High receiver pressure (CO2)

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

   The flash gas valve or its control does not work.

   Remedy: Check the control signal and the valve position. On a stepper motor valve such as the Danfoss CCM: measure the phase resistance (CCM: 52 Ω ±10 % per coil) and drive the valve to its reference position, fully closed ^[1]. Compare the receiver sensor reading with a gauge.

   🛠 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 an electronic expansion valve

   An electronic expansion valve (EEV) is only as good as its sensors, cable and driver. Check them in that order before you blame the valve itself.

   1. Read driver alarms and valve opening. Read the alarm log, the superheat setpoint, the measured superheat and the valve opening (%) on the driver or service tool. Note whether the valve is stuck at 0 % or 100 % or hunting.

      ✓ Check: You know what the driver measures and what it is trying to do with the valve.

   2. Check the sensors. Compare the suction pressure transducer with a calibrated gauge at the same point, and the suction temperature sensor with a reference thermometer next to it. Check that the sensor is clamped tight on clean pipe and insulated.

      ✓ Check: Both readings agree with the references within the sensor's tolerance; otherwise fix the sensor first.

   3. Isolate and measure the coil. Isolate the driver supply, lock off and prove dead. Disconnect the valve plug and measure the resistance of each winding (for a typical bipolar stepper, two windings), and from each winding to the valve body.

      ✓ Check: The windings read the same as each other and match the valve data sheet; there is no connection to the body. OL means an open winding.

   4. Check cable and connector. Check the cable for damage and the connector for moisture, corrosion and loose pins. Check that cable length and shielding follow the valve maker's limits.

      ✓ Check: The cable is intact, the connector dry and tight, and the cable within the specified length.

   5. Drive the valve manually. Reconnect and switch on. With the service tool, set the valve to manual and move it in steps from closed towards open while you watch suction pressure and superheat. Do not leave it open so far that liquid reaches the compressor. On CO2 systems do not trap liquid between a closed EEV and another closed valve.

      ✓ Check: Suction pressure and superheat change with each step; no response means a stuck valve or a dead driver output.

   6. Return to reference and automatic. Let the driver initialise the valve to its reference (closed) position and return it to automatic. If the driver has a back-up module that closes the valve on power failure, check its status.

      ✓ Check: The valve homes without alarms and superheat settles at the setpoint in automatic.

   Check CO2 high pressure and flash gas valves

   On a transcritical CO2 (R744) system the high pressure valve controls the gas cooler pressure and the flash gas bypass valve controls the receiver pressure. A faulty valve, driver or transducer gives poor capacity, high pressure trips or a safety valve that opens.

   1. Prepare for CO2 pressures. Use gauges and hoses rated above the PS of the high pressure side (see the nameplate; often above 100 bar), goggles and gloves. Never close valves so that liquid CO2 is trapped between them: the pressure rises quickly when it warms. CO2 displaces oxygen in low and enclosed spaces, so check that the gas detector and ventilation work.

      ✓ Check: Gauges are rated above the system PS and the machine room gas detector shows normal.

   2. Compare transducers with gauges. Connect the controller service tool and read gas cooler pressure, receiver pressure and gas cooler outlet temperature. Compare each pressure transducer with a calibrated gauge at the same point.

      ✓ Check: Each transducer reading agrees with the gauge within the accuracy stated in the transducer data sheet.

   3. Check the high pressure valve. Watch the valve opening (%) and the gas cooler pressure in the service tool while the gas cooler outlet temperature changes. Compare the pressure with the setpoint the controller calculates. Check the driver for alarms and, if the manual allows, drive the valve manually and listen or feel for movement.

      ✓ Check: The gas cooler pressure follows the controller setpoint and the valve opening changes smoothly, without hunting or alarms.

   4. Check the flash gas bypass valve. Compare the receiver pressure with the setpoint in the commissioning documents. Watch the valve opening as the load changes.

      ✓ Check: The receiver pressure stays close to its setpoint and below the receiver safety valve setting.

