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The Downflow Gas Furnace for Residential Heating


An Energy-Efficient Home Heater that Guards against Toxic Vent Gas

© 2012 by All rights reserved; content may not be copied, rewritten, or republished without author’s written permission. Author’s Google profile

A downflow gas furnace; photo courtesy Camzmac


There are 3 basic types of gas air furnaces that you might encounter. The name implies the difference between them, as you might expect. The name indicates the direction of the airflow as it makes its way through the furnace past the heat exchanger.

However, in all 3 cases the function remains the same; it keeps the home’s occupants comfortable.

Types of Gas-Fired Furnaces

The first type is the horizontal gas furnace. As you might expect, the air flows in a horizontal position. This type works in a crawl space where vertical space is at a premium.

The second type is the upflow gas furnace. In this case, the air flows from bottom to top. This has been the most commonly found heating furnace in the home.

The third type, and the topic of this article, is the downflow gas furnace. In this case, airflow occurs from the upper part of the unit to the base. Next, the heated air makes its way from the unit’s base through slab-mounted ductwork and is distributed through the home.

How Does the Downflow Gas Unit Work?

In this type of heater, natural gas is delivered to a combustion chamber. There it is combined with available air. This mixture uses a pilot light for ignition. That flame heats the furnace’s heat exchanger.

The heat exchanger is made up of metal tubing. The hot gases in the exchanger combine with the cooler air to be heated and pass over the heated surface of the exchanger. It then absorbs the heat provided by the exchanger.

The Importance of the Furnace Filter

Prior to the air entering the unit, before it is heated, it must pass through a filter. The purpose is to keep the furnace’s interior uncontaminated.

Two types of filters are commonly used, the washable filters and the disposable type. Washable filters do a better job but disposable filters mean less work when it comes to routine furnace maintenance.

A calibrated thermostat regulates the temperature of the living space when you set it to your desired temperature. As soon as ambient air temperature drops below the set limit, the natural gas valve supplying the furnace opens up and combustion begins.

As soon as your desired temperature is detected, the gas valve shuts off. There is just enough flow left to keep the heater pilot light lit.

Be Aware of the Dangers of Gas Furnaces

As combustion occurs, the vent gases appear in the form of toxic carbon monoxide. These vent gases are fatal to human beings and their pets. It is critical that they are vented to the exterior of your home.

Newer units that are now on the market are called ventless furnaces. They’re designed to produce a negligible amount of carbon monoxide because of their improved energy efficiency.

The amount of carbon monoxide produced is rated as having no appreciable effect on health.

Safety: the Carbon Monoxide Sensor Switch

Newer ventless units are fitted a sensor switch. It’s installed to detect the level of toxic vent gas in the home. They are designed to shut down the furnace when a minimum percent of gas is detected.

It is always wise to have a back-up indoor pollutant monitoring system to alert residents to not only carbon monoxide, but radon gas as well.

Regardless of which style of gas furnace you have, it is key to have your entire HVAC tested on a yearly basis to ensure that it is operating properly and scores below an acceptable vent gas level.

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© 2012 All rights reserved; content may not be copied, rewritten, or republished without author’s written permission.