Flow switch or freeze protection trips

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1. 1
   Flowchart A

   Too little flow: a blocked strainer, a closed or throttled valve, air in the circuit or a pump running too slowly.

   Remedy: If the pump does not run at all, check its supply first: voltage, fuse, contactor and inverter alarms. Otherwise clean the strainer, open the valves, vent the circuit and check pump speed and rotation.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check pump, strainer and flow switch on the water side

   Low water flow through the evaporator or condenser of a chiller or heat pump causes high pressure trips, freezing of the evaporator and lost capacity. Most causes are found at the pump, the strainer, air in the circuit or the flow switch.

   1. Check the pump in operation. With the unit running, measure the pump current and compare it with the nameplate. Read the differential pressure across the pump or across the heat exchanger and compare it with the design value or the pump curve. Check the rotation direction on three-phase pumps.

      ✓ Check: Current, rotation and differential pressure agree with the nameplate and design data.

   2. Isolate and lock off. Before opening anything on the water side near electrical parts, switch off the pump and the unit, lock off the isolator and prove dead at the pump terminals.

      ✓ Check: The voltage tester shows 0 V on all pump terminals.

   3. Depressurise and clean the strainer. Close the valves on both sides of the strainer and let off the pressure through a drain or vent before opening the cap: hot or glycol water under pressure can spray. Remove and clean the mesh, check the gasket and refit. Collect glycol water, do not drain it to the sewer.

      ✓ Check: The mesh is clean and undamaged, and the cap is tight with no drips after refilling.

   4. Vent the circuit. Refill to the correct static pressure and open the vents at the high points, starting with the lowest, until only water comes out. Check the pressure again and top up if it has fallen.

      ✓ Check: Each vent gives a steady stream of water without air and the system pressure is back at its setting.

   5. Check valves and the flow switch. Check that all isolating and balancing valves in the circuit are open as intended. Check that the flow switch paddle is the right length for the pipe size and sits in the flow direction, or that the differential pressure switch is set as in the unit manual.

      ✓ Check: Valves are in their working positions and the flow switch is fitted and set as the manual describes.

   6. Test the interlock. With the unit running, stop the pump (or slowly close a valve) and watch the unit. The flow switch must open and stop the compressor; the compressor must not be able to start without flow.

      ✓ Check: The compressor stops when the flow stops, and starts only when the pump is running and the flow switch has closed.

   Measure voltage and current under load

   Low voltage, phase imbalance or high current make motors overheat and trip. Measure at standstill, during start and in operation and compare with the nameplate. Live measurements in an open panel are for qualified personnel only.

   1. Prepare for live measurement. Use a CAT III/IV multimeter with insulated probes and a true-RMS clamp meter with inrush function. Wear safety glasses, no loose clothing, and keep hands clear of fans and belts that can start automatically.

      ✓ Check: Meter and leads are CAT III or CAT IV rated and the meter works on a known source.

   2. Measure the supply at standstill. Measure L1–L2, L2–L3 and L3–L1 (and phase to neutral where used) at the incoming terminals with the compressor stopped.

      ✓ Check: All voltages are within the range on the nameplate or data sheet (many compressors allow ±10 % of rated voltage).

   3. Measure voltage during start and in operation. Measure the same phases at the compressor contactor while the compressor starts and again after a few minutes of running. A large drop during start points to a weak supply, undersized cables or a loose connection.

      ✓ Check: The voltage stays within the permitted range also during start and in operation.

   4. Calculate the voltage imbalance. Imbalance (%) = largest deviation from the average of the three phase-to-phase voltages ÷ average × 100.

      ✓ Check: The imbalance is within the compressor manufacturer's limit (often about 2 %).

   5. Measure the current on each phase. Clamp each phase conductor separately: the inrush current at start and the running current after the system has stabilised.

      ✓ Check: The running current does not exceed the rated load amps (RLA) on the nameplate and is about equal on all three phases.

