HEI150 Wood Insert Fireplace Installation and Operation Manual
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9.6 Minimum Chimney
Height
The top of the chimney should
be tall enough to be above the
air turbulence caused when
wind blows against the house
and its roof. The chimney must
extend at least 1 m (3 ft.)
above the highest point of
contact with the roof, and at
least 60 cm (2 ft.) higher than
any roof line or obstacle within
a horizontal distance of 3 m
(10 ft.).
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.
