RT-SVD03G-EN 139
High Pressure and Low Pressure Lockout Circuits
High Pressure Cutout (HPC) and Low Pressure Cutout (LPC) circuits are designed to shut
compressor operation down when pressures are too low, such as loss of charge; or too high, such
as would result from dirty condenser coils.
The operational logic of these circuits is somewhat unique and requires explanation to fully
understand how they operate.
LPC Logic
LPC logic was changed with version 3.1 of the RTRM. Timing was changed from two minutes to 10
seconds to enhance compressor reliability due to no-flow issues for the TXV option during
compressor starts.
However, due to unexpected compressor lockout issues, the timing was again revised with version
6.0 of the RTRM. The timing function will be different depending on the outdoor temperature. If the
OA Temp is below 40ºF, the timing is one minute. If OA Temp is 40ºF or above, timing is 30 seconds.
For RTRM version 8.0 + the timing has been revised and is still dependent on outdoor ambient
temperature. If the outdoor temperature is less than 40ºF the LPC Bypass Delay will be set to 60
seconds. When the outdoor temperature is between 40 to 49.9°F the delay will be set to 30 seconds.
For all outdoor temperatures 50°F and above, there will be no delay in setting a LPC. These delays
will apply to all units.
LPC Sequence of Operation
On a call for cooling, there is an LPC bypass delay period (depending on OA Temp) where the LPC
switch is ignored. If there is a low charge condition or low pressure condition due to extreme cold
ambient conditions, the unit is allowed to run for the delay period to build up pressure.
If the switch is closed after the LPC bypass delay time, the compressor will continue to run. If the
switch is open, the compressor will shut down immediately. After three minutes, if there is still an
active call for cooling, the unit will once again start. If the switch is still open after the initial LPC
bypass delay time, the compressor will again shutdown. The unit will try to start four times. If the
switch is still open after the fourth try, the unit will shut down and be locked out, and will require
a manual reset to start the unit again.
If the switch is closed and the compressor continues to run for three minutes, the counter is reset
and the logic sequence starts over.
If the call for cooling goes away at any time during the routine, the counter is reset and the logic
sequence starts over.
Note: If the unit is configured as a Heat Pump and is actively heating with the outdoor ambient
temperature at or below 0ºF, or the unit is in an active defrost mode, the LPC inputs will be
ignored.
LPC Operation for 27.5-50T Units with Modulating Reheat
Units configured with Modulating Dehumidification will control compressors in regards to the
Compressor 1 and 2 Disable inputs slightly different than traditional units and will also include an
extra LPC switch connected to the RTDM. The following describes the implementation of this
function for these units:
• RTRM Compressor 1 Disable Input will function as today's units function.
• If active on a Dual Circuit Unit, only Circuit 1 will de-energize after the appropriate delays
during Active Cooling. Both Circuits will de-energize during Active Reheat (Circuit 2 after its
minimum on timer expires).
• If active on a Single Circuit Dual Compressor unit, Compressor 1 and Compressor 2 will de-
energize after the appropriate delays during Active Cooling and Active Reheat.
• RTRM Compressor 2 Disable Input will function as follows:
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