60
SSCBL-RPBL-IOM (10-22) 149159-C
ADJUSTMENTS—CONTINUED
Belts, Blowers, and Drives—Continued
Checking and Adjusting Belt Tension—Continued
3/4" (19 mm)
Figure 43. Checking Belt Tension
Adjusting Blower Speed
•
Blower speed may be adjusted to achieve the desired outlet temperature as long as the adjustment is within the
temperature rise and static pressure limits shown on the furnace rating plate . Motors are factory-set between
maximum and minimum blower speeds .
•
If the duct resistance is low, the blower may deliver too high an air volume . If the resistance is very low, the blower
may deliver excess air to overload the motor, causing the overload protector to cycle the motor . Reducing the
blower speed will correct these conditions . If ductwork is added to an installation, it may be necessary to increase
the blower speed . Decreasing blower speed increases outlet temperature . Increasing blower speed decreases
outlet temperature .
•
The system is set at the factory for the RPM required to meet the CFM and external static pressure specified on the
order . If estimated external static pressure is incorrect, or changes were made to the duct system, the blower RPM
may have to be adjusted . Motors are equipped with adjustable pitch pulleys that permit adjustment of blower speed .
•
Adjust the blower speed as follows:
•
To make adjustments to units with a <5-HP motor:
a . Shut OFF gas and electric power .
b . Loosen belt tension and remove belt .
c . For units with <5-HP motor:
(1) Loosen setscrew on side of pulley away from motor .
(2) To increase blower speed and decrease outlet temperature, turn adjustable half of pulley inward . To
decrease blower speed and increase outlet temperature, turn adjustable half of pulley outward . One turn
of pulley changes speed 8–10% .
(3) Tighten setscrew on flat portion of pulley shaft .
d . For units with
≥
5-HP motor:
(1) Loosen but do not remove two locking screws that are directly across from each other on outer locking ring .
(2) Adjust sheave to desired pitch diameter by turning outer locking ring . One complete turn of outer locking ring
results in 0 .233-inch change in pitch diameter . Increase diameter to decrease blower speed or decrease
diameter to increase blower speed .
⚠ CAUTION ⚠
Sheaves should not be adjusted in either direction to the point where movable and stationary
flanges are in contact.
(3) After completing adjustment, tighten both locking screws on outer locking ring .
e . Replace belt and adjust belt tension in accordance with
Checking and Adjusting Belt Tension
section .
f . Turn ON gas and electric power and light heater in accordance with instructions on lighting instruction plate .
g . Check motor amps using ammeter . Maximum motor amp rating on motor nameplate must not be exceeded .
h . When adjustment is complete, check for proper operation .
