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Carbon Monoxide and Combustion Safety

Carbon monoxide is the leading cause of accidental poisoning deaths in the United States. Most of these deaths can be prevented by installing carbon monoxide alarms in homes. For added safety, you can have carbon monoxide and combustion safety tests conducted in your home.

To keep combustion byproducts and other indoor air contaminants out of your home, choose sealed combustion water heaters and furnaces when replacing older models. Open-hearth wood-burning fireplaces should be replaced or retrofitted, and cracks and other openings between an attached garage and your living spaces should be sealed.

Carbon monoxide alarms

Carbon monoxide is an odorless, colorless and toxic gas produced by fuel-burning appliances such as stoves, water heaters, furnaces, unvented gas space heaters, clothes dryers and fireplaces, as well as from cars and some landscape equipment.

Carbon monoxide alarms are a simple, low-cost way to reduce the chance of death or illness from carbon monoxide exposure. Having a carbon monoxide alarm used to be voluntary, but as of July 1, 2011, single-family homes in California that have a fuel-burning heater, appliance or fireplace, or that have an attached garage, are required by law to have carbon monoxide alarms. All other homes, such as apartments and condos, are required to have carbon monoxide alarms as of January 1, 2013. Alarms should be tested regularly and replaced every three to five years, as they lose their sensitivity over time.

Carbon monoxide and combustion safety testing

Appliances and equipment that use natural gas, propane, oil, wood or other fuels produce combustion gases and other byproducts as the fuel burns. Fireplaces, water heaters, furnaces, gas or propane stoves, gas clothes dryers and other fuel-burning equipment do have flues or pipes that vent those combustion gases to the outdoors. However, under certain circumstances, those combustion gases can be drawn back down the flue and into your home. This is called combustion backdrafting, or spillage, and it creates a potential health hazard. Combustion byproducts can include carbon monoxide, sulfur dioxide, nitrogen oxides, water vapor and particulates.

Combustion backdrafting is more likely to occur in the winter, when homes are tightly closed up. Equipment with powerful exhaust fans, such as clothes dryers and kitchen range fans, can pull air out of the home, creating a pressure imbalance. As indoor air pressure falls below outdoor air pressure, the home becomes depressurized. If the home is sufficiently depressurized, outdoor air can be sucked into the home through chimneys, flues or vent pipes. If a fuel-burning appliance is operated under these circumstances, combustion gases can be drawn into the home.

To ensure that all of your fuel-burning appliances are operating safely and efficiently and not introducing carbon monoxide into your home, consider hiring a trained professional to test your home. The tester should conduct two types of tests: a carbon monoxide test that checks the operation of all gas appliances, and a combustion safety test to ensure that carbon monoxide will not backdraft into the home from an open combustion fireplace, water heater, stove or furnace.

These tests should be performed by a trained technician, such as a Building Performance Institute–certified professional (www.bpi.org). Combustion safety testing is a standard part of the basic energy efficiency and safety upgrades offered to California homeowners by Energy Upgrade California.

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Sealed combustion equipment for space and water heating

Conventional gas-burning furnaces, boilers and water heaters are open combustion devices (also known as atmospherically vented or naturally vented). These devices use air from the surrounding room for combustion, and are not ideal.

Sealed combustion units with direct venting are a safer option. They draw combustion air from the outdoors instead of drawing air from the surrounding room. Sealed combustion equipment also vents combustion gases directly to the outdoors so that they don't pollute the home. Sealed combustion equipment is designed to prevent potentially harmful combustion backdrafting.

When replacing open combustion equipment, choose energy efficient, sealed combustion models. For more information, see our GreenPointers on Heating and Cooling and Water Heating.


Burning wood in fireplaces is a major source of air pollution in the winter. In some areas, fireplaces generate up to one-third of outdoor air particulates on cold nights. In addition, conventional open hearth fireplaces suck air out of the house and send more heat up the chimney than they provide to the room. In fact, many fireplaces have efficiencies as low as 13%, yet many homeowners depend on them to meet some percentage of their heating needs.

In recent years, a number of cities and counties in California have enacted local ordinances that permit the installation of only gas-burning fireplaces or U.S. EPA-certified wood or pellet stoves. If you are planning to install a new fireplace, make sure it meets these standards, even if your community doesn't specifically require it.

There are no U.S. or state standards regulating the efficiency of fireplaces. However, efficiency listings are required in Canada, so efficiency information is available for many models also sold in the United States. If you must install a new fireplace, choose a gas-fueled model with a listed efficiency (based on data from Natural Resources Canada) that exceeds 60%. Make sure the model you choose uses combustion air vented directly into it from the outside.

If your home has a conventional wood-burning fireplace that you don't want to remove, consider retrofitting it with a gas insert that has sealed combustion; these products have much higher efficiencies than typical wood-burning fireplaces.

At a minimum, replace the old damper if it no longer seals the flue. This often happens due to mechanical failure, rust or soot buildup in the chimney. Another option is to install an inflatable fireplace plug (sometimes called a pillow or a balloon) just below the damper to reduce drafts.

For more information about retrofitting wood-burning fireplaces, see the Bay Area Air Quality Management District's website, www.baaqmd.gov; South Coast Air Quality Management District's website,www.aqmd.gov/healthyhearths; and the U.S. Environmental Protection Agency's website,www.epa.gov/burnwise.

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Reduce pollution entering the home from the garage

According to the U.S. Environmental Protection Agency, an attached garage is the biggest contributor to poor indoor air quality in a home. Car exhaust contains carbon monoxide, as well as carcinogenic contaminants that can migrate into living spaces through doors and cracks in walls and ceilings adjacent to the garage. Other pollutants commonly found in garages include gasoline, benzene from lawn mowers and power tools, pesticides, fertilizers and herbicides for gardens, toxic cleaning agents, and chemicals in paints and adhesives.

A carport or detached garage provides the most effective means of keeping garage pollutants out of the home. If your home has an attached garage, a number of improvements can be made to keep pollutants out of your home.

Use spray foam, weatherstripping and caulking to create an air barrier between the garage and living areas. Completely seal garage walls and ceilings adjacent to the interior. To be effective, this seal must be air tight. This includes:

  • Using tape and joint compound at all drywall joints
  • Painting walls and ceilings (carbon monoxide can penetrate unfinished drywall and seams)
  • Caulking the drywall at the bottom plate and at all framed openings
  • Installing a rubber/foam gasketed door frame (including side and top weatherstripping and door sweep)
  • Completely sealing all penetrations in the garage walls and ceilings using caulk or spray foam

For added protection, install an exhaust fan in the garage on the opposite wall from the door to the house. It can be triggered by a motion sensor or by an electric garage door and put on a timer to run after the door has been opened or closed.

More carbon monoxide and combustion safety tips

  • Range hood fans. If you have a gas oven or cooktop, make sure it has an exhaust fan that vents to the outdoors, and always run the fan when baking or cooking. Properly sized and operated range hood fans minimize the chance of backdrafting of fireplaces and other combustion appliances, and help remove combustion gases from gas-fired cooktops and ovens. See our GreenPointer on Kitchen and Bathroom Ventilation Fans for more information.


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