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8 The Venting System
8.1 General
The venting system, acts as the engine that drives your wood heating system. Even the
best fireplace will not function safely and efficiently as intended if it is not connected to a
suitable chimney.
The heat in the flue gases that pass from the fireplace into the chimney is not waste heat.
This heat is what the chimney uses to make the draft that draws in combustion air, keeps
smoke inside the fireplace and safely vents exhaust to outside. You can think of heat in
the flue gas as the fuel the chimney uses to make draft.
8.2 Suitable Chimneys
Your wood fireplace will provide optimum efficiency and performance when connected to a
6-inch diameter chimney.
8.3 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.).
8.4 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 fireplace, slow kindling of new fires, and smoke roll-out when the door
is opened for loading.
8.4.1 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.
Solution 2.5-ZC Fireplace Installation and Operation Manual
53
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