Combustion Appliance Safety Tips

Combustion appliances using natural gas, propane, oil, kerosene, or wood are often a more efficient and cost-effective way to produce heat than electricity. Examples include:

  • Furnaces

  • Space heaters

  • Fireplaces

  • Wood stoves

  • Water heaters

  • Range tops

  • Ovens

  • Clothes dryers

Combustion appliances have been used for many years in millions of homes. However, careful installation and maintenance of each unit is required to ensure safe and efficient operation, especially in today's energy-efficient, tightly sealed homes.

Problems Posed by Combustion Equipment: Combustion appliances burn fuel by using oxygen from supply air. They produce exhaust gases that should be directly vented to the outside to avoid introducing combustion by-products into the house. Exhaust gases may be released inside the house either knowingly-as in the case of unvented stoves, ovens, fireplaces, or space heaters-or unknowingly from leaky flues, cracked heat exchangers, or back-drafting. Three components of exhaust gases are especially troublesome when introduced indoors: carbon monoxide, water vapor, and nitrogen oxides.

Carbon monoxide (CO)is a toxic gas that is colorless, tasteless, and odorless. It can cause serious medical problems and is the cause of hundreds of deaths in U.S. homes each year!! CO is produced when insufficient combustion air is supplied to the appliance, the burner is improperly tuned, and/or the appliance is malfunctioning. See Figure 12.

Water vapor is present in large amounts in exhaust gases as a result of burning all fuels. When water vapor is introduced into a house, it increases humidity levels and can lead to condensation on window panes, exterior walls, and interior surfaces of wall cavities. High humidity and wet surfaces promote mold growth, wood deterioration, and other health and structural problems. Nitrogen oxides are usually present only in small amounts in exhaust gases, but they still present a health hazard to the inhabitants.

Figure 12: Effects of Carbon Monoxide Exposure

CO (ppm)




8 hours

Maximum exposure allowed by OSHA in the workplace over an 8-hour period.


2-3 hours

Mild headache, fatigue, nausea, and dizziness.


1-2 hours

Serious headache-other symptoms intensify. Life threatening after 3 hours.


45 minutes

Dizziness, nausea, and convulsions. Unconscious within 2 hours. Death within 2-3 hours.


20 minutes

Headache, dizziness, and nausea. Death within 1 hour.


All combustion appliances should be installed by knowledgeable technicians according to the manufacturer's installation instructions as well as following all national and local code requirements. As part of the installation process, CO measurements should be made to ensure the proper operation of the units and the safety and health of the occupants. CO should be measured in the combustion products of furnaces and water heaters before the diverter or before dilution occurs, and after the burner has run continuously for 5 minutes. Levels greater than 100 ppm (air free rather than as tested) indicate adjustments are necessary; levels less than 10 ppm usually are readily achievable. Ovens should be tested while operating on the BAKE setting, and range tops should be tested with a water-filled pan over the burner. CO levels greater than 50 ppm (air free) are not acceptable.

Install a Carbon Monoxide Detector:  CO detectors are highly recommended in homes with fuel-burning appliances. The detectors signal homeowners via an audible alarm when CO levels reach potentially dangerous levels. Some models have digital readouts of current CO levels, which are useful to the homeowner to monitor household air quality, while some less-expensive models indicate varying levels of CO with differing alarms. CO detectors are either plug in or hard-wired. They should be installed in rooms with a direct connection to combustion appliances, such as kitchens with fuel-burning stoves or ovens, areas near combustion closets for fuel burning heating systems, and rooms with fuel-burning space heaters.

Causes of Back-Drafting: Back-drafting occurs when combustion gases are pulled down the exhaust flue and into the house rather than being vented to the outside. Appliances with atmospheric vents rather than powered combustion systems are most susceptible to back-drafting. Clothes dryers, ventilation fans in bathrooms and kitchens, and whole-house fans are examples of household equipment that can reduce the air pressure surrounding the combustion appliance to levels that cause it to back-draft. The potential for back-drafting increases the closer the fan is to the appliance, when several fans run simultaneously, and when the appliance cannot be isolated from the fan through the use of a closed door or other means.

Leaks in heating and cooling ducts or poorly designed ductwork can also be a major cause of back-drafting. Leaks in supply ducts can create a negative pressure in the house (the pressure in the house is less than outside) because less air is being returned to the house through the supply ducts than is being removed from the house by the return ducts. If this negative pressure is large enough, it can prevent combustion gases from rising up the flue and cause combustion products to spill into the house.

A return register that is too close to the combustion appliance (especially if it is the only return register in the house) can create a negative pressure around the appliance that draws exhaust gases into the house. A leak in a return duct or return plenum that allows air surrounding the appliances to be drawn into the return system creates a similar effect.

Negative pressures can also be created in the main part of a house, where the combustion appliance often is located, when interior doors are closed. This can occur especially when there are only one or two return registers in the house and even in homes with tight ductwork. When the doors to rooms with supply registers are closed, it may be difficult for the air in these rooms to circulate back to a central return register. The pressure in the closed-off room increases, and the pressure decreases in the main part of the house open to the central return.

Preventing Back-Drafting:The potential for back-drafting can be reduced by:

  • Using appliances with powered combustion systems rather than atmospheric vents.

  • Installing appliances with atmospheric vents in a sealed combustion closet.

  • Sealing all supply and return duct leaks.

  • Installing return registers in all rooms with supply registers.

  • Making sure rooms without a return register have at least a 1-inch gap under the door (recommended only for rooms with one supply register) or a transfer grill to provide pressure equalization between rooms.

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