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Small space heaters are typically used when the main heating system is unavailable, inadequate, or when central heating is too costly to install or operate. In some cases, small space heaters can be less expensive to use if you only want to heat one room or supplement inadequate heating in one room. They can also boost the temperature of rooms used by individuals who are sensitive to cold without overheating your entire home.

Small space heaters work by convection (the circulation of air in a room) or radiant heating. Radiant heaters emit infrared radiation that directly heats objects and people within their line of sight, and are a more efficient choice when you will be in a room for only a few hours and can stay within the line of sight of the heater. They can also be more efficient when you will be using a room for a short period because they save energy by directly heating the occupant of the room and the occupant's immediate surroundings rather than the whole room.

Safety is a top consideration when using small space heaters. The U.S. Consumer Product Safety Commission estimates that more than 25,000 residential fires every year are associated with the use of space heaters, resulting in more than 300 deaths. In addition, an estimated 6,000 people receive hospital emergency room care for burn injuries associated with contacting the hot surfaces of room heaters, mostly in non-fire situations.

Space heater capacities generally range between 10,000 Btu and 40,000 Btu per hour, and commonly run on electricity, propane, natural gas, and kerosene (see wood and pellet heating for information on wood and pellet stoves).

When buying and installing a small space heater, follow these guidelines:

  • Newer model small space heaters have current safety features. Make sure the heater carries the Underwriter's Laboratory (UL) label.
  • Choose a thermostatically controlled heater, because they avoid the energy waste of overheating a room.
  • Select a heater of the proper size for the room you wish to heat. Do not purchase oversized heaters. Most heaters come with a general sizing table.
  • Locate the heater on a level surface away from foot traffic. Be especially careful to keep children and pets away from the heater.

Vented and Unvented Combustion Small Space Heaters

Small space heaters are classified as vented and unvented or "vent-free." Unvented combustion small space heaters are not recommended for use inside your home because of the risk of carbon monoxide poisoning.  They may also bring unwanted nitrogen oxide into the home; nitrogen oxide can cause asthma and other respiratory problems. These heaters can also introduce water vapor into the home, which can result in mold and other structural damage. Most states have banned unvented kerosene heaters for use in the home and at least five have banned the use of unvented natural gas heaters.

Vented, small space portable heaters are designed to be located next to an outside wall, so that the flue gas vent can be installed through a ceiling or directly through the wall to the outside. Look for sealed combustion or "100% outdoor air" units, which have a duct to bring outside into the combustion chamber. Sealed combustion heaters are much safer to operate than other types of space heaters, and operate more efficiently because they do not draw in the heated air from the room and exhaust it to the outdoors. They are also less likely to backdraft and adversely affect indoor air quality.

Less expensive (and less efficient) units use the room air for combustion. They do not have a sealed glass front to keep room air away from the heating elements and should not be confused with a sealed combustion heater.

In addition to the manufacturer's installation and operating instructions, you should follow these general safety guidelines for operating any combustion space heater:

  • Never use gasoline! For liquid-fueled heaters, use only the approved fuel. Follow the manufacturer's fueling instructions. Never fill a heater that is still hot. Do not overfill the heater -- you must allow for the expansion of the liquid. Only use approved containers clearly marked for that particular fuel, and store them outdoors.
  • Have vented space heaters professionally inspected every year. Improperly vented (and unvented), blocked, separated, rusted, or corroded small space heaters may emit dangerous levels of carbon monoxide (CO) and other gases into the home, causing sickness and death. Carbon monoxide can also be produced if the heater is not properly set up and adjusted for the type of gas used and the altitude at which it is installed.
  • Install a carbon monoxide detector if you use a small space heater.

Electric Small Space Heaters

Electric small space heaters can be more expensive to operate than combustion space heaters.  Although they avoid indoor air quality problems, electric small space heaters still pose burn and fire hazards and should be used with caution.

For convection (non-radiant) space heaters, the best types incorporate a heat transfer liquid, such as oil, that is heated by the electric element. The heat transfer fluid provides some heat storage, allowing the heater to cycle less and to provide a more constant heat source.

When buying and installing an electric small space heater, you should follow these general safety guidelines:

  • Electric heaters should be plugged directly into the wall outlet. If an extension cord is necessary, use the shortest possible heavy-duty cord of 14-gauge wire or larger. Always check and follow any manufacturer’s instructions pertaining to the use of extension cords.
  • Buy a unit with a tip-over safety switch, which automatically shuts off the heater if the unit is tipped over.