Optima Installation and Operation Manual
36
There are two reasons why the
chimney in the house at right will cold
backdraft when it is cold outside and
there is no fire burning in the stove.
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
low in the house will pull outside air
down the chimney, through the stove
and into the room. Even the finest
stove
will
not
work
well
when
connected to this chimney.
8.5
Supply of Combustion Air
In Canada, wood stoves are not required to have a supply of combustion air from outdoors (except in
mobile homes) because research has shown that these supplies do not give protection against house
depressurization and may fail to supply combustion air during windy weather. However, to protect
against the risk of smoke spillage due to house depressurization, a carbon monoxide (CO)
detector/alarm is required in the room in which the stove is installed. The CO detector will provide
warning if for any reason the wood stove fails to function correctly.
8.5.1 Air Supply in Conventional Houses
The safest and most reliable supply of combustion air for your wood stove is from the room in which it
is installed. Room air is already preheated so it will not chill the fire, and its availability is not affected
by wind pressures on the house. Contrary to commonly expressed concerns, almost all tightly-sealed
new houses have enough natural leakage to provide the small amount of air needed by the stove.
The only case in which the wood stove may not have adequate access to combustion air is if the
operation of a powerful exhaust device (such as a kitchen range exhaust) causes the pressure in the
house to become negative relative to outdoors.
Some jurisdictions in the United States require that wood stoves have a supply of combustion air from
outdoors. If you do install an air supply through the wall of the house, be aware that its pressure can
be affected during windy weather. If you notice changes in wood stove performance in windy weather,
and in particular if smoke puffs from the stove, you should disconnect the outdoor air duct from the
stove and remove the duct. In some windy conditions, negative pressure at the duct weatherhood
outside the house wall may draw hot exhaust gases from the stove backwards through the duct to
outdoors. Check the outdoor air duct for soot deposits when the full system is cleaned and inspected
at least once each year.