EGEE 102
Energy Conservation for Environmental Protection

Central Ducted Air Systems


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.

Households using forced air have been sending 30 percent or more of their energy dollars up the furnace flue, contributing to an additional 50 tons of carbon dioxide every year per household. Most conventional forced-air furnaces operate at very low efficiencies of about 50 percent. This is like utilizing only 50 percent of the energy that we buy and feed into the furnace.

Below are images of the main components of a forced air heating system. Can you identify each? Drag and drop the image onto its name.

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.

Click on the “play” button below to see how a forced-air heating system works.

How a Forced-Air Heating System Works

Click here to open a text description of how a forced-air heating system works

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.

Press the "play" button below to see how a gravity heating system works.

How a Gravity Heating System Works

Click here to open a text description of how a gravity heat system works

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.

Important Point!

In gravity heating systems, the ducts are larger than forced-air heating systems, and only warm air travels through them.