   5. Check the gas cooler fan control. Check that the gas cooler fans speed up as the outlet temperature rises and that the control sensor reads correctly (compare with a contact thermometer).

      ✓ Check: Fan speed follows the gas cooler outlet temperature and the sensor agrees with the reference thermometer.

   6. Check relief paths and record. Check that every pipe section that can be closed off by valves has a pressure relief device, and that safety valve outlets are open and unblocked. Record readings, alarms and any corrections.

      ✓ Check: No section can trap liquid without relief, and the readings are recorded.

   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.

2. 2
   Flowchart C

   There is more flash gas than the valve or the compressors can handle.

   Remedy: Check the gas cooler pressure setpoint and that the gas cooler and its fans work. If the problem recurs, review the capacity, for example with parallel compression.

   🛠 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 CO2 high pressure and flash gas valves

   On a transcritical CO2 (R744) system the high pressure valve controls the gas cooler pressure and the flash gas bypass valve controls the receiver pressure. A faulty valve, driver or transducer gives poor capacity, high pressure trips or a safety valve that opens.

   1. Prepare for CO2 pressures. Use gauges and hoses rated above the PS of the high pressure side (see the nameplate; often above 100 bar), goggles and gloves. Never close valves so that liquid CO2 is trapped between them: the pressure rises quickly when it warms. CO2 displaces oxygen in low and enclosed spaces, so check that the gas detector and ventilation work.

      ✓ Check: Gauges are rated above the system PS and the machine room gas detector shows normal.

   2. Compare transducers with gauges. Connect the controller service tool and read gas cooler pressure, receiver pressure and gas cooler outlet temperature. Compare each pressure transducer with a calibrated gauge at the same point.

      ✓ Check: Each transducer reading agrees with the gauge within the accuracy stated in the transducer data sheet.

   3. Check the high pressure valve. Watch the valve opening (%) and the gas cooler pressure in the service tool while the gas cooler outlet temperature changes. Compare the pressure with the setpoint the controller calculates. Check the driver for alarms and, if the manual allows, drive the valve manually and listen or feel for movement.

      ✓ Check: The gas cooler pressure follows the controller setpoint and the valve opening changes smoothly, without hunting or alarms.

   4. Check the flash gas bypass valve. Compare the receiver pressure with the setpoint in the commissioning documents. Watch the valve opening as the load changes.

      ✓ Check: The receiver pressure stays close to its setpoint and below the receiver safety valve setting.

   5. Check the gas cooler fan control. Check that the gas cooler fans speed up as the outlet temperature rises and that the control sensor reads correctly (compare with a contact thermometer).

      ✓ Check: Fan speed follows the gas cooler outlet temperature and the sensor agrees with the reference thermometer.

   6. Check relief paths and record. Check that every pipe section that can be closed off by valves has a pressure relief device, and that safety valve outlets are open and unblocked. Record readings, alarms and any corrections.

      ✓ Check: No section can trap liquid without relief, and the readings are recorded.

   Clean a condenser or gas cooler

   A dirty condenser or gas cooler raises the condensing pressure, the power consumption and the risk of high pressure trips. Clean it, straighten the fins and check that air can flow freely.

   1. Measure before you clean. With the system running, note the condensing pressure (or gas cooler outlet temperature on CO2) and the air-on temperature, so that you can see the effect afterwards.

      ✓ Check: Pressure and air temperature are recorded.

   2. Isolate and lock off. Isolate the condensing unit and fans, lock and tag the isolator and prove dead at the fan motor terminals. Fans can otherwise start automatically. On roofs, use fall protection.

      ✓ Check: 0 V at the fan terminals and the fans have stopped.

   3. Remove loose dirt. Brush off leaves and fluff from the air-entering side with a soft brush along the fins. Blow or rinse from the air-leaving side, against the normal airflow, so dirt is pushed out rather than in.

      ✓ Check: The surface of the coil is free of loose dirt.

