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Installation and Operation Manual - Inspire 2000 Insert
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 insert and into the
room. Even the finest insert will not work
well when connected to this chimney.
Figure 34: Chimney location in the house
10.8 Supply of Combustion Air
In Canada, wood inserts are not required to have a combustion air supply from outside. 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, a carbon monoxide
(CO) detector is required in the room where the insert is installed. The CO detector will provide
warning if for any reason the wood insert fails to function correctly.
The safest and most reliable supply of combustion air for a 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.
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