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.