Furnaces are devices designed for heating. They come in many forms including fuel combustion furnaces, electric furnaces, and induction furnaces. The name furnace comes from the Greek word “fornax,” meaning oven.

A home furnace is a permanently installed appliance that moves heat into an interior space using air, steam or hot water. Heaters that use hot water to heat a space are typically referred to as boilers. Home furnaces rely on a wider range of fuel sources including electricity, natural gas, liquefied petroleum gas, fuel oil, and in some cases coal or wood. Industrial furnaces work in a similar fashion as a home furnace and are often used to provide heat for a specific process in addition to heating a business environment.

Furnaces are generally classified into four categories depending on their efficiency and overall design. The earliest furnaces were known as natural draft furnaces and relied on rising warm air to channel heat through steel or cast iron heat exchangers in brick and masonry buildings.

Forced-air furnaces were developed in the 1950's and used electric blowers to move heat through the original natural draft systems. The earlier systems were large and bulky, which lead to the development of forced draft furnaces, a more compact system that utilizes steel heat exchangers and multi-speed blowers.

The final category of furnaces is the high efficiency condensing furnace, which can achieve efficiencies well over 90%. Condensing furnaces include a a sealed combustion area and a secondary heat exchanger. Because condensing furnaces remove heat from the exhaust gasses, the unit actually condenses water vapor and other chemicals as it operates.

In addition to the four common furnace categories above, Furnaces can be classified into three types. Single stage furnaces air have only one stage of operation (on or off_ and always run at the highest speed and deliver the hottest air. Two-stage furnaces, however, can operate at full or half speed. And modulating furnaces can regular heat output and speed in a continuous fashion as required, thereby saving a great deal of energy.

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