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Air Starvation |
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All fires must have oxygen to
burn. Air contains approximately 20% oxygen. Therefore five times as much air
is needed than the oxygen required for proper combustion. Additionally, open
fires also take considerable quantities of air to vent the smoke up the
chimney. Modern homes tend to have solid
floors, tight fitting doors and draught stripped windows. Lack of ventilation
to the room is therefore a common cause of smokiness, more particularly in
modern or modernized homes. Trying to burn either solid
fuel or gas with insufficient ventilation will have two effects: If insufficient oxygen is
available to the fire, incomplete
combustion will take place. In simple terms, the products of complete
combustion are Carbon Dioxide and water vapour. Lack of oxygen however will
produce Carbon Monoxide, an odourless and highly poisonous gas. Secondly, lack of ventilation
will mean insufficient air is available to replace that being drawn up the
flue and needed to clear the smoke and fumes from the fire. Result, smokiness
or spillage of the fumes, including the Carbon Monoxide, into the room. THIS IS A VERY
DANGEROUS SITUATION. If the fire works well when the
room door is open, but smokes when the door is shut, the problem is air
starvation. This is not a fault in the construction of the fireplace or flue,
but a lack of room ventilation. Ventilation requirements are laid down in the
Building Regulations part J and the Gas Safety Regulations 1984, (and BS 5440
pt 2 for gas fires). To cure this problem,
additional air must be brought into the room, preferably without introducing unacceptable cold
draughts. Either vent directly through an outside wall, or vent into the hall
(or a conservatory) and then to outside. If the fire works well with the door
to the hall open, this has proved that there is sufficient ventilation from
the main house. Also, a vent from the hall to outside is usually more
acceptable than a vent from the living room
direct to outside. If the room has a suspended wood floor with air
bricks round the outside of the house, then a simple floor grille cut into
the floorboards to one side of the hearth or chimney breast is a good solution.
Grilles must have a sufficient free open area. Aim for half the cross section
of the flue as a minimum. Do not vent air up immediately in front of the fire
opening. |