Troubleshooting guide

Page 10 of 28 / OM 931-3
Low Pressure Reset
The low pressure switch fault is subject to intelligent reset. If
the pressure recovers, it resets three times within a 24-hour
time period. If further faults are detected within the 24 hours,
the controller must be manually reset as described in the High
Pressure Reset section above.
Low Suction Line Temperature Fault
Board LED Status – Low Suction Line T
emperature Fault
Y
ellow
Green
Red
Flash
Of
f
Of
f
Heating Mode
When the suction line temperature falls below 28°F on stan-
dard equipment (6.5°F on Geothermal) the low temperature
fault generates the following:
■ The reversing valve de-energizes. The compressor and fan
continue to operate in cooling mode for 60 seconds, which
results in a defrost mode. This defrosts any ice that may
have accumulated in the water-to-refrigerant coil, because
of a lack of condenser water ow in heat mode.
■ In heat mode the low temperature fault is subject to
Intelligent Reset.
■ The fault terminal A de-energizes while the unit is
in defrost mode. It will stay de-energized until the
temperature recovers to 38°F for standard equipment
(18°F for geothermal). A to C will be used to indicate an
alarm signal.
Cooling Mode
When the suction line temperature falls below 28°F standard
equipment (6.5°F geothermal) in cool mode the:
■ Compressor de-energizes.
■ The fan continues to run for 60 seconds.
■ Alarm output de-energizes.
■ When the suction line temperature recovers to 38°F
standard equipment (18°F on geothermal) the low
temperature fault continues and the unit will be locked
out.
Fan Only Mode
When the suction line temperature falls below 28°F standard
equipment (6.5°F geothermal) in cool mode the:
■ Fan de-energizes.
■ Alarm output de-energizes.
■ In Fan Only mode, the low temperature fault is subject to
Intelligent Reset.
When in remote shutdown (E terminal grounded), thermostat
and control inputs have no affect upon unit operation. No
faults or modes have higher priority than remote shutdown.
Remote shutdown and brownout condition have the same level
of priority. See "Priority of Faults and Modes" on page 12
and Table 3 on page 12.
When the unit is in remote shutdown mode, the following
occurs:
1. The compressor de-energizes.
2. The reversing valve de-energizes.
3. The fan de-energizes.
4. Fault terminal (A) de-energizes (fault). A to C will be used
to indicate an alarm signal.
When the E terminal is no longer grounded the unit will auto-
matically return to normal operation.
Note: The remote shutdown input (E) will suspend unit
operation. Disconnect power when servicing the unit/
controller.
High / Low Pressure Faults (HP/LP)
Normally closed high and low refrigerant pressure switches
help protect the water source heat pump from excessively
high or low refrigerant pressures. The MicroTech III unit con-
troller monitors these switches individually. If the compressor
is running and the HP circuit is open, the controller enters a
pressure fault mode. If the compressor is running and the LP
circuit is open after a time delay (default of 30 seconds, ad-
justable if a communication module is present) the controller
enters a low pressure fault mode. The fan operates normally
during a pressure fault condition.
Board LED Status – High/Low Pressure Faults
Y
ellow Green
Red
HP
Of
f
Of
f
Flash
LP
Of
f
Of
f
Solid
Only brownout and remote shutdown modes have higher pri-
ority than the pressure fault mode. See "Priority of Faults and
Modes" on page 12 and Table 3 on page 12.
When the unit is in high pressure fault mode the following
occurs:
1. The compressor de-energizes.
2. The fault terminal (A) de-energizes (fault). A to C will be
used to indicate an alarm signal.
3. After the shutdown is complete the IV/PR(H8) output will
change state. (On to Off / Off to On).
High Pressure Reset
After the HP circuit is closed, the unit does not return to
normal operation until the control board is reset. The unit is
locked out in this manner until the unit can be serviced.
The control board is reset by a short interruption of unit pow-
er, by holding down the tenant override button for more than
10 seconds, or via the Building Automation System (BAS).