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As the air intake is reduced, the burn rate decreases. This
has the effect of distributing the thermal energy of the fuel
over a longer period of time. In addition, the flow rate of
exhaust through the appliance and flue pipe slows down,
which increases the duration of the energy transfer of the
exhaust gases. As the air intake is reduced, the flame slows
down.
If the flames diminish to the point of disappearing, the air
intake has been reduced too early in the combustion cycle
or the wood used is too wet. If the wood is dry and the air
control is used properly, the flames should decrease, but
remain bright and stable.
On the other hand, too much air can make the fire uncontrollable, creating very high temperatures
in the unit as well as in the chimney and seriously damaging them. A reddish glow on the unit
and on the chimney components indicates overheating. Excessive temperatures can cause a
chimney fire.
5.7
Fire Types
Using the air intake control is not the only way to match the insert heat output to the desired
temperature in the house. A house will need far less heating in October than in January to
maintain a comfortable temperature. Filling the firebox full in fall weather will overheat the space.
Otherwise, the combustion rate will have to be reduced to a minimum and the fire will be smoky
and inefficient. Here are some suggestions for building fires suitable for different heating needs.
5.7.1
Flash Fire
To build a small fire that will produce a low heat output, use small pieces of firewood and load
them crisscross in the firebox. The pieces should only be 3" to 4" in diameter. After raking the
coals, lay two pieces parallel to each other diagonally in the firebox and lay two more across
them in the other direction. Open the air control fully and only reduce the air after the wood is
fully flaming.
This kind of fire is good for mild weather and should provide enough heat for up to four hours.
Small fires like this are a good time to use softer wood species and avoid overheating the house.
5.7.2
Low and Long Output Fires
For a fire that will last up to eight hours but will not produce intense heat, use soft wood and
place the logs compactly in the firebox. Before reducing the air intake, the load will have to burn
at full heat for long enough for charring the surface of the logs. The flame must be bright before
letting the fire burn by itself.
5.7.3
High Output Fires
When heating needs are high during cold weather, the fire should burn steadily and brightly. This
is the time to use larger pieces of hardwood. Place the biggest pieces at the back of the firebox
and place the rest of the pieces compactly. A densely built fire like this will produce the longest
combustion this insert is capable of. Special attention must be paid when building fires like this
since if the air intake is reduced too quickly, the fire could smoulder. The wood must be flaming
brightly before leaving the fire to burn.
Figure 14: Air Intake Control
Page 21
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