   6. Record and act. Write voltages, imbalance and currents in the service report. High voltage drop: tighten and check connections under isolation. Imbalance or high current: trace the supply or the motor before restarting repeatedly.

      ✓ Check: Readings are documented and any fault is traced to supply, connection or motor.

   Check an inverter (VFD) and read its alarms

   When a speed-controlled compressor, fan or pump will not run or trips, the inverter usually tells you why. Read its alarms first, then check supply, signals and output before you replace anything.

   1. Read and note the alarm log. Before you reset or switch off, read the active fault and the fault history on the drive keypad or service tool. Note code, time and the values stored with it (current, DC bus voltage, frequency). Look up each code in the drive manufacturer's manual.

      ✓ Check: You have a written list of codes with their meaning from the manual, and you know which fault occurred last.

   2. Check the key parameters. Compare motor data (rated voltage, current, frequency), minimum and maximum speed, ramp times and current limit with the compressor or fan manufacturer's settings list. Compressors often have a minimum speed for oil return and lubrication.

      ✓ Check: The parameters match the manufacturer's list; any differences are noted before you change them.

   3. Measure the supply voltage. With a CAT III/IV multimeter and insulated probes, measure phase to phase on L1, L2 and L3 at the drive input, at standstill and in operation. Do not wear loose clothing near open live parts.

      ✓ Check: All three voltages are within the input range in the drive manual and close to each other; a missing or low phase explains supply alarms.

   4. Check run, enable and speed signals. At the control terminals, measure the run/enable input and the speed reference (0-10 V or 4-20 mA) while the controller asks for operation. Follow the safety chain (pressure switches, motor protection) if the enable is missing.

      ✓ Check: The drive display shows a run command and the reference value that the controller sends; if not, the fault is upstream of the drive.

   5. Measure output voltage and current. With the drive running, measure U-V, V-W and W-U with a meter in low-pass (VFD) mode, and the current in each output phase with a true-RMS clamp meter. Compare with the frequency on the display and the motor nameplate.

      ✓ Check: The three output voltages and currents are balanced, and the current is within the motor rating at that speed.

   6. Isolate and wait for the DC bus. Before you disconnect or tighten anything: isolate, lock off and tag. Wait at least the discharge time on the drive label, then measure DC voltage across the DC bus terminals (+/-). Check terminals for heat damage and tighten them. If you megger the motor, disconnect the motor leads from the drive first; never insulation test through the drive.

      ✓ Check: The meter shows no dangerous DC voltage on the bus before you touch anything, and the motor test is done with the drive disconnected.

   7. Restart and watch. Reconnect, close the cover, switch on and start the unit. Follow speed, current and alarms during start and a full run cycle.

      ✓ Check: The drive ramps up to the requested speed without new alarms, and the fault log shows no new entries.

2. 2

   The pump motor, contactor, fuse or inverter has failed.

   Remedy: Measure voltage and current on the pump motor and read the inverter alarms.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Measure voltage and current under load

   Low voltage, phase imbalance or high current make motors overheat and trip. Measure at standstill, during start and in operation and compare with the nameplate. Live measurements in an open panel are for qualified personnel only.

   1. Prepare for live measurement. Use a CAT III/IV multimeter with insulated probes and a true-RMS clamp meter with inrush function. Wear safety glasses, no loose clothing, and keep hands clear of fans and belts that can start automatically.

      ✓ Check: Meter and leads are CAT III or CAT IV rated and the meter works on a known source.

   2. Measure the supply at standstill. Measure L1–L2, L2–L3 and L3–L1 (and phase to neutral where used) at the incoming terminals with the compressor stopped.

      ✓ Check: All voltages are within the range on the nameplate or data sheet (many compressors allow ±10 % of rated voltage).

   3. Measure voltage during start and in operation. Measure the same phases at the compressor contactor while the compressor starts and again after a few minutes of running. A large drop during start points to a weak supply, undersized cables or a loose connection.

      ✓ Check: The voltage stays within the permitted range also during start and in operation.

