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Escape 1800-I Insert Installation and Operation Manual
43
9.7.1 Why the Chimney Should Penetrate the Highest Heated Space
When it is cold outside, the warm air in the house is buoyant so it tends to rise. This tendency
of warm air to rise creates a slight pressure difference in the house. Called ‘stack effect’, it
produces a slightly negative pressure low in the house (relative to outside) and a slightly
positive pressure zone high in 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
low in the house will compete against the desired upward flow in the chimney.
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 insert.
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 insert and into
the room. Even the finest insert will
not work well when connected to this
chimney.
9.8 Supply of Combustion Air
In Canada, wood inserts are not required to have a supply of combustion air from outdoors
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 insert is installed. The
CO detector will provide warning if for any reason the wood insert fails to function correctly.
9.8.1 Air Supply in Conventional Houses
The safest and most reliable supply of combustion air for your wood insert 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 a
small amount of air needed by the insert. The only case in which the wood insert 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.
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 insert performance in windy
weather, and in particular if smoke puffs from the insert, you should disconnect the outdoor
air duct from the insert and remove the duct. In some windy conditions, negative pressure
at the duct outside the house wall may draw hot exhaust gases from the insert 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.
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