Trane ReliaTel - Verifying Proper Air Flow; Units with Direct Drive Indoor Fan with MMC

44 RT-SVD03G-EN
Verifying Proper Air Flow
Units with Direct Drive Indoor Fan with MMC
Much of the systems performance and reliability is closely associated with, and dependent upon
having the proper airflow supplied both to the space that is being conditioned and across the
evaporator coil. The indoor fan speed is changed by adjusting the output voltage from the MMC
to the direct drive fan. Before starting the SERVICE TEST, set the minimum position setpoint for the
economizer to 0% using the setpoint potentiometer located on the Economizer Control Module, if
applicable.
ReliaTel Control: Momentarily jump across the Test 1 & Test 2 terminals on LTB1 one time to
start the Minimum Ventilation Test.
Electro Mechanical Control: Perform the proper test mode connections.
Once the supply fan has started, determine the total system airflow (CFM) by (ReliaTel/
Electromechanical):
1. Measure the DC voltage across pins Vt and com on the MMC. Using the fan rpm table shown
on the access panel label or in the unit Service Facts, determine RPM correlated to measured
voltage.
2. Measure the amperage at the supply fan contactor and compare it with the full load amp (FLA)
rating for the evaporator motor stamped on the unit nameplate.
a. Calculate the theoretical BHP using (Actual Motor Amps/ Motor Nameplate Amps) X Motor
HP.
b. Using the fan performance tables in the unit Service Facts, plot the actual RPM (step 1) and
the BHP (step 2a) to obtain the operating CFM.
3. If the required CFM is too low, (external static pressure is high causing motor HP output to be
below table value),
a. Relieve supply and/or return duct static.
b. Change indoor fan speed and repeat steps 1 and 2.
• To Increase Fan RPM; Push & Turn the knob on the MMC counter-clockwise. Measure the
DC voltage across pins Vt and com on the MMC. Using the fan rpm table in the unit
Service Facts, determine RPM correlated to measured voltage.
• To Decrease Fan RPM; Push & Turn the knob on the MMC clockwise. Measure the DC
voltage across pins Vt and com on the MMC. Using the fan rpm table in the unit Service
Facts, determine RPM correlated to measured voltage.
4. If the required CFM is too high, (external static pressure is low causing motor HP output to be
above table value), change indoor fan speed and repeat steps 1 and 2.
5. To stop the SERVICE TEST, turn the main power disconnect switch to the "Off" position or
proceed to the next component start-up procedure.