   4. Calculate the voltage imbalance. Imbalance (%) = largest deviation from the average of the three phase-to-phase voltages ÷ average × 100.

      ✓ Check: The imbalance is within the compressor manufacturer's limit (often about 2 %).

   5. Measure the current on each phase. Clamp each phase conductor separately: the inrush current at start and the running current after the system has stabilised.

      ✓ Check: The running current does not exceed the rated load amps (RLA) on the nameplate and is about equal on all three phases.

   6. Record and act. Write voltages, imbalance and currents in the service report. High voltage drop: tighten and check connections under isolation. Imbalance or high current: trace the supply or the motor before restarting repeatedly.

      ✓ Check: Readings are documented and any fault is traced to supply, connection or motor.

   Check an inverter (VFD) and read its alarms

   When a speed-controlled compressor, fan or pump will not run or trips, the inverter usually tells you why. Read its alarms first, then check supply, signals and output before you replace anything.

   1. Read and note the alarm log. Before you reset or switch off, read the active fault and the fault history on the drive keypad or service tool. Note code, time and the values stored with it (current, DC bus voltage, frequency). Look up each code in the drive manufacturer's manual.

      ✓ Check: You have a written list of codes with their meaning from the manual, and you know which fault occurred last.

   2. Check the key parameters. Compare motor data (rated voltage, current, frequency), minimum and maximum speed, ramp times and current limit with the compressor or fan manufacturer's settings list. Compressors often have a minimum speed for oil return and lubrication.

      ✓ Check: The parameters match the manufacturer's list; any differences are noted before you change them.

   3. Measure the supply voltage. With a CAT III/IV multimeter and insulated probes, measure phase to phase on L1, L2 and L3 at the drive input, at standstill and in operation. Do not wear loose clothing near open live parts.

      ✓ Check: All three voltages are within the input range in the drive manual and close to each other; a missing or low phase explains supply alarms.

   4. Check run, enable and speed signals. At the control terminals, measure the run/enable input and the speed reference (0-10 V or 4-20 mA) while the controller asks for operation. Follow the safety chain (pressure switches, motor protection) if the enable is missing.

      ✓ Check: The drive display shows a run command and the reference value that the controller sends; if not, the fault is upstream of the drive.

   5. Measure output voltage and current. With the drive running, measure U-V, V-W and W-U with a meter in low-pass (VFD) mode, and the current in each output phase with a true-RMS clamp meter. Compare with the frequency on the display and the motor nameplate.

      ✓ Check: The three output voltages and currents are balanced, and the current is within the motor rating at that speed.

   6. Isolate and wait for the DC bus. Before you disconnect or tighten anything: isolate, lock off and tag. Wait at least the discharge time on the drive label, then measure DC voltage across the DC bus terminals (+/-). Check terminals for heat damage and tighten them. If you megger the motor, disconnect the motor leads from the drive first; never insulation test through the drive.

      ✓ Check: The meter shows no dangerous DC voltage on the bus before you touch anything, and the motor test is done with the drive disconnected.

   7. Restart and watch. Reconnect, close the cover, switch on and start the unit. Follow speed, current and alarms during start and a full run cycle.

      ✓ Check: The drive ramps up to the requested speed without new alarms, and the fault log shows no new entries.

3. 3

   The flow switch cable or terminal is loose.

   Remedy: Tighten the terminals and measure across the contact with the pump running.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check pump, strainer and flow switch on the water side

   Low water flow through the evaporator or condenser of a chiller or heat pump causes high pressure trips, freezing of the evaporator and lost capacity. Most causes are found at the pump, the strainer, air in the circuit or the flow switch.

   1. Check the pump in operation. With the unit running, measure the pump current and compare it with the nameplate. Read the differential pressure across the pump or across the heat exchanger and compare it with the design value or the pump curve. Check the rotation direction on three-phase pumps.

      ✓ Check: Current, rotation and differential pressure agree with the nameplate and design data.

   2. Isolate and lock off. Before opening anything on the water side near electrical parts, switch off the pump and the unit, lock off the isolator and prove dead at the pump terminals.

