Space heaters are typically used either to supplement an inadequate existing heating system or to save energy and defray energy costs in one of the situations described above. In either of these cases, look for a heater that has a design and safety features that match your needs.
To pick a design that works well in a certain situation, it is important to understand the different types of space heaters that are available.
Radiant heaters. This type of heater provides warmth in the same way that the sun’s rays or a warm fireplace does: by emitting radiated heat in a specific direction. They can be powered either by electricity or by a fuel (typically propane, natural gas, or kerosene). Electric radiant heaters use a variety of heating elements, including quartz tubes, carbon tubes, metal coils, and halogen lamps (Figure 1). Some radiant heaters use reflectors to focus the heat; others simply allow the heat to dissipate in the direction the heater is facing. Radiant heaters are better suited to heating objects (usually people) than entire spaces, and they work best in areas with open architecture (not many doors or other obstructions). Objects must be within the heater’s line of sight to feel the heat. Over time, radiant heaters will increase air temperature as the people and objects that absorb the radiant heat directly transfer some of it to the air. Fuel-fired heaters typically require some degree of fresh makeup air for the space to avoid buildup of combustion pollutants.
Figure 1: Radiant heater
A typical radiant heater, utilizing a reflector to direct heat outward.
If you plan on being in a room for only a short period, a radiant heater is the best option for space heating. They can be more efficient over a short period than other heater types because radiant heaters avoid the energy needed to heat the entire room by instead directly warming the occupant of the room and the occupant's immediate surroundings. Radiant heaters are sometimes referred to as quartz, infrared, parabolic, ceramic, halogen, and reflective heaters.
Convection heaters. The main difference between a convection heater and a radiant heater is air flow. Convection heaters rely on the the natural pattern of rising hot air to heat a room. They are designed to heat an entire room by heating the air, rather than focusing the heat on any particular object, and so they work best in rooms that can be sealed off (usually by closing a door). One example is a portable electric baseboard-style space heater. With a baseboard heater, air circulates through the room and enters the baseboard heater on the floor. The air is then warmed by passing across the electric element in the heater and rises through the room. The cold air in the room descends to enter the heater, and the cycle is repeated. The most efficient convection heaters are oil- and water-filled heaters (Figure 2). These employ a heating element in a bath of oil or water. The oil or water remains heated while the unit is on and heats the surrounding air by way of natural air movement through the unit, similar to an electric baseboard heater. These oil- or water-filled units look like a portable radiator. Fuel-fired convection heaters are also available. Like their radiant heater cousins, they require a fresh air supply. These are often used in semi-enclosed spaces, such as a patio or barn.
Figure 2: Convection heater
A typical convection heater, using air movement through the unit to heat the room.
Combination heaters. Many space heaters employ both radiant and convection heating techniques. These combination heaters are sometimes regarded as having more flexible operating use requirements than conventional heaters. One example of this type of heater is a ceramic radiant heater that uses an internal fan to distribute the heat it generates (Figure 3). Combination heaters heat specific objects in addition to the air surrounding them, although they don't do either as well as a heater designed solely with either radiant or convection technology would. Combination heaters are also called heater fans.
Figure 3: Combination heater
A typical combination heater, with controls on top and an internal fan that circulates heat throughout the room.
Unit heaters. One type of space heater that dominates use in the commercial and industrial sectors is the unit heater, which is basically a forced-air furnace without ducts (Figure 4). It is a self-contained, permanently surface-mounted heater that can operate on either electricity or natural gas, though natural gas is the more common fuel used. As with all gas-fired combustion equipment, unit heaters must be vented to the outdoors to exhaust the combustion gas. These units require some installation work, but they can also provide a much greater amount of heating for a space than a typical portable space heater. Unit heaters are good for heating an add-on room, workshop, or garage without needing to add ductwork. They are most commonly used in warehouses, car dealerships, auto service bays, and other large open buildings. Even with their ability to create a large amount of heat for open areas, they are sometimes criticized for creating large temperature differences in a space, as well as for the noise they produce.
Figure 4: Unit heater
A typical gas-fired unit heater, with an exhaust pipe shown on the back side.
Utility heaters. These are used mainly at outdoor worksites and in other areas where spot heating is either helpful or necessary. They are generally oil- or propane-fueled, and they use a cylindrical shape to direct heating in a specific direction (Figure 5). They can be used in auto bays and other industrial areas that are often open to the elements. They are effective for outdoor worksite use where electrical outlets may not be convenient, although some units come fitted with an electric ignition that requires an outlet.
Figure 5: Utility heater
A typical oil-fired utility heater, with the fuel reservoir located in the base of the unit.
Because the efficiency of an electric space heater is essentially 100 percent, all of the electricity it consumes will be given off as heat. Though there are no federal minimum efficiency standards for gas-fired space heaters, the Energy Policy Act of 2005 (EPAct 2005) included component requirements for unit heaters that went into effect on August 8, 2008. They must now be equipped with an intermittent ignition device and have power venting or an automatic flue damper.
However, ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standard 90.1, the Energy Standard for Buildings Except Low-Rise Residential Buildings, does specify minimum efficiency levels for gas- and oil-fired unit heaters. This is significant, because 90.1 is often adopted by local building codes, either directly or indirectly through local adoption of the International Energy Conservation Code, which includes many 90.1 specifications. According to 90.1-2007, both gas- and oil-fired unit heaters must have a minimum combustion efficiency of 80 percent. Combustion efficiency is defined as 100 percent efficiency minus flue losses.