Vented vs. Unvented Space Heaters
Electric space heaters that plug in and are ready to go are the simplest way to bring supplemental heat to a home, office, or workshop. However, if you are considering a portable combustion heater that burns natural gas, propane, kerosene, or oil, you must also pay attention to the issue of venting. Be aware that Consumer Reports does not recommend any of these heater types for indoor use, except in an emergency, and that unvented kerosene heaters in residences are illegal in some states and local jurisdictions.
What is Venting?
Vents provide a path for the combustion exhaust to safely exit the home so toxic chemicals aren't trapped inside and air quality remains high. Furnaces have vents, as do wood burning stoves and other small room heaters that are installed on an outside wall. Chimney flues serve a similar function.
Without vents, nitrogen oxides, carbon monoxide, water vapor, and various chemicals that result from burning fuel can be trapped inside and have harmful effects on people in the building. Combustion heaters also add moisture to the air which if not vented out can lead to mold, rot, and bacteria which can damage the house and potentially cause health problems for its occupants.
Many portable space heaters that use gas, oil, or kerosene are marketed as "vent-free" or "ventless;" their use is controversial -- and sometimes illegal. Be aware of the legal requirements for your location before purchasing an unvented heater. Good places to start investigating include your local fire department, consumer safety agencies, or construction and building inspection agencies in your state and town. If your state has a Department of Public Safety it may also be a good starting point for determining your local laws related to space heaters.
In addition to potential legal issues with using unvented space heaters, some insurance companies negate claims if an illegal unvented space heater was in operation at the time of a fire - even if the space heater has nothing to do with the cause of the fire. Clearly check any relevant insurance policies before purchasing an unvented gas or kerosene heater.
While some combustion heaters are marketed as space heaters, they are not the portable variety that most consumers think of when looking for supplemental heat. Instead, vented heaters can be smaller than a household furnace and heat just a room, but they will require permanent placement next to or into an exterior wall and the construction of a flue or vent leading to the outside of the house. If such vents do not already exist, hiring a contractor to build them can greatly increase the cost of the heater.
Even with an existing flue, a professional should retrofit the new heater to the older vent system. Be aware that the National Fuel Gas Code (NFPA 54) issued by the National Fire Protection Agency recommends that an entire vent system is upgraded to meet current codes if any new elements are added. Adding a new vented heater to an existing home, especially an older house, may require additional work on other flues and vents in the house before the new work passes inspection.