Gas Furnace

What is a Gas Furnace?

A gas furnace is a home heating appliance used by many to produce warm air during the cold winter months. Most common in regions of the country that experience frigid temperatures that often drop below freezing, it provides an economical means to heat your home. A gas furnace uses natural gas or propane as an energy source to heat air. The fuel source is available through a gas main or delivered and stored in onsite tanks. Gas furnaces are considered to be among the most efficient types with some models operating at a 95 percent efficiency rate. Translated, it means that only five percent of the energy is lost, or not used to provide heated air.

A residential gas furnace consists of three main components, all of which include several parts. The first section includes the heat exchanger, burner, vent and draft inducer. The second section consists of the necessary safety and control devices. The third section includes the blower, plenum and adjoining ductwork. All of the components work in unison to heat and circulate conditioned air throughout your home.

Forced Air Heating System

A gas fired furnace is considered a forced air heating system. As the temperature in your home drops below the thermostat setting, the system activates to raise the inside temperature. Once activated, fuel and air combine in the burner where an electronic igniter or pilot light ignites them. The hot gases rise up, through the heat exchanger to heat the air. Gases and exhaust fumes are diverted to the home’s exterior via a flue, or pipe, often through an exterior wall or the roof.

Larger furnaces might incorporate two burner systems to heat larger volumes of air. This would by typical in homes over 2000 square feet or in some commercial buildings. The furnace continues to fire until your home’s temperature rises to the programmed settings on the thermostat after which time it will shut down. When the temperature drops below the thermostat settings again, the furnace system is reactivated and starts the whole process over again.

Blower Assembly

Air circulation is achieved by means of an electric fan referred to as the blower assembly. Its job is to draw fresh air into the system and distribute warm, conditioned air throughout the home. A return vent — a large, flat vent located in the ceiling, wall or floor — is the point where the fresh air enters the furnace system and is usually located close by.

The heated air is distributed through a series of air ducts and vents connected to the plenum attached to the furnace that channel the heated air to specific areas within your home. The plenum acts as a junction point for both the gas furnace and its adjoining ductwork. Often a square box, it attaches to the furnace on one end, while the ductwork circuit connects to one or several of the remaining five sides of the square. The plenum is commonly fabricated out of galvanized sheet metal, or fiberglass duct board.

Gas Furnace


As the fresh air enters the furnace system, it must pass through a filter to remove dirt, dust, dander and other contaminates that could harm the system. The filter is often located within the return vent but sometimes fitted directly to the furnace assembly itself. It is very important to clean or replace the filter on a regular basis to maintain peak system efficiency. A dirty filter reduces airflow causing your system to work harder to circulate the air. This puts unneeded stress on its components and leads to higher energy consumption, which translates into higher energy costs for you. A stressed or overworked furnace system is also prone to mechanical failures and malfunctions leading to costly repairs.


Some furnace systems incorporate a humidifier, located close to the appliance to mitigate the drying of heated air, which can cause respiratory discomfort. A simple way to look at it is as the air is heated, moisture within the air dissipates. The humidifier replenishes the air’s moisture content that is lost during the heating process to enhance the overall comfort levels within the home. Most humidifiers incorporated into a gas furnace system are connected directly to a water line, which will supply a continuous flow of water to the device. The humidifier should also include a drainpipe to channel any condensation that accumulates during the process to your home’s exterior. Older humidifiers might require that you fill them with water as needed and empty a condensation tray manually.


To maintain your gas furnace system and reduce the likelihood of costly repairs in the future, it is recommended that you schedule annual maintenance and cleanings with a certified heating and air contractor. This should be done before the winter season sets in, when HVAC contractors are likely to become busy. This will give you peace of mind and assurance that your system is running at peak efficiency when the cold weather hits.

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