The byproducts of combustion cannot be allowed to accumulate
inside the home. Their removal must not endanger or damage the
structure or fail to replace the oxygen supply used in the process.
A properly specified and installed venting system will meet these
requirements:
Convey all combustion products to the outside atmosphere.
Prevent fire hazards due to overheating of nearby walls and
framing. Sustained high temperatures can dry out and reduce the
ignition temperatures of combustible materials near venting components.
If the ignition temperature is reached, even without the presence
of flame, the material will burn. Venting systems are designed,
tested, and listed to prevent ignition of nearby combustibles
when they are installed properly. The minimum distance they may
be safely placed away from combustibles (known as clearance to
combustibles) is specified by the venting manufacturer and must
be followed carefully to insure safety. Close clearances to combustibles
are achieved by design factors such as intervening air spaces
and dilution of flue gases (and reduction of their temperatures)
by the draft hood. NOTE: Always follow the venting manufacturer's
clearances.
Prevent damage from condensation of water vapor in the flue
gases to the appliance, vent, building, and furnishings. Burning
100 cubic feet of natural gas produces about 200 cubic feet of
water vapor. Condensation occurs when flue gases are cool enough
to reach their dew point (the temperature at which water vapor
becomes liquid). Condensed water vapor can cause problems in the
house, the appliance, and the venting system.
Provide adequate oxygen supply for the appliance and occupants
of the home. There must be adequate amounts of air available
to replace combustion, excess, and dilution air. Incomplete combustion
and consequent danger to occupants can result from inadequate
supplies of fresh air. The size and use of the area served by
the appliance, as well as the tightness of house construction,
are factors that must be considered.
When planning an installation, or performing service work in
the field, please remember that there are many causes of negative
pressure in a home, including vented appliances such as forced
air furnaces, hot water tanks, clothes dryers, bathroom fans,
and kitchen fans.
