Chiller alarm codes

Choose the alarm type from your chiller controller and get the most likely causes and the first checks to make, in a sensible order. The guidance works for all brands, because it starts from the physical problem behind the alarm.

How to read a chiller alarm

Every chiller controller reports faults in its own way. One manufacturer shows a number, another a short code with letters, a third a full text line in the alarm log. Behind all of them stands a small set of physical problems: the pressure on the high or low side is outside the safe range, the water does not flow, the motor or the oil is in danger, or the controller does not trust one of its signals. The alarm finder above groups the alarms in this way, so you can start the right checks even if you do not have the manual at hand.

Before you reset anything, write down three things: the exact alarm text and number, the time it appeared and the values the controller stored at the moment of the trip. Most modern controllers keep a history with pressures and temperatures. That snapshot is often worth more than any measurement taken an hour later, when the unit has cooled down and the conditions are different.

Manual and automatic reset

Safety devices that protect against a dangerous condition, such as the high pressure switch, usually need a manual reset. Operating alarms, such as a short flow interruption, often reset automatically and the controller restarts the compressor after a delay. A controller that blocks the unit after three or more trips per hour is doing its job. If a unit trips again soon after each reset, the fault is still there and every new start adds wear to the compressor.

Alarm group	Typical device	First measurement	Who should act
High pressure	HP switch, discharge transducer	Condensing temperature against ambient or condenser water	Operator checks fans and coil, technician for refrigerant side
Low pressure	LP switch, suction transducer	Superheat and subcooling	Certified technician
Freeze protection	Leaving water probe, evaporating temperature	Flow, glycol concentration	Operator for flow, technician for refrigerant side
Flow	Flow switch, differential pressure switch	Pump differential pressure, strainer	Maintenance team
Motor protection	Thermal protector, PTC module, overcurrent relay	Phase voltages and currents	Electrician with refrigeration experience
Phase	Phase monitoring relay	Voltage and phase sequence	Electrician
Oil	Oil pressure or level switch	Oil level, superheat	Certified technician
Sensor or communication	Probes, transducers, bus modules	Resistance, supply voltage, wiring	Technician with the wiring diagram

Pressure alarms tell you about heat transfer

A high pressure alarm almost always means that the condenser cannot reject the heat. On air-cooled units we most often find a fouled coil, a fan that stopped or a fan speed controller that keeps the fans too slow. On water-cooled units the usual reasons are low condenser water flow or a fouled heat exchanger. Only after these are ruled out does it make sense to suspect an overcharge or non-condensable gases.

A low pressure alarm is the mirror image on the evaporator side. If superheat is high and subcooling is low, the circuit is most likely short of refrigerant and there is a leak to find. If superheat is high but subcooling is normal, look at the expansion valve, the filter drier and the liquid line solenoid valve. If both are normal and the pressure is still low, check the water flow and the evaporator. Our refrigerant pressure-temperature table converts the gauge readings into saturation temperatures for these checks.

Water side alarms

Flow and freeze alarms are linked. A chiller with weak flow often trips on freeze protection first, because the leaving water cools down faster than the flow switch reacts. Before changing any setting, clean the strainer, vent the system and check the pump. If the system runs on glycol, measure the concentration: water that was added after leaks dilutes the mixture and the freeze margin disappears. The glycol calculator shows how much concentrate brings the mixture back to the required protection.

Electrical alarms

Motor protection and phase alarms need a meter, not guesswork. Measure the three phase voltages under load, the current on each phase and compare them with the nameplate. Voltage imbalance of a few per cent already raises the winding temperature noticeably. Never bridge a phase monitor to get the unit running: a scroll compressor that runs backwards for a short time can be damaged.

When to call a service company

• The alarm repeats after a reset or appears several times a week.
• The fault concerns the refrigerant circuit: leaks, low pressure, oil or high discharge temperature.
• The compressor makes unusual noise, or you smell burnt insulation in the panel.
• Somebody suggests bridging a safety device or changing safety setpoints.

When you send us an enquiry, attach a photo of the nameplate, the alarm text and, if possible, the alarm history screen. We measure first and then decide what to repair. More about our approach is on the chiller service page.

Frequently asked questions

Why does the guide not list code numbers for each brand?

The same number means different things on different controllers and software versions, and a wrong code list sends people in the wrong direction. The alarm text and the unit manual are the reliable source; the physical cause behind the alarm is what this page helps with.

Can I reset a high pressure alarm myself?

Yes, once, after checking that all condenser fans or the condenser pump run and the coil is clean. If it trips again, leave the unit off and have the refrigerant side checked. Repeated resets under high pressure stress the compressor.

What does it mean when a chiller trips only on hot afternoons?

The condenser works at its limit. Typical reasons are a partly fouled coil, a weak fan or hot air recirculation. The capacity margin that the unit had when new has been used up.

Is a sensor fault urgent?

It depends on the input. A failed leaving water probe can stop the whole unit or disable freeze protection, so it needs attention the same day. A failed outdoor air probe often only changes the control behaviour.