Supply Of Combustion Air - Drolet Fox Small Owner's Manual

When it is cold outside, the warm air in the house is buoyant so it tends to rise. This creates
a slight pressure difference in the house. Called 'stack effect', it produces a slightly negative
pressure in the lower part of the house (compared to the outside) and a slightly positive pressure
zone in the high part of the house. If there is no fire burning in a heater connected to a chimney
that is shorter than the warm space inside the house, the slight negative pressure in the lower
part of the house will compete against the desired upward flow in the chimney. This occurs for
the two following reasons:
First, the chimney runs up the outside of
the house, so the air in it is colder and
denser than the warm air in the house.
And second, the chimney is shorter than
the heated space of the house, meaning
the negative pressure in the lower part
of the house will draw cold air down the
chimney, through the stove and into the
room. Even the finest stove will not work
well when connected to this chimney.
6.6

Supply of Combustion Air

For the stove draft to work correctly, the room must have an outside or fresh air inlet, with
a minimum of 5" diameter, from the house exterior to the room, that is at least sufficient to
replenish the volume of air that comes out of the chimney flue. In well insulated houses an air
inlet must be fitted through the outer wall that is not exposed to the prevailing winds, depending
on the surrounding conditions of the house. If a vent is installed, it must be fitted in such a way
that it cannot be blocked. A fresh air intake register with a airtight damper may be installed to
help prevent any uncomfortable air draft.
When the stove and the chimney are completely cold, it may be necessary before starting up
to provide an external air supply by opening a door or a window for a short period. A house
constructed or renovated in a waterproof manner is prone to not having the air exchange required
for the proper functioning of a wood heating appliance.
In that case, avoid, during startups, to use appliances that evacuate air outside of the house,
such as:
• Cookstove hood
• Bathroom vent
• Air exchange system
The supply of fresh combustion air can be done in several ways, provided they comply with CSA
B365 and NFPA211.
In Canada, wood stoves are not required to have a combustion air supply from outside, except
for mobile homes. Research has shown that outside air supply do not compensate for the
depressurization of the house and may not be sufficient to provide a supply of combustion air in
windy weather. However, to reduce the risks against smoke spillage due to house depressurization,
Wood Stove - Owener's Manual
• Ventilated central vacuum cleaner
• Dryer
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