Fueled by gas, electricity, or oil, a forced-air distribution system is just what the name implies. Air is forced from the furnace through ducts to registers in various rooms. Besides warming the air, the blower system that distributes the warmed air also returns the cold air to the furnace so it can be rewarmed and distributed to the rooms again.
A forced-air system is also efficient for distributing cool air from a central air conditioner with the same ducts, registers, and blower. There is little that can go wrong with a forced-air system. The big problems typically include noise and blockage of airflow, usually caused by dirt or by furniture or draperies blocking the registers. Forced-air systems should be cleaned and maintained regularly.
Floor registers are slip-fit into ducts or are held by retaining screws on the frame of the register. Wall and ceiling registers are also held in place by retaining screws on the frame of the register. Duct joints are usually slip-fit and held with sheet-metal screws or duct tape. The ducts are supported by wire or metal strap hangers nailed or screwed to wooden framing members such as studs and rafters. All of these parts are easy to disassemble. Lay them out in order as you work so you'll be able to reassemble them properly.
A problem can arise with this type of system in which the temperature of different rooms varies widely. Find out how to solve this problem in the next section.