- The chiller shuts down and alarm AL174 is displayed for Circuit 1, signifying a Low Refrigerant Pressure condition. Often this alarm occurs shortly after the compressor starts (after a brief delay) if the pressure doesn't build up sufficiently.
- You might observe the compressor trying to run then stopping, or it may not start at all if the pressure is sensed too low. In many designs there is a ~45 second delay after compressor start before a low-pressure alarm is allowed, to avoid nuisance trips. The fact it tripped means pressure stayed low beyond that.
- If there are sight-glasses or pressure gauges on the unit, the refrigerant suction pressure will be very low (possibly in vacuum) when this alarm triggers. Sometimes you may also see the evaporator barrel starting to frost (if low pressure is due to no flow of water). Other alarms like freeze protection might accompany it.
Inadequate Water Flow or Low Load
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).
Refrigerant Leak (Low Charge)
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.
Expansion Valve or Control Issue
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.
Cold Ambient/Start-up in Cold Weather
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.
Verify and Restore Water Flow
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.
Reset the Alarm
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).
Watch the Pressure on Restart
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.
Inspect for Refrigerant Leaks
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.
Ambient Temperature Considerations
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.
Professional Help Required:
- Suspected refrigerant leak: If you have any indication that refrigerant might be low (e.g., AL174 continues after basic checks, or you've observed oil around connections), call a certified HVAC technician. Handling refrigerant requires special training and tools.
- Alarm keeps tripping despite normal flow: If water flow is fine and the chiller still hits low pressure, there could be a problem with the refrigerant circuit components (expansion valve, low-pressure switch calibration, etc.).
- Reoccurring trips in cold weather: If your environment is very cold and the chiller isn't equipped for it, a technician can install low ambient controls or adjust settings.
- General uncertainty or multiple faults: Low-pressure alarms can be tricky because they're both a potential cause (leak) or an effect (of other issues). If you're not certain what's causing AL174, it's safest to bring in a technician.
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.