Specifications

4
As mentioned earlier, a well-designed and
well-functioning venting system is more important for
successful wood and gas burning than a good stove
or replace.
Chimneys operate based on the simple principle that
warm air rises because it has a lower density than
cold air. Warmer ue gases in the chimney create
buoyancy called “draft”, and the rising gases create
a partial vacuum at the bottom of the chimney and
appliance outlet.
What are thought to be “draft problems” may actually
be chimney performance problems and are seldom
caused by inadequate draft. They are more often
caused by poor design and caused by the chimney
itself.
There are two major factors to consider when
working with chimneys: “draft” and “ue gas volume”
(or ow). “Draft” is the force, which causes the ue
gases to be exhausted. The “ue gas volume” is the
amount of the products of combustion from the heat-
ing appliance passing through the system as a result
of the draft.
The sizing of a chimney should be based on the ow
requirements, as any chimney size has a certain
capacity in regards to the amount of ue gases it is
able to remove safely. A good analogy is drinking
straws. Blowing into a small straw is more difcult
than blowing into a large straw, and you can blow
more air through the large one. A chimney works the
same way: A greater volume of air can ow through a
large ue easier than a small one.
Two major factors inuence the chimney capacity:
“draft” and “ow resistance”. “Flow resistance” is a
result of friction and always exists between the
moving gases and the chimney through which they
ow. Bends and elbows cause ow resistance,
but even a straight pipe can cause ow resistance
through friction.
The surface of the pipe is also important. A
corrugated or rough surface causes more ow
resistance than a smooth surface.
For a venting system to work properly, the draft
MUST exceed the ow resistance. Otherwise there
will be no movement of ue gases.
2.1 Chimney Performance Problems
The principles of ow and draft apply to all heating
appliances.
Open heaters (open replaces, BBQ’s etc.) require a
large chimney capacity, as they exhaust a relatively
large volume of ue gases. They allow more
dilution air, or excess air, to be introduced. As a
result, they draw more room air, which lowers the
ue gas temperature. A lower ue gas temperature
means less draft is required for operation.
Closed heaters (stoves, replaces with doors, inserts,
ovens etc.) burn more slowly and with much less
dilution air. Thus there is less ue gas volume at
a higher temperature, so this means more draft is
required.
Open heaters are more susceptible to draft
performance problems, but the following venting
problem descriptions apply to all heating appliances:
2. Venting Problems and Possible Solutions