      ✓ Check: The voltage tester shows 0 V on all pump terminals.

   3. Depressurise and clean the strainer. Close the valves on both sides of the strainer and let off the pressure through a drain or vent before opening the cap: hot or glycol water under pressure can spray. Remove and clean the mesh, check the gasket and refit. Collect glycol water, do not drain it to the sewer.

      ✓ Check: The mesh is clean and undamaged, and the cap is tight with no drips after refilling.

   4. Vent the circuit. Refill to the correct static pressure and open the vents at the high points, starting with the lowest, until only water comes out. Check the pressure again and top up if it has fallen.

      ✓ Check: Each vent gives a steady stream of water without air and the system pressure is back at its setting.

   5. Check valves and the flow switch. Check that all isolating and balancing valves in the circuit are open as intended. Check that the flow switch paddle is the right length for the pipe size and sits in the flow direction, or that the differential pressure switch is set as in the unit manual.

      ✓ Check: Valves are in their working positions and the flow switch is fitted and set as the manual describes.

   6. Test the interlock. With the unit running, stop the pump (or slowly close a valve) and watch the unit. The flow switch must open and stop the compressor; the compressor must not be able to start without flow.

      ✓ Check: The compressor stops when the flow stops, and starts only when the pump is running and the flow switch has closed.

   Trace the control and safety chain

   When a compressor or fan does not get a start signal, follow the control voltage through the safety chain in the wiring diagram to the point where it disappears. Live measurements in an open panel are for qualified personnel only.

   1. Get the wiring diagram. Find the diagram for the unit and mark the path from the control supply through fuses, safety devices (HP/LP switches, motor protection, thermal protection) and controller outputs to the contactor coil. Use a CAT III/IV meter with insulated probes.

      ✓ Check: The diagram matches the unit's model and serial plate, and you know the order of the devices.

   2. Check the control supply. Measure the voltage at the secondary of the control transformer or power supply and check the control fuses.

      ✓ Check: The rated control voltage (e.g. 24 V AC or 230 V AC) is present and the fuses are intact.

   3. Follow the chain device by device. Measure from each terminal in the chain to neutral or control common, in the order of the diagram. Never bridge a safety switch to make the system run.

      ✓ Check: You find the point where voltage is present on the input but 0 V on the output: that device is open.

   4. Find out why the device is open. Read the device: is the HP switch tripped and does it have a manual reset, is the motor protection tripped, is a door or flow switch open? Check the cause (pressure, current, temperature) before resetting.

      ✓ Check: The cause of the trip is known, not only the device.

   5. Check the controller output and relays. With a call for operation active, measure the output at the controller relay terminal. A relay that is energised but has no voltage on its output, or that stays closed without a call, is faulty.

      ✓ Check: The output follows the call: voltage when the controller calls and none when it does not.

   6. Repair and test. Isolate and lock off before replacing a part or tightening terminals. Restore power, reset and check that the chain closes and the contactor pulls in.

      ✓ Check: The system starts on call and stops when each safety device opens.

4. 4

   The controls stop the pump before the compressor, or start the compressor before there is flow (missing pump overrun or interlock).

   Remedy: Check the start and stop sequence: the pump should start first and run on after the compressor stops, and the compressor must be interlocked with the flow switch.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check pump, strainer and flow switch on the water side

   Low water flow through the evaporator or condenser of a chiller or heat pump causes high pressure trips, freezing of the evaporator and lost capacity. Most causes are found at the pump, the strainer, air in the circuit or the flow switch.

   1. Check the pump in operation. With the unit running, measure the pump current and compare it with the nameplate. Read the differential pressure across the pump or across the heat exchanger and compare it with the design value or the pump curve. Check the rotation direction on three-phase pumps.

      ✓ Check: Current, rotation and differential pressure agree with the nameplate and design data.

   2. Isolate and lock off. Before opening anything on the water side near electrical parts, switch off the pump and the unit, lock off the isolator and prove dead at the pump terminals.

