Fueled Appliances
Smokey Flues
While dealing with the minimum free areas for air replacement, a not uncommon problem is that of the ‘smoky fire’ and flue gases pulled down the chimney of a gas-fired boiler when the extract units are switched on in a kitchen or public house bar.
It should be pointed out that the National Gas Boards insist on permanent air inlets in rooms in which gas boilers are fitted to ensure a sufficient supply of air for combustion, the free area of the inlet being twice the cross-sectional area of the flue pipe. For a 127mm diameter flue pipe the free area of this inlet would be 252cm² to which should be added the free area necessary for the extract unit.
The following table gives the minimum free area for grilles to suit various combinations of flue pipe and fan diameters:
Fan Impeller Dia(mm) ——————-TOTAL MINIMUM FREE AREA OF GRILLE
————————————————–IN CM.sQ. WITH FUEL DIA. OF
————————————————-100mm— 125mm— 150mm— 200mm
150———————————————- 425——- 515—— 620——-905
230 ———————————————-815 ——-905 ——1005—– 1290
300——————————————— 1460 ——-1550—— 1650—- 1935
The same method can be used in any room with exhaust units fitted if there is a complaint of ineffectivenesss of the system. An extractor fan is not a vacuum pump and will only work if there is an adequate air supply to match the volume extracted. If this replacement air is not available, the fan will work at a reduced rate or possibly even run stalled, i.e., without actually moving any air.
However most fans are powerful enough to overcome the natural updraught in an open-flued combustion appliance and pull the product of combustion back into the room. This can lead to carbon monoxide poisoning which is fatal. The Building Regulations Approved Document F1 (1995 Edition) addresses this.
Interaction of mechanical extract ventilation and open-flued combustion appliances
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 BS5440 : Part 1, Clause 4.3.2.3 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), PO Box 37, Bishop’s Cleeve, Gloucestershire, GL52 4TB.
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:
(a) the solid fuel open-flued appliance is a primary source of heating, cooking or hot water production.
. or
.
(b) 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.
Source:Vent Axia Ventilation Hand Book