An upflow furnace is a heating device that draws in air from near floor level in a building, heats it and then discharges the warmed air through a vent or plenum at the top of the machine. The warmed air is then fed into ductwork that distributes it throughout the building. This type of furnace is useful for installation below the area to be heated.
Warm air is lighter than cool air, so it tends to rise. Cooler air, conversely, tends to sink. An upflow furnace takes advantage of these basic laws of nature. As the circulating indoor air cools and sinks under the lighter warm air, it seeks the lowest level of the structure — the basement floor. If the upflow furnace's intake is near the basement floor, it will be well supplied with the coolest air in the building.
The heavy, cool air is drawn in by the upflow furnace and heated — either by a gas or oil burner; an electric, toaster-like coil; or a hollow coil containing a warm, circulating liquid. Whatever the method of heating, the warmed air will rise up through the furnace, into the ducts and then further on into the rooms throughout the building. Some heating systems are designed to rely only on the tendency of warm air to rise in order to circulate the heat. These are known as natural convection furnaces, and they can save money on energy costs by not using an electrical fan to move the warmed air.
Most modern upflow furnaces require a built-in fan to circulate the warmed air. These forced-air or forced convection systems are able to distribute the air through long runs of duct and reach rooms that natural convection can't reach. A forced-air upflow furnace also responds much more quickly to a call for heat when the thermostat is dialed up.
The upward direction of air circulating through a furnace makes it an upflow furnace. By comparison, furnaces also come in downflow and horizontal versions. A downflow furnace is best suited for attic or rooftop installation, and a horizontal furnace is suited for a ranch home or single-level building. The choice of one configuration over another is dictated by which type will operate most efficiently in a particular installation.