Common Causes

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 cooktop fans, clothes dryers, fireplaces and woodstoves, and fuel fired furnaces or water heaters.

Other than negative pressure, another problems which may arise are "backdrafting" and "spillage."  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 (aka backdrafting).

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, induced draft combustion 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.

Health Effects

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.

Prevention

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:

• Ensure all fireplaces, wood stoves, and other combustion appliances have their own designated outside combustion air supply.
• Make sure all exhaust chimneys work properly and are securely sealed.
• Where possible, use double walled insulated chimneys on furnaces and water heaters.
• Try to limit the use of high powered exhaust devices, in particular cook top exhaust fans, unless directly connected to a make-up air supply.
• Chimney failure caused by mechanical problems or blockage can also be a cause of major leakage into the home.