24
Unit Start-up
Verifying Proper Air Flow
(Units with Direct Drive Indoor Fan)
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 motor is factory wired to operate on low
speed in the cooling and heating mode. It can be rewired for
high speed operation should the application require it. Refer
to the wiring diagram that shipped in the unit.
The indoor fan motors are specifically designed to operate
within the BHP parameters listed in the fan performance
tables of the unit Service Facts. By understanding that these
motors will safely work within these conditions, before an
oversized motor is required, will allow the air distribution
system to be set up properly and diagnostics enhanced
should a problem occur.
When verifying direct drive fan performance, the tables must
be used somewhat differently than those of belt driven fans.
Fan performance diagnostics can be easily recognized when
these tables are used correctly.
Before starting the SERVICE TEST, set the minimum
position setpoint for the economizer to 0 percent using the
setpoint potentiometer located on the Economizer Module
(UEM), if applicable.
Using the Service Test Guide in Table 5-1, momentarily jump
across the Test 1 & Test 2 terminals on LTB1 one time to
start the Minimum Ventilation Test.
With the fan operating properly, determine the total system
external static pressure (inches w.c.) by;
1. Measuring the supply and return duct static pressure,
2. Using the accessory pressure drop table in the Service
Facts, calculate the total static pressure drop for all of the
accessories installed on the unit; i.e., curb, economizer,
etc.
Note: Static pressure is based on desired CFM and
may not be actual static pressure.
3. Add the total accessory static pressure drop (step 2) to the
duct external static pressure (step 1). The sum of these
two values represents the total system external static
pressure.
4. Measure the amperage at the supply fan contactor and
compare it with the full load amp (FLA) rating stamped on
the motor nameplate.
a. Calculate the theoretical BHP
Actual Motor Amps X Motor HP
Motor Nameplate Amps
b. Using the fan performance tables in the unit Service
Facts, plot the total external static pressure (step 3)
and the BHP (step 4a) to obtain the operating CFM.
c. When plotted, if the two values can not be
interpolated correspondingly, the static pressure will
most likely be the least accurate measurement.
Because of the direct drive motor operation, the RPM
performance is relatively constant making the
operating current a very reliable diagnostic tool.
Example: TCD060 single phase, low speed.
Actual Motor Amp (5.25)
= .99%
Motor Nameplate Amps (5.3)
0.99 X Motor HP (0.6) = .59 BHP
The actual external static pressure is approximately
0.45” w.c., airflow equals 2100 CFM.
If the static pressure reading was higher, motor
current would have to be lower proportionately to get
an accurate CFM measurement in direct drive
applications.
5. 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 to “High” and repeat steps 1
through 4.
6. If the required CFM is too high, (external static pressure is
low causing motor HP output to be above table value), in-
crease supply and/or return duct static.
7. To stop the SERVICE TEST, turn the main power discon-
nect switch to the “Off” position or proceed to the next
component start-up procedure.
