- The chiller has stopped and alarm AL171 is shown for Circuit 1, indicating a High Pressure refrigerant alarm. The red alarm LED may be on, and the circuit will not run until reset.
- You may hear the condenser fan(s) running heavily before the trip, or conversely, one of the fans might not have been running at all. Often, a high-pressure trip is accompanied by the condenser area being very hot – you might feel excessively hot air around the chiller or see the fan cutting in and out rapidly.
- In some cases, a pressure gauge on the system (if visible) would show high discharge pressure. If the unit was running, it likely shut off abruptly once the pressure switch activated. You might also find the high-pressure manual reset switch (if equipped) has popped out and needs resetting.
Reduced Airflow Through Condenser
The most common cause is a blocked or dirty condenser coil, which prevents proper heat dissipation. Dirt, debris, or clogged fins on the coil lead to heat buildup and high refrigerant pressure. Similarly, if the condenser fans are obstructed or running at reduced speed, the effect is the same.
Condenser Fan Failure
If one or more condenser fans stopped working (e.g., fan motor failure or overload like AL366 on one fan), the remaining fans might not handle the heat load, causing pressure to rise.
High Ambient Temperature or Inadequate Clearance
Extremely hot weather can push pressures high. Also, if the chiller is too close to walls or other equipment, recirculating hot air, the condenser can't reject heat effectively, leading to an HP alarm.
Refrigerant Overcharge or Non-Condensables
Less commonly (and to be checked by technicians), having too much refrigerant in the system or air/non-condensable gases can elevate pressure. This isn't usually the first suspect unless recent maintenance involved adding refrigerant.
Shut Off the Chiller Power
For safety, turn off the chiller before attempting any inspection. High pressure conditions can be dangerous, and you will likely need to access the condenser area.
Inspect and Clean the Condenser Coil
Check the condenser coils for dirt, dust, or obstructions. Clear any debris and clean the coil fins gently (e.g., with a soft brush or coil cleaner). Ensure nothing is blocking airflow into or out of the coils (like leaves, paper, or other debris).
Verify All Condenser Fans Are Operating
With the power back on, observe the fans when the chiller starts (or manually test them if possible). Replace or repair any non-working fan. If a fan motor overload tripped (e.g., you also have AL366 alarm), reset that as well. Balanced airflow is crucial.
Improve Airflow and Conditions
If the unit is in an enclosure or near walls, open doors or create space for ventilation. In high ambient conditions, every bit of airflow helps – you might use temporary fans to augment airflow as a short-term fix, but ultimately ensure the chiller has proper clearance per manufacturer specs.
Reset the AL171 Alarm
Once the underlying issue is addressed (coil cleaned, fans running), reset the high-pressure alarm. On many systems, the high-pressure cutout might be a manual reset switch on the compressor or in the electrical panel that needs to be pressed after the pressure drops to a safe level. Then you may need to press the alarm reset on the controller as well.
Monitor Operation Carefully
After reset, keep an eye on the chiller's pressure readings if available. The condenser should expel warm (not searing hot) air when working correctly. The fans should run smoothly, and the pressure should remain within normal operating range.
Professional Help Required:
- Recurring high-pressure trips: If the AL171 alarm happens again shortly after cleaning the coil and confirming fans are working, do not continually reset it. Persistent high pressure could indicate refrigerant issues or pressure switch calibration problems.
- If you found the condenser coil was clean and fans operational, yet AL171 still tripped, there may be a deeper problem such as a failing compressor or blockage in the refrigeration circuit.
- High ambient environment solutions: When ambient temps are extreme, a tech might recommend additional cooling or scheduling operations during cooler periods.
- Safety concerns: If you're uncomfortable resetting a high-pressure switch or hear the pressure relief valve venting, call a technician immediately.
Dealing with refrigerant pressure is sensitive. Technicians have the expertise to safely handle high-pressure refrigerant problems and restore the system without risking equipment damage or personal injury.