YORK YLAA — Low Suction Pressure

The panel prints Low Suction Pressure; the fault tables print the same name in capitals. It is a fault/inhibit code name in the Native BACnet, Modbus, and N2 communications data map, reported through the Sys 1 / Sys 2 Fault Code points, and the communications fault-code list assigns it code 8. On the system points, Low suction pressure will display if the system is not locked out.

The Suction Pressure Cutout is a software cutout that protects the chiller from evaporator freeze-up if the system tries to run with a low refrigerant charge or with a restriction in the refrigerant circuit.

Its behaviour around a start is staged. When a system starts, the cutout is set to 10% of the programmed value and is ramped up to the programmed cutout point over the following 3 minutes; if suction pressure drops below the ramped cutout point at any time during those 3 minutes, the system stops. For the first 30 seconds of run time the cutout is ignored completely, to avoid nuisance shutdowns — especially on units using a low pressure switch instead of the suction pressure transducer. If, after the first 3 minutes, suction pressure falls below the programmed cutout setting, a “transient protection routine” takes over: it again sets the cutout to 10% of the programmed value and ramps it up over the next 30 seconds.

• Suction pressure cutout settings that do not match the recommended settings.
• Low refrigerant charge.
• Fouled filter dryer.
• Defective TXV.
• Reduced flow of chilled liquid through the cooler.
• Defective suction pressure transducer or low pressure switch, or faulty wiring.
• Defective LLSV.

1. Compare the programmed suction pressure cutout settings with the recommended settings and adjust them as required.
2. Check the refrigerant charge; repair a leak if necessary and add refrigerant.
3. Check the filter dryer and change the dryer/core if it is fouled.
4. Check the TXV and replace it if defective.
5. Check the chilled-liquid flow through the cooler: verify GPM against the Operational Limitations (Imperial) in the Installation section, check the pump, clean the pump strainer, and purge the chilled liquid system of air.
6. Check the suction pressure transducer / low pressure switch and its wiring; replace the faulty component or wiring. The Service and Troubleshooting section gives the pressure/voltage formula for this check.
7. Check the LLSV and replace it if defective.

• Low Suction Pressure Cutout Heating (HP) — the point lists print it with BACnet object AI37 and point list code ADF 44, and as AV33 with point list code ADF 33; engineering units PSI/BAR, values carried with x10 scaling.
• Low Suction Pressure Cutout Cooling — the point lists print it with BACnet object AI38 and point list code ADF 45, and as AV34 with point list code ADF 34; engineering units PSI/BAR, values carried with x10 scaling.

Before restarting, note the manual's warning: Repeated starts after resetting a low suction pressure fault will cause evaporator freeze-up. Whenever a system locks out on this safety or any safety, the manual calls for immediate steps to identify the cause. On the restart after a reset, the cutout again starts at 10% of the programmed value, ramps up over the next 3 minutes, and is ignored for the first 30 seconds.