YORK YCAV — LOW SUCTION PRESSURE FAULTS CAUSED BY LOW SUBCOOLING

The panel reads LOW SUCTION PRESSURE FAULTS CAUSED BY LOW SUBCOOLING. The manual prints the message in only this one form and places it in its LOW SUCTION PRESSURE FAULTS group for this chiller.

Per the manual, this low suction pressure fault points at the refrigerant side of the running system: the refrigerant charge is low, or there are non-condensables in the system. The manual gives no other cause for this message. Its diagnostic route runs through two observations — the liquid line sight glass and the “liquid line subcooling” — which together separate an undercharged system from one containing non-condensables.

• Low refrigerant charge — the manual states that low charge is often responsible for low suction pressure faults.
• An undercharged system, or non-condensables in the system — indicated when there is no liquid, or bubbles are consistently observed, in the liquid line sight glass.
• Non-condensables in the system — indicated when high subcooling is noted along with bubbles in the sight glass.

1. Run the suspect system to full speed.
2. Look at the liquid line sight glass for bubbles or a loss of liquid. A full column of liquid should normally be observed except during transients, such as fan cycling or a rapid speed increase.
3. If there is no liquid, or bubbles are consistently observed in the sight glass, the system is undercharged or there are non-condensables in it.
4. Check the “liquid line subcooling” of the system; it should normally be 7 to 12° F on a 95° F day.
5. In colder ambients, turn fans OFF to approximately duplicate those conditions before reading the subcooling.
6. If high subcooling is noted along with bubbles in the sight glass, there are non-condensables in the system.
7. To verify correct charge, check subcooling along with verifying there are no bubbles in the sight glass. Do not weigh the charge.
8. When checking “liquid subcooling”, always measure the liquid pressure as close to the feed valve as possible on the liquid line, and measure the liquid line temperature as close as possible to the feed valve.

• Liquid line subcooling reference: normally 7 to 12° F on a 95° F day.
• Sight glass reference: a full column of liquid is the normal observation; bubbles or a loss of liquid are expected only during transients such as fan cycling or a rapid speed increase.