical ventilation and air-conditioning in buildings. The relevant clauses are 2.32.1,,,, and 3.2.6; or

c. _ BS 5250: 1989 Code of practice for the control of condensation in buildings. The relevant clauses are 6, 7, 8, 9.1, 9.8, 9.9.1 to 9.9.3 and Appendix C; or

d. BRE Digest 398 Continuous mechanical ventilation in dwellings: design, installation and operation. For the design of either:
— continuous balanced (supply and extract) mechanical ventilation to be provided throughout the dwelling; or
— continuous mechanical extract ventilation to be provided in kitchens, utility rooms, bathrooms and sanitary accommodation.