How Furnaces Work

Although the process of applying flame to heat the air may seem simple, modern furnaces use a number of techniques to make the process safe and efficient. Here's how it works:

Your thermostat controls the operation of your furnace. As the temperature in the room falls, the thermostat sends an electrical signal to the furnace via a (typically low volt) wire.

When the furnace receives the signal from the thermostat, it sends fuel to the burners, which are metal elements designed to produce an even, controllable flame. An electronic spark ignites the fuel, and sensors check to make sure all of the burners are lit. If not, the system will shut itself down.

Once the burners are operating, they heat a metal unit called the heat exchanger. The heat exchanger transfers the heat of the flame to the surrounding air more efficiently than the flames would on their own. A timer starts a fan called a blower after the heat exchanger has had time to warm up. The blower is responsible for moving air through the heating system.

The furnace is connected to a series of metal tubes that run through the walls and floors of your home. These metal tubes are called ducts. The ducts that supply cool air to the furnace are called returns and the ducts that carry warm air to the rooms are called heat registers. Since warm air rises, registers located near the floor will generate the most heat.

The heating system works by drawing cool air in through the returns, passing it over the heat exchanger and pushing warm air out into the rooms of the house using the heat registers. Once the temperature reaches the desired level, the thermostat sends another signal to shut off the furnace.