Forced-air gas heating systems are used in central air heating/cooling systems for houses. Sometimes the system is referred to as "Forced hot air". Generally, they have a furnace heated by natural gas that pushes hot air through duct-work, then through vents to heat a building. A climate-control device (e.g., thermostat) regulates and controls the usage of the furnace. A digital thermostat can be programed to activate the gas furnace at certain times. For example, a resident may want the temperature in their dwelling to rise 15 minutes before returning from work. This will conserve energy, expenses and natural resources. The natural gas is fed to buildings from a main gas line submerged a few feet beneath the earth, most likely running under and parallel to a street. The duct work supplying the hot air (and sometimes cool air if an AC unit tied into the system) can be insulated, or not insulated. Residential and commercial buildings located in rural and remote areas do not often use forced hot air systems. This is due to the financial impracticality of running natural gas lines many miles past areas of relative dense habitation. Usually these rural and remote buildings utilize oil heat, which is delivered by a truck. When reading ads for houses for sale in the MLS, this is abbreviated as FAG.