How Does a Downdraft Furnace Work?

Warm Air Rising

Residential homes usually contain one of two kinds of furnaces: updraft or downdraft. Updraft furnaces are installed in the basement, and air is pushed up through the house. Downdraft furnaces are installed on the main living level. Cool air enters the top of the furnace and warmed. Then the warm air is allowed to rise naturally from floor vents. Although these furnaces are based on the scientific fact that warm air rises, they are still called "downdraft" furnaces.

Natural gas

In downdraft furnaces, cool air enters the furnace from the top of the unit. As it does so, it passes through a filter, which keeps dust and pollutants from entering the furnace. A gas valve opens to let gas in when triggered by the thermostat to do so. The air mingles with the gas in a combustion chamber, where it is then ignited by the pilot light. A flame results, which heats up something called a heat exchanger.

The heat exchanger is composed of a series of metal tubes, which keep hot for a long time. Air continues to enter the furnace, flowing over the hot tubes. As it flows over them, it warms up. This warm air is then pumped through the ducts and into the house.

Thermostat

The thermostat is what controls a downdraft furnace. The residents of the house set it to a temperature that they feel is comfortable. Once the thermostat senses that the temperature has dropped below this level, it opens the gas valve and lets the gas into the furnace, where combustion occurs.

After the temperature reaches the optimum level, the gas valve will automatically close, and warm air will cease to flow through the furnace and house.

Resources

• More about how downdraft and other furnaces work.