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XTD 1.9 Insert Installation and Operation Manual
9.7 The Relationship Between the Chimney and the House
Because the venting system is the engine that drives the wood heating system, it must
have the right characteristics. The signs of bad system design are cold backdrafting when
there is no fire in the insert, slow kindling of new fires, and smoke roll-out when the door is
opened for loading.
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.
47
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