Gas Furnace Heating Basics

In gas furnaces and gas heaters there are burners that mix and burn fuel in a combustion chamber surrounded by a heat exchanger. The heat exchanger picks up heat from the flame and burning gases in the combustion chamber and transfers that heat to a heating fluid (air, water or steam). The combusted or burned gases leave the combustion chamber through a passageway or flue.

The efficiency of a combustion heater depends on combustions or burning that didnt complete, air lost in the flue, losses at beginning and end of the burners cycle, and losses from the ducts.

The combustion fuels primarily used to burn are hydrocarbons which are molecules that are made of hydrogen and carbon. Combustion or burning is rapid oxidation, oxygen combines with the carbon and hydrogen, splitting the hydrocarbon molecule. Carbon dioxide and water vapor are the results of the burning reaction. Most of the flames heat will transfer to the heat exchanger and the heat exchanger will transfer that heat to the heating fluid.

There are sealed and open combustion chambers. The heat exchanger and flue are open to surrounding air for open combustion chambers. Most homes have open combustion heaters but sealed combustion chambers are more safe and are usually more efficient. Sealed combustion chambers have no openings to the heat exchanger or flue, instead they use a sealed tube sometimes combined with the flue, and brings air in from outside the home.

Burners mix fuel and air, and burns the mixture. Atmospheric, induced draft and power burners are the three types of burners. Atmospheric burners are the most common type of gas burner. A pilot light, hot surface ignitor, or sparking electrode will ignite the mixture. Secondary air inside the combustion chamber around the flame gives oxygen for almost complete burning.

Gas Furnace with Central Air Conditioning

Central a/c systems will have an air handler or furnace, thermostat, blower fan, duct work, filters, registers and grilles.

Forced air furnaces accommodate central cooling and ventilation, using the same air handler and distribution system. The thermostat turns the unit on when the temperature goes below the temperature your thermostat is set for. Supply and return ducts connect to the furnace cabinet and the blower moves air over the heat exchanger to heat the air. Air filters in filter grilles keep the furnace clean. While these systems have the highest efficiency for combustion heaters, the air distribution systems (the fans and ductwork) often leak air, spread dust and cause overly dry indoor air. These systems are hard to zone or hard to divided into parts controlled by separate thermostats. The forced air furnace is the most common of central heaters.

(Click The Image Below To Get A Closer Look)

how gas furnace works


Krigger, John, and Chris Dorsi. “Chapter 6 Heating.” Ed. Mary Coster.Residential Energy: Cost Savings and Comfort for Existing Buildings. Ed. Margaret Regan. 5th ed. 324 Fuller Avenue, Helena MT 59601: Saturn Resource Management, 2009. 137-59. Print.

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