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Introduction to Air Furnaces

written by: Mike Aguilar • edited by: Lamar Stonecypher • updated: 3/5/2010

Air furnaces have developed from the earliest stoves to the modern direct fired heaters, with different configurations and locations of the heat exchangers and blowers. They are located at basements or first floors depending upon their utilization, and direction of air is upward or downwards.

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    What is an Air Furnace?

    An air furnace is a device that transfers heat released from one side of the surface of a heat exchanger to circulating airflow for the purpose of providing heating.

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    The History of Furnaces

    Here are some of the early types of air furnaces:

    The Stove was the initial type of air furnace. Although common and useful, they did not present a healthy and comfortable environment, and were soon discarded and replaced. A stove is comprised of an area that is heated by the combustion of some source of energy like wood, oil, or coal. The fire is created in a compartment which is attached with a pipe, and a flow of air is created by drawing clean air through the fuel compartment as the fuel is burnt. Radiant heat sufficient for cooking and room warming was provided, but a large amount of the heat energy escaped through the exhaust flue.

    The Pipeless Furnace was similar to today's forced air furnaces, but they used no pipes or ducting. For this reason, they are also known as "single register" furnaces. In this design, warm air proceeds in the upward direction through openings in the floor, while return air also passes through the same floor opening.

    In the Forced Air furnace design the circulation of air was improved by mechanical methods, which also increased the rate of heat transfer.

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    Modern Air Furnaces

    In order to improve the efficiency of heat transfer, research was carried out and numerous types and configurations of air furnaces have been developed, some of which are mentioned below:

    • The blower and furnace are located in independent locations, and air is released in an upward direction. This arrangement is used for installations at basements and at first floors.
    • The heat exchanger is located at the top and the blower is placed at the bottom. Air flows in the ascending direction. Also used for basements and first floors.
    • The heat exchanger is placed below while the blower is at the top, and air flows downwards. Used for first floors where air is released downward.
    • The heat exchanger and blower are positioned horizontally, and the discharge of air is in horizontal direction. Used mainly for basements and top story installations.
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    Types of Air Furnaces

    Various types of air furnaces may use wood, coke, coal or kerosene. Here are some of the types commonly used today:

    • Space Heater: These are fired by oil, gas, wood or coal, and the circulation is by gravity with the possible assistance of fans. Their efficiency is good due to incorporation of thermostats.
    • Furnace At Floor: This system is attached to the beams in the basement, with automatic controls and firing systems. The air is heated appreciably.
    • Furnace At Wall: This mechanism is fixed in the wall of a room to avoid utilization of room area which can be beneficially used for other purposes.
    • Heater Direct Fired: They can be with or without a circulating fan located at the rear of the heat exchanger. These systems are used mostly in commercial establishments.
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    Selection Of Air Furnaces

    While selecting an air furnace, it should be ensured that its capacity is commensurate with the heat losses in the building prevailing during winter weather conditions. Heat loss should be designed for the complete structure, including the basement. It should also be ensured that the blower has sufficient capacity to manage air flow rate for summer cooling.