      ✓ Check: The voltage tester shows 0 V on all pump terminals.

   3. Depressurise and clean the strainer. Close the valves on both sides of the strainer and let off the pressure through a drain or vent before opening the cap: hot or glycol water under pressure can spray. Remove and clean the mesh, check the gasket and refit. Collect glycol water, do not drain it to the sewer.

      ✓ Check: The mesh is clean and undamaged, and the cap is tight with no drips after refilling.

   4. Vent the circuit. Refill to the correct static pressure and open the vents at the high points, starting with the lowest, until only water comes out. Check the pressure again and top up if it has fallen.

      ✓ Check: Each vent gives a steady stream of water without air and the system pressure is back at its setting.

   5. Check valves and the flow switch. Check that all isolating and balancing valves in the circuit are open as intended. Check that the flow switch paddle is the right length for the pipe size and sits in the flow direction, or that the differential pressure switch is set as in the unit manual.

      ✓ Check: Valves are in their working positions and the flow switch is fitted and set as the manual describes.

   6. Test the interlock. With the unit running, stop the pump (or slowly close a valve) and watch the unit. The flow switch must open and stop the compressor; the compressor must not be able to start without flow.

      ✓ Check: The compressor stops when the flow stops, and starts only when the pump is running and the flow switch has closed.

   Check controller settings and alarm log

   Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.

   1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.

      ✓ Check: You have a list of alarms with times and know what each code means.

   2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.

      ✓ Check: The controller clock shows the correct local date and time.

   3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.

      ✓ Check: The service report holds a complete copy of the original settings.

   4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.

      ✓ Check: Every deviation is noted with the original and the recommended value.

   5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.

      ✓ Check: All safety limits are within the manufacturer's values.

   6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.

      ✓ Check: The system runs as expected and the alarm does not return.

5. 5
   Flowchart D

   The freeze protection sensor or the flow switch is wrongly set, badly placed or faulty.

   Remedy: Compare the sensor with a calibrated thermometer and check the flow switch setting and function.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Check a sensor against a reference

   A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.

   1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.

      ✓ Check: The reference reading is steady for a few minutes.

   2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.

      ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).

   3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.

      ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.

   4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.

      ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.

   5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).

      ✓ Check: The sensor sits firmly at the correct point and is insulated.

   6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.

      ✓ Check: Controller and reference agree within the tolerance.

   Check pump, strainer and flow switch on the water side

   Low water flow through the evaporator or condenser of a chiller or heat pump causes high pressure trips, freezing of the evaporator and lost capacity. Most causes are found at the pump, the strainer, air in the circuit or the flow switch.

   1. Check the pump in operation. With the unit running, measure the pump current and compare it with the nameplate. Read the differential pressure across the pump or across the heat exchanger and compare it with the design value or the pump curve. Check the rotation direction on three-phase pumps.

      ✓ Check: Current, rotation and differential pressure agree with the nameplate and design data.

   2. Isolate and lock off. Before opening anything on the water side near electrical parts, switch off the pump and the unit, lock off the isolator and prove dead at the pump terminals.

      ✓ Check: The voltage tester shows 0 V on all pump terminals.

   3. Depressurise and clean the strainer. Close the valves on both sides of the strainer and let off the pressure through a drain or vent before opening the cap: hot or glycol water under pressure can spray. Remove and clean the mesh, check the gasket and refit. Collect glycol water, do not drain it to the sewer.

      ✓ Check: The mesh is clean and undamaged, and the cap is tight with no drips after refilling.

   4. Vent the circuit. Refill to the correct static pressure and open the vents at the high points, starting with the lowest, until only water comes out. Check the pressure again and top up if it has fallen.

      ✓ Check: Each vent gives a steady stream of water without air and the system pressure is back at its setting.

   5. Check valves and the flow switch. Check that all isolating and balancing valves in the circuit are open as intended. Check that the flow switch paddle is the right length for the pipe size and sits in the flow direction, or that the differential pressure switch is set as in the unit manual.

