Unvented Heating Appliances

General Information

During the long cold winter months, Maine residents rely on a variety of strategies to heat their homes to a comfortable level. In addition to the use of a furnace that provides heat to the entire home, a variety of supplemental heating units are available to consumers to do spot heating of specific areas. These supplemental units can be split into several categories: electric, vented wood stoves, vented fossil fuel devices, and unvented fossil fuel heating devices.

This fact sheet only addresses unvented fossil fuel heating devices. Unvented heating devices are appliances that are not vented to the outdoors. The fossil fuel typically used in these devices is propane.

Effect on the Indoor Environment

The major pollutants released during combustion are carbon monoxide, nitrogen dioxide, sulfur dioxide and particulates. Carbon monoxide is an odorless, colorless, tasteless, and nonirritating gas that can be deadly. The manufacturer’s instructions for unvented heating appliances should indicate the need to provide for make-up air when the heating appliance is in use. This means that property owners need to ensure that enough fresh air is entering the home so that the combustion byproducts do not reach unhealthful levels. Homeowners often struggle with the concept of opening a window in order to provide for the safe operation of their supplemental heating device. During cold days, owners are tempted to close up the make-up air, thus increasing the amount of carbon monoxide in the building.


Another significant byproduct of the fossil fuel combustion process is water. Unvented fossil fuel heating devices will release a large amount of moisture into the room. In the combustion process, every pound of fuel burned puts approximately two pounds of water into the indoor air. If heating devices are located in poorly vented rooms or where circulation is limited, the moisture in the air can condense, become trapped, and cause mold, rotting wood, and cause the attic insulation to get wet. If severe enough, the condensation can drip through and damage the ceiling below. Homeowners are urged to provide sufficient ventilation so that moisture in the home does not reach unhealthful levels. This can be done by opening a window ½ inch and monitoring the relative humidity. A good rule of thumb: maintain relative humidity between 30-50%.

Health Effects

Mildew, mold, and dust mites, as well as other pests or microorganisms will increase in number in moist warm areas. The presence of these contaminants can be of concern, particularly for those with allergies, asthma, or chronic lung diseases. The Maine Indoor Air Quality Council recommends that relative humidity is 30 to 50 percent. Humidity can be checked by using a hygrometer. Hygrometers are available through most hardware and electronics retail stores.

Carbon monoxide, while colorless and odorless, can be deadly. Carbon monoxide detectors are available at any retail outlet that sells smoke detectors. The Maine Indoor Air Quality Council recommends that both a functioning carbon monoxide detector and a functioning smoke detector be located in any area using a combustion heating appliance (vented or unvented). Evacuate the home immediately and contact your local fire department if either of these devices is activated. Contact the Maine Poison Control Center if carbon monoxide poisoning is suspected.

Protect Yourself from Adverse Health Effect

In summary, property owners should protect themselves from the risk of adverse health effect from the use of these appliances by:

  • Adhering to the manufacturer’s instructions for the safe operation of the unit.
  • Providing for sufficient ventilation while the unit is in operation.
  • Purchasing, installing, and using carbon monoxide detectors in the home (vacate home immediately and contact your local fire department if detector alarm goes off.)
  • Monitoring relative humidity in the home.

Maine specific resources and contacts

Maine Bureau of Health Indoor Air Program, (207) 287-5743

MIAQC Fact Sheet on Combustion By-Products

Northern New England Poison Control Center

If you find someone who is unconscious, is having convulsions or any difficulty breathing, call 911. If you believe someone has been poisoned, remain calm and call the poison center toll free: 1-800-222-1222

Web Links for additional information.

U.S. Consumer Product Safety Commission. “The Senseless Killer”. 1993. GPO Publication No. 1993-0-356-764. [https://www.epa.gov/indoor-air-quality-iaq/indoor-air-pollution-ntroduction-health-professionals ]