Ducted air systems are the most common type of central heating and cooling used. If a home has a central air conditioner, heat pump, or furnace, it is a ducted air system. There are two main types: Forced-air Heating Systems and Gravity Heating Systems.
Forced Air Heating Systems
Almost 35 million homes in America are heated by natural gas-fired, forced-air heating systems- by far the most popular form of central heating.
With a forced-air system, a furnace warms air, an air conditioner cools air, or a heat pump either warms or cools air, then a blower forces the air through the system. Therefore, the same duct system can be used for both heating and cooling.
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The Main Components of a Forced Air Heating System
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- Main Duct
- Flexible Supply Branch
- Supply Branch
- Return Register
- Supply Register
In a forced-air heating system, room air (cooler) is drawn by a fan or a blower through return air registers and ductwork, and passes through a filter (to remove any dust particles) into a furnace, where the air is heated. The warmed air is then blown back to rooms through a system of supply ducts and registers.
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How a Forced-Air Heating System Works
The process begins when cold air is pulled into the return register. The air passes through a filter into the furnace to be heated. The now hot air travels up through the main duct and into the supply branches. Each supply branch has a supply register that allows the hot air to enter the room.
Gravity Heating System
With a gravity furnace, convection currents (caused by the natural tendency of heated air to rise) carry heated air through the system from a furnace that is located on or below the main floor level. Gravity systems, somewhat older, do not have blowers, and tend to have very large air ducts; they can only deliver warmed air.
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How a Gravity Heating System Works
In a gravity heating system, a furnace located in the basement heats air that rises and travels to each room, creating a convection current that circulates the warm and cool air.
In gravity heating systems, the ducts are larger than forced-air heating systems, and only warm air travels through them.