Low refrigerant pressure often ties back to the chilled water flow being too low or the cooling load being very small. If water isn't moving through the evaporator (due to a pump issue or closed valve), the refrigerant will over-absorb heat and pressure will drop. This is why the first corrective step is often to check coolant water flow and ensure it's unobstructed (clear filters, open valves).
A significant loss of refrigerant will result in low pressure in the system, especially when the compressor runs. If the chiller has been losing charge, eventually the low-pressure switch will prevent operation.
If an expansion valve is stuck too closed or a control malfunction starves the evaporator of refrigerant, the pressure can drop. However, in many cases the system would likely hit freeze alarm first if the expansion valve issue was severe.
If the chiller is being started in very cold ambient conditions, the refrigerant pressure can be naturally low. Many chillers have low ambient control strategies; without them, a cold start might trip LP alarm until things warm slightly.
Immediately check the chilled water circuit for flow. Ensure the pump is on and that all valves are open, and inspect the water filter/strainer for clogs. Low or no flow will cause both low pressure and potentially freeze alarms. If you find an issue (like a closed valve or dirty filter), fix that and allow the system a few minutes to stabilize.
Once adequate flow is confirmed and any obvious issues are fixed, reset the AL174 low-pressure alarm. On many units, you hold the alarm reset for 5 seconds to clear it. The compressor will likely attempt to restart after reset (assuming conditions are normal).
If you have access to gauge readings via the controller or installed gauges, observe the suction pressure as the compressor runs. It should stay above the low-pressure cutout threshold (varies by model, e.g., might need to stay above ~2-3 bar depending on refrigerant). Also monitor that the water flow is steady and the evaporator isn't freezing.
If the low-pressure alarm occurs again even with good water flow, cautiously look for signs of refrigerant leak: oil stains on piping, hissing sounds, or known leak sites. Do not attempt to repair refrigerant leaks yourself, but identifying a suspected leak can be useful information for the technician.
If this happened on a very cold morning, you might try warming up the area or throttling the condenser dampers (if any) to raise pressure slightly on start. This is a niche scenario; in general the system should have controls for it, but older systems might not.
The tech will recover any remaining refrigerant, repair the leak, and recharge the system to the proper level. This will prevent possible damage to the compressor from running in abnormal conditions.
