Unvented combustion heaters use indoor air for combustion and vent the combustion by-products directly into the room. They usually burn kerosene or natural gas. Kerosene burning heaters are typically portable and can be moved around where and when needed. However, most building scientists and indoor air quality professionals do not recommended ventless heaters in homes where moisture problems exist, or where small children and elderly persons live or where the heater is likely to be operated for more than 2 hours per day. Under NO circumstances should you ever install ventless heaters in airtight houses, mobile homes, or trailers.
Occupant safety is a major concern when operating an unvented combustion heater.
Poorly made, maintained, or operated units have caused fires and other combustion-related accidents. Depending upon the level of exposure, these pollutants and the lack of oxygen can cause eye irritation, headaches, dizziness, fatigue, respiratory problems, and possibly death. Newer unvented heaters have sensors intended to automatically shut off the burner when the oxygen level in the room falls below a safe level. Because of these problems, a number of states have banned the use of unvented heaters. States where these appliances are legal suggest opening a window slightly to provide fresh air.