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I-SCE (09-18) PN207697R9
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.
At final adjustment, amperes should not exceed motor nameplate amp rating. The installation must be adjusted to
obtain a temperature rise within the range specified on the furnace rating plate.
The belt drive on these units is equipped with an adjustable pulley that permits blower speed adjustment. Adjust the
blower speed as follows:
1. Shut off gas and electric power.
2. Loosen belt tension and remove belt.
3. Loosen setscrew on side of pulley away from motor.
4. 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%.
5. Tighten setscrew on flat portion of pulley shaft.
6. Replace belt and adjust belt tension in accordance with Checking and Adjusting Belt Tension.
7. Turn on gas and electric power and light heater in accordance with instructions on lighting instruction plate.
8. Check motor amps using ammeter (maximum motor amp rating on motor nameplate must not be exceeded).
When adjustment is complete, check for proper operation.
Blower Rotation
Each blower housing is marked for proper rotation. Rotation may be changed on single-phase motors by rewiring in
the motor terminal box. Rotation may be changed on three-phase motors by interchanging two wires on the three-
phase supply connections.
Optional Variable Frequency Drive
If the system is equipped with an optional variable frequency drive, the motor will operate at speeds determined by
the electrical frequency: 60 Hertz (Hz) is maximum speed. Speed must be within the temperature rise range of a
Model SCE Series 6 heater: 30–90°F.
Follow the variable frequency controller manufacturer’s instructions that are packaged with the heater (in the owner’s
envelope) to program the variable frequency drive settings. The formula for motor speed is as follows:
N = 120 × f/p
where N is speed, f is frequency, and p is number of poles
A 3600-rpm motor has two poles and an 1800-rpm motor has four poles.
For example, for an 1800-rpm motor at 60Hz, N = 120 × 60/4 = 1800 (1800 is synchronous speed, assume 2% slip).
The motor will run between 1750 and 1790 rpm at full load depending on design. For the same motor run at 45Hz,
120 × 45/4 = 1350 (1350 rpm less 2% slip equals about 1300 rpm).
OPERATING VALVE
⚠ WARNING ⚠
The operating valve is the prime safety shutoff. To ensure positive closure, all gas supply lines
must be free of dirt or scale before connecting the unit.
All furnaces are equipped with a 24-volt combination valve that includes the automatic, electric on/off valve controlled
by the room thermostat, the pressure regulator, the safety pilot valve, and the manual shutoff valve. The standard
gas valve allows for single-stage control from a single- stage, 24-volt thermostat.
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