Fire places and Other Wood Burning Appliances. To burn wood a source of
oxygen is required. For most homes this source is found inside the room occupied by
the appliance (see Fig. lA) .The rate at which air is drawn into the firebox depends
on three things:
1. The size of the appliance.
2. The stage of the burn cycle.
3. The control decisions of the operator.
During the early stages of the burn cycle, a large amount of air is needed for
efficient combustion. It is not unusual for a moderately sized wood heater to
require over 40 c.f.m. of air. If the home is typical and inadequately tightened,
this means 40 c.f.m. of cold and often dry outside air will be drawn into the house.
In fact, the normal wood heating installation will, over the entire burn cycle,
cause a net energy loss to the home through induced infiltration. The house will
feel overly warm for a short while, but by the time the last ember has died, all the
energy will be lost up the chimney. If, on the other hand, the home is properly
weatherized, the stove will be starved for air and creosote will form and back
drafting of smoke into the house will occur.
How then are the problems of inefficiency and indoor pollution handled?
There are two minimum requirements for a safe and efficient installation. First, a
source of combustion air must be brought in from outside the structure. This is
surprisingly easy in most cases be they cook stoves, masonry fireplaces, or freestanding
units. Figures IB and 2 show simple through the floor and wall strategies
that work for most freestanding units. Figure 3 depicts an unusual set of
circumstances encountered only once in the hundreds of installations I have
inspected. The intake vent penetrates the roof and is finished by a standard roof
jack vent.