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CARBON MONOXIDE POISONING: Downdrafting (Backdrafting)


ISU Extension Publication #: AEN-165
Author: Dr. Thomas Greiner, Dept. of Agricultural and Biosystems Engineering
Iowa State University
Date: 7/96

What is Carbon Monoxide?

Carbon monoxide (CO) is a colorless, odorless, tasteless, deadly gas. You can't see, smell or taste it. Carbon monoxide is slightly lighter than air and quickly spreads throughout an entire house.

What causes carbon monoxide production?

Carbon monoxide gas is produced when fossil fuel burns incompletely because of insufficient oxygen. During incomplete combustion, the carbon and hydrogen in the fuel combine to form  carbon dioxide, water, heat, and deadly carbon monoxide. In properly installed and maintained appliances gas burns clean and produces little carbon monoxide. Anything that disrupts the burning process  burners or results in a shortage of oxygen or burners, can increase  carbon monoxide production. Wood, coal, and charcoal fires always  produce carbon monoxide, as do gasoline engines.

What kinds of appliances and equipment can produce carbon monoxide?

Any appliance or implement which burns a fossil fuel. Examples include:

  • Gas and oil furnaces, boilers, and water heaters
  • Gas, oil, and kerosene space heaters
  • Gas and wood kitchen ranges, ovens and fireplaces
  • Gas lawnmowers, snowblowers, chainsaws, weedeaters, generators and water pumps
  • Cars, trucks, motorcycles, mopeds
  • Charcoal grills, candles, gas lanterns and tobacco smoke
  • Smoldering wood or coal fires or glowing charcoal

What causes carbon monoxide not to vent to the outside?

Incorrectly installed venting systems and chimneys, chimneys plugged by bird nests or tree leaves, deteriorating chimneys, chimneys too short to vent correctly, appliances with no venting system, and house air flow patterns and downdrafting can all cause vent failure.

What is downdrafting (backdrafting)?

Downdrafting, also called backdrafting, is flow reversal in a  vent pipe or chimney. Outside air enters the house through the vent.  During downdrafting, hot combustion gases spill into the house instead  of flowing up the vent. Anything that moves air out of a house and  depressurizes the house can cause downdrafting. This includes exhaust fans, any vented heating appliance, fireplaces, even holes in the  ceiling. Vents in tight homes are especially susceptible to downdrafting.

How does a chimney or vent work?

Warm air is lighter than cold air and normally rises. A  chimney relies on this buoyant effect to carry combustion gases out  of the house. Unfortunately, buoyant forces are weak and can easily be overcome by other air movements, such as those created by bathroom exhaust fans, clothes dryers, and air leaks in the house. Also high  winds around the outside of the building or a chimney which is too short can create a downdraft. Downdrafting usually disappears as the  appliance warms up. It can, however, last for several hours.

Is making the vent taller the best way to fix a  backdrafting problem?

Sometimes. Taller chimneys do have a stronger draft. Short chimneys are sometimes downdrafted by wind blowing against the roof.  If the downdrafting problem is the result of depressurization or faulty vent design, though, extending the chimney will not help.

How can air leaks downdraft a vent?

All air moving out of a house must be replaced with air leaking into the house. Holes located high in the house, like those in the ceiling on the top floor, allow warm air to leak out. Holes low in the house, like those in the basement, allow cold air to leak in. The high leaks tend to "depressurize " the house and increase the risk of downdrafting.

Does "tightening up" houses contribute to  downdrafting?

Yes. Tighter houses have fewer holes leaking air into the  house. With fewer holes, appliances may not have sufficient air for combustion or for venting. Venting failures, along with less fresh air, increase the risks of carbon monoxide poisoning.

Can the furnace fan contribute to depressurization and downdrafting problems?

Definitely. The air handling blower on a furnace is powerful--much more powerful than the natural draft of the chimney. Improperly  installed or leaky ductwork on the air return side can downdraft the chimney. In an Iowa case a family was hospitalized with carbon monoxide poisoning because they had removed the cover on the return side of the blower.

Does adding combustion air pipes to the furnace room solve the problem?

Not always. Combustion air pipes to the outdoors allow additional air in most of the time, but can be overcome by wind and intermittently fail. Fan-powered combustion air kits which operate whenever the appliance burner is on force air into the furnace room for combustion and for venting and are more reliable than combustion air pipes. Fan-powered combustion kits can be professionally installed on water heaters and furnaces. Be cautious though, adding a fan to only one appliance, can increase the risks of spillage from other heating appliance vents.

What kind of changes reduce depressurization and downdrafting?

Some ways to reduce depressurization include sealing holes in  the upper portion of the house and increasing air leakage in the lower levels; adding combustion air and make-up air openings; shutting off  exhaust fans; sealing return ducts in the basement; closing return registers in the basement; opening supply registers in the basement;  opening doors between rooms; and closing fireplace dampers.

Can an air-to-air heat exchanger be used to bring in the needed air for appliances?

No. Heat exchangers are balanced systems, and exhaust the same amount of air as they bring in. They furnish fresh air for you to breath, but they do not keep the house from being depressurized.

If I suspect a downdraft problem what should I do?

Call a qualified heating contractor to check the house and the heating systems. Small temperature sensitive strips called "Backdraft Indicators" can be attached to the draft diverter to  detect downdrafting. These strips have no audible alarm and should be  used along with a carbon monoxide detector.

For more information request other notes in the Carbon Monoxide  series. Agricultural Engineering Notes (AEN's) are informally  published information releases on topics of current concern to Iowans.

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