YORK YVAA — SYS X LOW SUCTION PRESSURE

The control panel shows SYS X LOW SUCTION PRESSURE as a system safety. X is the system number: the panel shows the number of the system that tripped in its place. The manual lists the message in its system safeties (faults) group for the YVAA.

The manual identifies this message as the programmable Low Suction Pressure Cutout. It protects the chiller from an evaporator freeze-up if the system attempts to run with a low refrigerant charge. The safety trips when suction pressure falls below the programmed Suction Pressure Cutout — with one startup allowance: the cutout is ignored at system start for the programmed Suction Pressure Startup Bypass Time, and immediately after that time expires it is linearly ramped from 10% to 100% of the programmed Suction Pressure Cutout. The system shuts down if suction pressure falls below the ramped cutout at any time during the ramp, or below the programmed cutout at any time after the ramp.

• Suction pressure falls below the ramped Suction Pressure Cutout while the startup ramp is running — after the Suction Pressure Startup Bypass Time expires and while the cutout is being ramped from 10% to 100% of its programmed value.
• Suction pressure falls below the programmed Suction Pressure Cutout after the ramp has completed.
• The system attempts to run with a low refrigerant charge — the condition the manual names for this cutout, which is why it protects the chiller from an evaporator freeze-up.

• Suction Pressure Cutout — the programmable trip threshold. Once the startup ramp has finished, the system shuts down whenever suction pressure falls below it.
• Suction Pressure Startup Bypass Time — the programmable time at system start during which the cutout is ignored.
• Startup ramp — immediately after the bypass time expires, the cutout is linearly ramped from 10% to 100% of the programmed Suction Pressure Cutout.
• Lockout window — a lockout is caused on the third fault within 90 minutes.

Every occurrence of this safety shuts the system down; a third fault within 90 minutes causes a lockout. To clear it, the manual gives one instruction: lockout cleared by cycling the respective system switch OFF and ON — the system switch of the system shown in the message. No other reset for this safety is described.