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OPERATION GUIDELINES
in particular around windows and doors, penetrations cre-
ated for pipes and wiring as well as fireplaces, water heat-
ers and furnace flue pipes. If a person would measure the
flow of air inside a typical home, he would be amazed at
the amount of movement that occurs even though the
condition is not generally noticeable.
Based on the 'stack principle', the house itself acts as if it
were a short, fat chimney with air moving from the bottom
of the structure to the top.
Typical Scenario
If we take an example of a typical home with its natu-
ral stack effect and add weather proofing which makes
a structure even tighter, place vents in the structure for
exhaust fans for bathrooms and stoves, add a large attic
fan plus flues required for venting water heater, furnace
and fireplace gases, we create an opportunity for a great
air tug-of-war.
During winter months, the air within the home is generally
much warmer and less dense than the outside air and,
as a result, the cooler outside air seeks ways to work into
the structure. The easiest link to the interior of the home
is generally through flues and other openings connected
directly to the inside.
A typical comment comes from the person who states,
"I don't seem to get a good draft and when I open my
damper, cold air rushes in." In this case you are imme-
diately tipped off to the probable cause of the problem
- depressurization. The flue system is simply a conduit
through which incoming air is drawn into the home to
equalize pressure.
87D0174
SSTB11 Woodburning Fireplace
Depressurization is a contributing factor to back drafting
75% of the time and is the sole cause of back drafting
40% of the time. The greater the exhaust capacity and
the tighter the building envelope, the more a house will
depressurize.
The combination of forces of exhausting air from the house
can be so strong that it is possible for hot flue gases from
the fire in the fireplace to be overpowered resulting in a
down draft during or at the end of the fire cycle. The com-
peting factors are the exhaust devices located throughout
the home which work in concert with the natural draw of
the house causing a depressurized environment pulling
at the bottom of the chimney, thereby reducing the net
chimney draft.
Air is pulled down the chimney where it provides make-up
air (replacement air). The cool back drafting air coming
down the chimney will not allow all of the hot combustion
gases to rise up the chimney, therefore, chimney drafting
is curtailed resulting in the escape of gases and smoke
into the living area.
It doesn't take a lot of activity to create a depressurization
effect. For example, a typical gas furnace requires 1,450
cubic feet of air per hour for proper combustion and an-
other 4,000 cubic feet of air per hour to maintain a draft.
All this air has to come from somewhere.
A surprising amount of outdoor air manages to leak into a
house by infiltration but the point is, infiltrating air can not
be depended upon as a reliable source of combustion air.
Add to this factor a fireplace that needs combustion air
to operate in a properly insulted, air tight home and you
quickly realize how a problem can occur.
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