      ✓ Check: Valves are in their working positions and the flow switch is fitted and set as the manual describes.

   6. Test the interlock. With the unit running, stop the pump (or slowly close a valve) and watch the unit. The flow switch must open and stop the compressor; the compressor must not be able to start without flow.

      ✓ Check: The compressor stops when the flow stops, and starts only when the pump is running and the flow switch has closed.

6. 6
   Flowchart B

   The setpoint is too low for the fluid, or the glycol concentration is too low.

   Remedy: Measure the glycol concentration with a refractometer and set the setpoint according to the manufacturer.

   🛠 How do I do it? Step by step

   Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.

   Measure glycol concentration

   Too little glycol and the fluid can freeze in the evaporator or heat exchanger and burst it. Measure the freezing point with a refractometer and adjust the mix.

   1. Check the refractometer. Wear gloves and goggles. Clean the prism and check the zero with a drop of distilled water, as the instrument maker describes.

      ✓ Check: The refractometer reads zero with distilled water.

   2. Take a representative sample. Take the sample from a point where the fluid circulates (not a dead leg or the top of a tank) after the pump has run for a while, into a clean bottle.

      ✓ Check: The sample is clear and comes from the circulating fluid.

   3. Measure. Put a few drops on the prism, close the cover and read the scale against the light. Use the scale for the right glycol type (ethylene or propylene).

      ✓ Check: The boundary line is sharp and you read the freezing point on the right scale.

   4. Compare with the requirement. Compare the freezing point with the design value. It should be well below the lowest evaporating or fluid temperature the system can reach; the margin is set by the designer or the equipment manufacturer.

      ✓ Check: The freezing point is at or below the design value.

   5. Adjust the mix. If the concentration is too low, add the same glycol type and brand (with inhibitors) and let it mix in. Never mix ethylene and propylene glycol. Use propylene glycol where food can be affected.

      ✓ Check: After mixing in, a new sample shows the design freezing point.

   6. Record and set the frost protection. Record the concentration and freezing point in the plant logbook and check that the frost protection setpoint suits the measured freezing point.

      ✓ Check: The values are recorded and the frost protection setpoint has a margin above the freezing point.

   Check controller settings and alarm log

   Many faults that look like hardware faults are wrong settings or alarms that keep repeating. Read the alarm log and compare the settings with the manufacturer's parameter list before you change anything.

   1. Read the alarm log. Open the alarm menu with the keypad or service tool and note every recent alarm code with date and time. Look up each code in the manufacturer's manual.

      ✓ Check: You have a list of alarms with times and know what each code means.

   2. Check the clock. Check the controller's date and time. A wrong clock shifts scheduled defrosts and night setback and makes the alarm log misleading.

      ✓ Check: The controller clock shows the correct local date and time.

   3. Record all settings before changing anything. Write down, photograph or export setpoints, differentials, neutral zone, minimum run and off times, anti-short-cycle timer, defrost interval and duration, and the operating mode.

      ✓ Check: The service report holds a complete copy of the original settings.

   4. Compare with the parameter list. Compare each value with the manufacturer's parameter list and the plant documentation. Look in particular for a differential that is too small (short cycling), timers set to zero and setpoints that do not match the application.

      ✓ Check: Every deviation is noted with the original and the recommended value.

   5. Check the safety limits. Compare low and high pressure cut-outs and temperature limits in the controller with the manufacturer's values. Never set a safety limit beyond the manufacturer's values, and the high pressure cut-out must stay below the safety valve set pressure.

      ✓ Check: All safety limits are within the manufacturer's values.

   6. Change one value at a time and test. Correct one setting at a time, check the operating mode, and let the system run long enough to see the effect. Record every change in the service report.

      ✓ Check: The system runs as expected and the alarm does not return.

7. 7
   Flowchart C

   Low evaporating pressure: a refrigerant shortage or a restriction makes the evaporator too cold.

   Remedy: Troubleshoot the low pressure.

   Continue with: System trips on low pressure →