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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.
Destination 1.5 Insert Installation and Operation Manual
41
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