YORK YCAV — SYS X LOW SUCTION PRESSURE

The panel shows SYS X LOW SUCTION PRESSURE on the system display; X is the system number. The manual prints the full line as SYS X YYYYYYYY LOW SUCTION PRESSURE, where YYYYYYY indicates a "FAULT" condition or "LOCKOUT" — so the message tells you both what tripped and whether the system is merely in fault or locked out.

This is a software safety that triggers a controlled ramped shutdown of the system: the control has detected that suction pressure fell below the programmable cutout. It is the programmable secondary back-up for the flow switch — it protects against operation with low refrigerant charge and helps protect the chiller from evaporator freeze-up. Because it is a software function with staged timing (below), it does not trip on the first instant of low pressure after a start.

• Suction pressure falls below the programmable cutout — typically set at 24 PSIG for chilled water applications.
• Low refrigerant charge.
• Low flow.

1. Check the refrigerant charge — the safety exists specifically to protect against operation with a low charge.
2. Check for low flow — low flow is listed as a cause, and this cutout serves as the secondary back-up for the flow switch.

• Cutout point: programmable; typically set at 24 PSIG for chilled water applications.
• First 30 seconds of run time: the cutout is ignored.
• Next 3 minutes of run time: the cutout point ramps linearly from 10% of the cutout value up to the programmed cutout.
• After the first 3 minutes 30 seconds (3.5 minutes) of run time: a transient sets a 30-second timer with a linearly ramped cutout starting at 10% of the programmed cutout — if the suction pressure does not stay above the ramped cutout over those 30 seconds, the system faults.
• The manual gives the ramp as Cutout = ((Programmed Cutout x Run Time) / 200) - 1.2 PSIG

The shutdown itself is a controlled ramped shutdown with no manual action needed in the ordinary case: the system auto-restarts once the 120-second anti-recycle timer times out. It locks out after 3 faults on the same system within 90 minutes. On LOCKOUT the system will not restart until the operator clears the fault using the keypad; the manual also gives the lockout clear action as cycling the system switch OFF and ON.