Air Delivery and Blower Adjustment
The furnace blower is used to force air through the heat exchanger in order to remove the heat produced by the burners. This removal of heat by the blower serves a twofold purpose: (1) It distributes the heat through the supply ducts to the various spaces in the structure; and (2) it protects the surfaces of the heat exchanger from overheating.
Most blowers are factory-set to operate with a specific air tem- perature rise. For example, Janitrol Series 37 gas-fired furnaces are certified by the American Gas Association to operate within a range of 70 to 100°F air temperature heat rise. In other words, the temperature of the air within the supply ducts may be 70 to 100°F higher than the air temperature in the return duct. A temperature rise of 85 to 90°F with approximately 0.1-in external pressure in the area beyond the unit and filter is generally considered adequate for most residential applications.
The furnace air delivery rate should be adjusted at the time of installation to obtain a temperature rise within the range specified on the furnace rating plate. Many furnace manufacturers recommend that the temperature rise be set below the maximum to ensure that temperature rise will not exceed the maximum requirement should the filter become excessively dirty.
Although the blower and motor are factory-set to operate within a specific temperature-rise range, the unit can be field adjusted to deliver more or less air as required. The air delivery rate is adjusted by changing the blower speed. On direct-drive motors, this is accomplished by moving the line lead to the desired terminal. Belt- driven motors are adjusted by changing the effective diameter of the driver pulley (see Direct-Drive Blower Adjustment and Belt- Drive Blower Adjustment).