Small Vented and Unvented Combustion Space Heaters Space heaters are typically used when the main heating system is inadequate or too costly to install or operate. In some cases, small space heaters can be less expensive to use if you only want to heat a small part of a building. Space heater capacities generally range between 10,000 Btu to 40,000 Btu per hour. Some space heaters work by only radiant heat transfer, but most rely on both radiation and convection. A radiant heater does not have a fan.
Radiant heaters are generally more comfortable and quieter to operate.
Space heaters are also classified as vented and unvented. Vented units are designed to be permanently 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. The better-made vented units sometimes have a duct to bring outside air to the burner and a sealed glass cover to keep room air away from the fire. Sealed combustion heaters are much safer to operate than other types of space heaters. They are also less likely to backdraft and adversely affect indoor air quality.