Backdrafting (a.k.a. Combustion Gas Spillage) occurs when a naturally vented appliance loses the chimney effect, which normally carries dangerous combustion by-products (carbon dioxide, nitrogen dioxide and carbon monoxide) up the chimney. Backdrafting can occur with furnaces, fireplaces, woodstoves, and water heaters when air pressure in the house is lower than outside air pressure. Backdrafting is more likely to occur when the wind speed is too low to create suction by the Venturi effect at the top of the chimney.
Running a furnace, clothes dryer, bathroom exhaust fans, or a kitchen exhaust fan can cause the negative indoor air pressure that leads to backdrafting, as they move air out of the house, which must be replaced from the outside. If the doors and windows are all closed and very tight, the replacement air may come down a chimney and can cause backdrafting of combustion appliances.
How do we prevent backdrafting?
Try to start fireplaces and woodstoves when no other devices are removing air from the house. Once a draft is established, it is less likely to change into a backdraft, particularly in a properly vented and maintained device. If possible, have the water heater and furnace draw their intake air directly from the outside. If you see smoke coming back down a chimney, you have a problem that needs immediate attention.
An influx of combustion gases occurs whenever the exhaust system comes
under a negative pressure, and
thus negative pressure situations are the cause of combustion gas
spillage. Negative pressure occurs when indoor air pressure is less than
that of outdoor air pressure, drawing outside air into the home. Two
different types of forces work on a home and affect air pressure:
external forces and internal forces.
External Forces: those that occur naturally and usually are
determined by weather conditions. The two major external forces at work
on a home are:
[1] Wind, which can cause a positive pressure on the windward side of a
house and negative pressure on the leeward side and sides parallel to
the flow, affecting a home's pressure balance.
[2] Temperature, which changes pressure because of its effect on air
density. If temperatures vary greatly between indoor and outdoor, this
can create what is known as the "stack effect" in which cold air enters
easily through lower level openings and, once heated, rises and exits
through openings in the upper levels of the home. This creates a
negative pressure at the lower levels and a positive pressure in the
upper ones.
Internal Forces: those which occur within the home to create
pressure differences. Any device which has an air exhaust can cause
depressurization of a home, including bathroom fans, downdraft cook-top
fans, clothes dryers, fireplaces and woodstoves, and fuel fired furnaces
or water heaters.
Fuel fired appliances rely on the
natural tendency of their exhaust gases to be warmer than surrounding
air, thereby creating a natural updraft in the chimney for the gases to
escape. Whenever house depressurization occurs, the chimney must work
against the suction created by other exhaust devices. If this suction is
great enough, the flow of combustion gases may be reversed, which causes
combustion gas spillage (also known as backdrafting). An air pressure sensor can
be used to warn you
Usually the chimney will warm up after a short time, creating a stronger updraft to pull the gases out, but if the negative pressure inside the home is strong enough it will continue to spill into the home for as long as the appliance remains running. However, not all combustion appliances are susceptible to this combustion gas spillage. Those which are vulnerable to spillage problems include: naturally aspirated fuel-fired appliances and all fireplaces or wood burning stoves, even those classified as ‘airtight.’ Those not susceptible are: direct vent combustion appliances, sealed combustion appliances, and non fuel-fired appliances.
Most commonly, heating appliances will produce by-products of mainly
water and carbon dioxide. However, if the combustion from these
processes does not have enough oxygen, carbon monoxide is formed, which
can be an extreme health hazard even at low concentrations. When natural
gas furnaces are used, the carbon monoxide concentration in the exhaust
is lower, unless the gases become recycled back through the burner.
However, if heating oil is used instead of natural gas or propane, it
will commonly produce high amounts of carbon monoxide. Wood-burning
fireplaces and stoves always give off some amount of carbon monoxide,
especially during 'ember fires.'
Even if exhaust gas is not a significant problem, spillage may still
pose a health risk. When using gas and propane, a major problem is the
presence of nitrogen dioxide, which can burn and scar lung tissue even
in small quantities. Other problematic pollutants include sulphur
dioxide, which creates health risks similar to nitrogen dioxide, and
benzene, which is a harmful carcinogen produced in wood smoke.
Backdrafting and spillage are both caused by negative pressure in the home, so the two most effective solutions are [1] to try and create a neutral pressure in the home and [2] use sealed combustion appliances whenever possible. Here are some common specifics for preventing combustion gas spillage:
