I have used vent-free gas heating for over 20 years with no problems. I have one in my house for emergency heat if the power goes out, a vent-free gas fireplace in the living room we use every year and a vent-free gas heater in my shop I use every year during the winter months when I need it. I also have carbon monoxide detectors in the house and workshop. They have never gone off.
In 1980, the ANSI standard for vent-free gas heating products was revised to require each unit to be equipped with an oxygen detection safety sensor (ODS). The ODS is remarkably equivalent in function/reliability to what a circuit breaker is to electrical current. The ODS automatically shuts off the unit in the unlikely situation that carbon monoxide is elevating and there is oxygen depleting in the vicinity of the unit (regardless of the CO source). The ODS is tamper resistant! If there is any attempt to override, modify, or tamper with the unit, the ODS will shut off the gas supply and disarm the ignition system and the appliance cannot be operated.
The U.S. Consumer Product Safety Commission (CPSC) has stated that it is not aware of any documented incident in the CPSC In-Depth Investigation (IDI) database of fatal CO poisoning associated with an ODS-equipped vent-free gas heating product. These appliances have earned an outstanding safety record.
In 1996 the Vent-Free Gas Products Alliance, members of GAMA, an Association of Appliance and Equipment Manufacturers, commissioned an independent research project to be conducted by the American Gas Association Research Division (AGAR). The objective was to measure the primary by-products of gas combustion from vent-free gas products against the most relevant indoor air quality standards and/or guidelines.The contributors to indoor air quality that were tested were oxygen, carbon monoxide, carbon dioxide, nitrogen dioxide, and water vapor (humidity). The researchers took into consideration the climate in the five Department of Energy heating regions in the U.S., various types of housing construction, and varied volumes of space to be heated. After running hundreds of thousands of computer based scenarios, the results were confirmed by the American Gas Association test house which was modified for all factors.
The researchers concluded that ?vent-free gas heating products performed well within nationally recognized guidelines for indoor air quality. This research proves that vent-free gas heating products meet applicable emissions requirements even when used over extended time periods, among sensitive populations, and with units whose maximum heat output exceeds the requirements of the space.