Carbon Monoxide Poisoning: Unvented Gas Space Heating Appliances (AEN-204)

An unvented heating appliance, also called a vent-free appliance, has no chimney vent and is located in the space being heated. Combustion products are discharged into the heated space rather than exhausted to the outdoors through a chimney. Typical fuels are natural gas and LPG (propane). While convenience is the major advantage of vent-free gas heating products, the degradation of indoor air quality is a concern. In some areas state and local codes regulate unvented heaters.

What are some of the advantages of vent-free heating products? Because no vent pipe or chimney is required vent free heaters are easy and inexpensive to install. Many heaters do not require electricity allowing them to be used for emergency heat. Most are quiet, easy to use, and efficient. In most locations in the country gas is a relatively inexpensive fuel. Many heaters are designed to create a comfort zone in a single room, reducing heating costs.

What types of vent-free heating appliances are available? Small portable units for use in a single room are manufactured for emergency heating–others are made to serve as a permanent heat source. Optional blower kits and ductwork allow them to heat two or more rooms. Available styles include cabinet mounted units, wall mounted units, fireplaces, and fireplace inserts with a variety of flame patterns including blue flame, infrared, yellow flame, and glowing logs. Sizes range from under 5,000 Btu/hour to over 38,000 Btu/hour.

How efficient are direct vent heating appliances? While the flame burns with over 99 percent efficiency not all the heat is available. Approximately 9.6 percent of the heating energy available in gas comes from burning the hydrogen which produces water vapor. To recover this heat, the water must be condensed. With an unvented heater, the water vapor condenses on room surfaces, which increases the potential for wood rot, peeling paint, plaster failure, and mold growth. To prevent wet windows and walls, the vapor must be prevented from condensing, reducing the maximum efficiency of the heater to 90.4 percent.

Unvented heaters release pollutants into the house. Once combustion pollutants are in the house outside air must be provided to dilute and remove the contaminants. The additional ventilation needed to reduce pollutants to tolerable concentrations depends on the tightness of the building and the health of the occupants. Heating cold outside ventilation air reduces the thermal efficiency of the heater. The National Fuel Gas Code, commonly used in determining outside air requirements, is based on 1.0 air change per hour (ACH). Providing 1 ACH to a 10 x 20 foot room in Iowa would require over 1,000 Btu/hour, which is over 25% of the output of a 3800 Btu/hr heater.

What pollutants are released into a room when an unvented heater is used? The main combustion products formed when gas burns are:

  • Carbon dioxide (CO2), a non-toxic gas formed during complete combustion of carbon based fuels.
  • Carbon monoxide (CO), a toxic gas formed if combustion is incomplete.
  • Nitrogen dioxide, a toxic gas.
  • Water vapor.

What about carbon monoxide risks? CO, a colorless, odorless, tasteless, non-irritating poison, is highly toxic and can cause death or permanent brain and organ damage. CO poisons more people than all other poisons combined. When properly maintained and adjusted, gas heaters produce low amounts of carbon monoxide. One cause of carbon monoxide poisoning from unvented heaters– incomplete combustion caused by lack of air–has been virtually eliminated in newer heaters by use of Oxygen Depletion Sensors (ODS). Unfortunately, the ODS does not respond to incomplete combustion caused by improper gas pressure; dust, dirt, or rust on the burner; incorrect placement of artificial logs in a gas fireplace; or disruption of the burner by air currents. CO poisoning from unvented heaters remains a concern.

What is the hazard of carbon dioxide? CO2 is not toxic. At high concentrations it can cause sleepiness, headache, and contribute to the “stuffy” feeling of closed houses. All unvented gas heaters, even when burning properly, produce large concentrations of carbon dioxide, raising the levels of in the house. What is the hazard of nitrogen dioxide? NO2 is a toxic gas that at lower concentrations is an upper respiratory irritant causing cough, sore throat, headache, vertigo, and nausea. Some NO2 is always produced in a burning flame, the amount depending on burner size and design. NO2 is linked to an increase of asthma which occurs in winter.

What is the hazard of water vapor? If a home is too dry, water vapor is not a hazard and some operation of an unvented heater will provide needed moisture. If the home is already too wet, any operation of an unvented heater will increase the moisture problems. The amount of water produced from burning a fuel is large, with over 4.8 gallons produced in 24 hours of operation of a 28,000 Btu/hour natural gas heater.

Can the health hazards of an unvented heater be reduced? The most effective method to reduce the hazards is to discontinue use of the unvented heater by switching to vented gas or electric appliances. Where unvented gas appliance use is permitted the following are suggested:

  1. Use only approved gas heaters with ODS pilots.
  2. Follow all operation and maintenance instructions carefully.
  3. Clean the burner yearly, or more often, as required in the owners manual.
  4. Do not use an oversized heater. The Gas Appliance Manufacturers Association (GAMA) recommends limiting the amount of pollutants by correctly sizing the heaters. A 3840 Btu/hr heater is the largest that should be used in a tight 10 x 20 foot room located in Iowa.
  5. Do not operate for more than 4 hours at a time. Unvented gas heaters are designed for supplemental use only.
  6. Do not use unvented heaters in bedrooms, bathrooms, or confined spaces.
  7. Provide adequate ventilation, as required in the owner’s manual. If the home has weatherstripped doors and windows an outside air source will likely be required.
  8. Provide even more ventilation, or discontinue unvented heater use, if the pollutants cause health problems.
  9. Install a U-L or IAS listed carbon monoxide detector. Because low concentrations of carbon monoxide can cause health problems, purchase a detector advertised as a “sensitive” detector or one with a digital display.

Will a “properly sized” heater be large enough to heat the room in cold weather? Unvented heaters are meant primarily as supplemental heat sources. A GRI study conducted for the Gas Appliance Manufacturers Association noted that “it may not be possible to create a comfortable temperature” with a correctly sized heater. Alarger heater, in cold weather, will pollute the air beyond allowable standards.

What type of heater is recommended? A vented gas heater, a direct-vent wood stove or an electric heater will all provide enough output to safely heat a room for long periods of time. Combustion products from vented gas heaters or woodburning fireplaces also contain CO, CO2 , NO2, and water vapor. Improper design or installation can allow combustion products to enter the house. However, if the vent system operates correctly, all the pollutants will escape to the outdoors through the vent pipe. Pollutant levels will not increase.

Prepared by
T.H. Greiner, Ph.D., P.E.
Extension Agricultural Engineer

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File: sep98\AEN-204