How Do Downflow Gas Furnaces Work

The downflow gas furnace uses the natural tendency of heat to rise in order to efficiently diffuse heat throughout the home. With this heating setup, the upper portion of the furnace heats the air, then it is directed downward through the home. The ductwork is located in the home's foundation slab instead of at the top of the walls. Once the heated air is pumped out of the register, it naturally drifts upward to heat the entire room.

Types of gas furnaces

There are three basic types of home heating systems. The most common is the upflow gas furnace. In this setup, the air is heated by a furnace at the bottom of the house and then directed upward through a duct system. Homes that lack the vertical space for an upflow furnace will usually have a horizontal gas furnace. These can be installed into smaller areas such as the crawlspace below the floorboards.

Downflow gas furnaces: Basic components

A downflow gas furnace is made up of a combustion chamber, heat exchanger, and thermostat. Natural gas is pumped into the combustion chamber, where it mixes with air and is ignited by the furnace's pilot light. The flame that is created warms up the downflow furnace's heat exchanger. The heat exchanger consists of a series of metal tubes that will retain the heat created by the combustion chamber. The cooler air from the room is directed through the heat exchanger and into the ductwork that runs throughout the house.

Downflow gas furnaces: Air filtration

The air is run through a filter before it is allowed to enter the heating chamber of a downflow gas furnace. The filter keeps dust and other contaminate particles from being redistributed through the house again. Furnace filters are available in both disposable and washable varieties. Disposable filters are less expensive and require less maintenance than the washable type, but washable filters provide the best filtration results.  

Downflow gas furnaces: Thermostats and carbon monoxide sensors

The temperature produced by a downflow gas furnace is set by the system's thermostat (500RF). The temperature sensor reads the air temperature in the room, opening the gas valve if it is below the user's desired setting. Once the thermostat reads the proper temperature again, the natural gas valve is closed again and the downflow furnace will turn off. Many modern home heating systems also include built-in carbon monoxide sensors that shut down the furnace when dangerous levels are detected. Of course, this does not eliminate the need for carbon monoxide detectors in other areas of the home.

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