   4. Clean with coil cleaner and water. Protect fan motors and electrical parts from water. Apply a coil cleaner suitable for the fin material, follow its safety data sheet (gloves and goggles) and rinse with low-pressure water against the normal airflow. Never use a high-pressure washer, which flattens the fins.

      ✓ Check: The rinse water runs clear and the fins are undamaged.

   5. Straighten fins and check clearances. Straighten bent fins with a fin comb of the right pitch. Check that nothing blocks the air inlet or outlet and that warm air cannot recirculate to the inlet (walls, snow, other units).

      ✓ Check: Fins are straight and the free distances around the unit match the installation instructions.

   6. Restart and compare. Remove your lock, restore power and let the system stabilise. Compare the condensing pressure and the air-on temperature with the values before cleaning.

      ✓ Check: The condensing temperature is lower at a similar air temperature.

   Check a condenser or evaporator fan motor

   A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.

   1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.

      ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.

   2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.

      ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).

   3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.

      ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.

   4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.

      ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.

   5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.

      ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.

   6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.

      ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).

3. 3
   Flowchart B

   The compressors that should handle the flash gas are stopped.

   Remedy: Troubleshoot the compressor that will not start. If the whole plant has stopped (power failure), troubleshoot the standstill pressure.

   Continue with: Compressor will not start →

4. 4

   The system has been at standstill (power failure or stop), so the pressure rises as the refrigerant warms up.

   Remedy: Troubleshoot the standstill pressure.

   Continue with: High standstill pressure in a CO2 system →

5. 5
   Flowchart D

   Fault in the receiver sensor or setpoint, or in the high pressure valve.

   Remedy: Check sensors and setpoints in the controller and that the high pressure valve is controlling.

   🛠 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 CO2 high pressure and flash gas valves

   On a transcritical CO2 (R744) system the high pressure valve controls the gas cooler pressure and the flash gas bypass valve controls the receiver pressure. A faulty valve, driver or transducer gives poor capacity, high pressure trips or a safety valve that opens.

   1. Prepare for CO2 pressures. Use gauges and hoses rated above the PS of the high pressure side (see the nameplate; often above 100 bar), goggles and gloves. Never close valves so that liquid CO2 is trapped between them: the pressure rises quickly when it warms. CO2 displaces oxygen in low and enclosed spaces, so check that the gas detector and ventilation work.

      ✓ Check: Gauges are rated above the system PS and the machine room gas detector shows normal.

   2. Compare transducers with gauges. Connect the controller service tool and read gas cooler pressure, receiver pressure and gas cooler outlet temperature. Compare each pressure transducer with a calibrated gauge at the same point.

      ✓ Check: Each transducer reading agrees with the gauge within the accuracy stated in the transducer data sheet.

   3. Check the high pressure valve. Watch the valve opening (%) and the gas cooler pressure in the service tool while the gas cooler outlet temperature changes. Compare the pressure with the setpoint the controller calculates. Check the driver for alarms and, if the manual allows, drive the valve manually and listen or feel for movement.

      ✓ Check: The gas cooler pressure follows the controller setpoint and the valve opening changes smoothly, without hunting or alarms.

   4. Check the flash gas bypass valve. Compare the receiver pressure with the setpoint in the commissioning documents. Watch the valve opening as the load changes.

      ✓ Check: The receiver pressure stays close to its setpoint and below the receiver safety valve setting.

   5. Check the gas cooler fan control. Check that the gas cooler fans speed up as the outlet temperature rises and that the control sensor reads correctly (compare with a contact thermometer).

      ✓ Check: Fan speed follows the gas cooler outlet temperature and the sensor agrees with the reference thermometer.

   6. Check relief paths and record. Check that every pipe section that can be closed off by valves has a pressure relief device, and that safety valve outlets are open and unblocked. Record readings, alarms and any corrections.

      ✓ Check: No section can trap liquid without relief, and the readings are recorded.

   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.