The blower is contained in the furnace itself and is the source of pressure that moves the air through the ducts. The typical residential furnace blower develops a static pressure (that is a push on the air that it moves) of 1/2-inch water column, which is equal to 1/56 -inch of a PSI. If you can imagine inflating your bike tires with 1/56 -inch of a PSI, you see that the blower really exerts very little pressure, and it is important for the system that supplies and returns the air through the house be free of obstruction.
Blowers are typically
. The belt-driven ones are easy to identify because the motor sits beside the fan, and a belt and pulleys drive the fan. The direct-drive units are neater looking, because the motor is mounted inside the body of the fan. There is no belt with direct-drive systems; the shaft at the end of the motor drives the fan.
Both types of fans can change speeds. The belt-driven fan speeds can be adjusted by changing the pulley sheaves (flanges). The fan speed is usually a quarter to one half of the 3450rpm (typical) motor speed. The direct-drive blowers may have from one to six speed settings. The blower and motor turn at the same speed (1750 rpm is a typical maximum speed). Different wire terminals on the motor can be used to obtain different speeds.
The blowers are usually an integral part of the furnace. They typically range from 1/4 to 3/4 horsepower, depending on the size of the furnace.
Some of the blowers and motors require regular lubrication. Some have factory-sealed bearings and lubrication is not required.
On a conventional gas-fired, forced-air furnace, an air temperature rise of 70F to 100F across the heat exchanger is typical. On original installations, most installers shoot for the low end of this. The blower should be sized and adjusted to deliver the appropriate temperature rise. For example, 70F air into the furnace should yield 140F to 150F air at the supply plenum.