Lennox MS8 SERIES Installation And Service Procedures page 48

DISCHARGE LINE TEMPERATURE SENSOR (RT5)
High compressor discharge temperature can result from either of the following conditions:
* Low refrigerant charge
* Restricted refrigerant flow
When the discharge line temperature drops below 194°F (90°C) and the compressor has been OFF for more than 3 minutes,
the system will resume normal operation.The discharge line sensor continuously monitors the discharge temperature and
communicates this to the microprocessor. The microprocessor takes these measurements and changes the frequency of the
compressor to meet the system load. If the discharge temperature increases to the ranges noted in the above chart, the
microprocessor will adjust the frequency or cycle the compressor OFF (table 14).
MONITORING OF OUTDOOR DISCHARGE (RT5) LINE TEMPERATURE
Condition
If outdoor discharge line temperature is greater than 208.ºF (98ºC)
If outdoor discharge line temperature is greater than 217ºF (103ºC)
If outdoor discharge line temperature is greater than 230ºF (110ºC)
If outdoor discharge line temperature is less than 194ºF (90ºC)
When compressor is cycled "OFF", the indoor fan will continue to run at a preset speed in air conditioning or in heating it will
cycle "OFF" 60 seconds after compressor.
TEMPERATURE SENSOR FAULTS
Refer to the error code tables for details concerning fault conditions and equipment behavior.
TESTING TEMPERATURE SENSOR RESISTANCES
Sensors are connected to the indoor and outdoor controls through field-replaceable harnesses. These sensors provide
indoor ambient air (RT1), outdoor ambient air (RT4), indoor coil (RT2), outdoor coil (RT3) and outdoor discharge line (RT5)
temperatures to the indoor and outdoor controls to identify fault conditions. As the temperature changes, the resistance
across these sensor changes. Tables 15 through 17 indicates how the resistance varies as the temperature changes for all
reference temperature sensors.
MS8C / MS8H (208-230V)
DISCHARGE LINE TEMPERATURE PROTECTION SEQUENCE (RT5)
Indoor Fan Operation
FIGURE 62
TABLE 14
Action
Change in compressor speed is not allowed.
Compressor will run at reduced speed.
Compressor will stop.
After anti-short cycle (3 minutes) delay, the compressor and outdoor fan
will resume operation.
Page 48