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One way to measure the temperature rise across the furnace is to insert temperature
measuring devices (e.g. thermometers) into the return air duct and into the supply air
duct about 12 inches from the furnace. After the furnace has been firing continually for
over 20 minutes, read the temperature difference between the two (2) thermometers.
The temperature difference should not exceed 100ºF, nor be less than 70ºF. A
temperature rise of 85ºF is considered to be optimum for comfort. For better energy
efficiency, a temperature rise closer to 70ºF is recommended.
The blower (fan) speed fan is adjusted by changing the fan motor winding energized by
the control system. The furnace is set on the lowest fan speed, “L”, at the factory, refer
to Table 2. To adjust the fan speed, follow this procedure.
a. Turn off all electrical power to the furnace at the disconnecting switch.
b. Remove the blower compartment access door.
c. Move the orange wire from the low speed terminal block, marked “L”, to the next
higher fan speed (medium low), marked “ML” on the terminal block, refer to
Figure 8. Other available higher fan speeds are medium high, marked “MH” and
high, marked “H”.
NOTICE: The red wire is normally positioned on the high fan speed, for
cooling applications. However, the red wire may be moved to another
terminal block position, as required.
d. Replace the blower compartment access door.
e. Restore electrical power to the furnace at the disconnecting switch.
With the furnace operating, measure the air temperature rise across the furnace again.
If the value does not fall in the range of 70 to100ºF, repeat the above procedure.
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Figure 8: Blower Motor Terminal Blocks and Wiring