A space heater is a device for heating an enclosed area. Space heating is generally employed to warm a small space, and is usually held in contrast with central heating, which warms many connected spaces at once. Permanently installed space heaters may burn natural gas, propane, fuel oil, or wood pellets; use electricity for resistance heating; or use a heat pump that can also provide air conditioning. For portable use, electric heaters are most appropriate because gas heating can be very dangerous without a permanent flue.
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Types of heaters
Space heaters can be divided into those that transfer their heat to the required space primarily by convection, or by radiation.
Convective heaters
In convection heaters, heating elements either warm the air directly or heat oil or another filler, which in turn transfers heat to the air. The air then warms the objects and people in the space. Convective heaters are suitable for providing constant, diffuse heat in well-insulated rooms. Oil heaters warm up slowly but do not reach dangerous surface temperatures; wire-element heaters, which may be fan assisted, produce warm air more quickly but may pose a fire hazard. One particular type of convective heater is the fan heater.
Radiative heaters
The main advantage of radiative heaters is that the infrared radiation they produce is absorbed directly by clothing and skin, without first heating the air in the space. This makes them suitable for warming people in poorly insulated rooms, or even outdoors. It also allows for greater distance between the people and heater.
Some of the earliest electric heaters were radiative, consisting of Nichrome heating wires held by ceramic or mica insulation at the focal point of a (usually) polished metal reflector. The cost was very low since nothing else, not even a switch, was needed. Later models included a wire guard preventing accidental contact with the heating wires or the hot ceramic.
However the metal reflectors needed to be fairly heavy gauge, and if the size was reduced then the metal housing would get too hot to be safe. In the mid-20th century the cheapest heaters were radiative, but with the heating wires stretched relatively closely across a larger thin metal reflector separated from a thin metal housing. A small fan blew just enough air between the housing and the reflector to cool them; the main output to the room was the radiative heat and not the heated air. Having the heating wires stretched across a larger area meant that fewer expensive ceramic insulators were needed, and the small fan was cheaper than a larger or heavier housing.
Quartz heaters are radiative heaters which were more efficient, in the amount and directionality of heating, by using coiled heating wire inside unsealed quartz tubing. The wires could thus operate at a higher temperature than practical with ceramic-supported wires or be thinner. If the heating elements were at a higher temperature, then proportionally more energy was radiated compared to open-wire heaters.
Halogen heaters comprise tungsten filaments in sealed quartz envelopes, mounted in front of a metal reflector in a plastic case. They operate at a higher temperature than Nichrome wire heaters but not as high as incandescent light bulbs, radiating primarily in the infrared spectrum. They convert up to 86% of their input power to radiant energy, losing the remainder to conductive and convective heat. The halogen cycle reduces the darkening of the quartz envelope and lengthens the life of the filament.
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Power sources
Many of these space heaters, including those of oil-filled radiators and natural stone heaters, are plugged into an electric power source, most commonly a mains outlet. The power ratings of appliances are measured in kW, which allows an easy estimation of operation cost per hour, as energy is billed in kWh.
Safety issues
The two primary health risks from heaters are the risk of fire and the risk of carbon monoxide poisoning. The latter risk applies to gas and kerosene heaters but not electric heaters. The risk of fire from heaters may be mitigated by low surface temperatures (as found on oil-filled convective heaters), or by switches that cut power in the event of the device inadvertently being tipped over (often found in the bases of halogen heaters), or by thermal cut-out switches. Natural Stone Heaters do not pose a fire or carbon monoxide poisoning risk, and can be a safer alternative. However, the surface temperatures of the stone heaters can be considerable, though they may not cause an instant burn as the heat transfer is slow. For that reason they are usually mounted high on walls or ceilings, away from the reach of infants.
Within the United States, Underwriters Laboratories maintains standards UL 1278 for portable electric space heaters, and UL 1042 for portable and fixed baseboard electric heaters. The General Services Administration used to maintain Specification W-H-193 for electric space heaters, but this was cancelled in 1995 in favor of the UL standards. Additional information on portable heater safety may be found at the Department of Energy Energy Efficiency website. Also, the "EPA does not currently label space heaters as an ENERGY STAR qualified product...".
Source of the article : Wikipedia
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