High discharge temperature or hot compressor

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

   Excessive suction superheat: too little refrigerant, a restriction or a badly set expansion valve.

   Remedy: Follow the low-pressure troubleshooting and set the superheat 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 superheat

   Superheat shows whether the evaporator is fed with the right amount of refrigerant: too high starves the evaporator, too low risks liquid reaching the compressor. Measure at the evaporator outlet (for the expansion valve) and at the compressor (to protect it).

   1. Let the system stabilise. Run at normal load for at least 10–15 minutes, not during or right after defrost. Wear goggles and gloves (cold pipes can cause frostbite); with A2L/A3 refrigerants keep ignition sources away.

      ✓ Check: Suction pressure and pipe temperature no longer drift.

   2. Read the suction pressure. Connect the gauge to a port at the evaporator outlet if there is one; a port at the compressor shows a lower pressure because of the pressure drop in the suction line. Select the correct refrigerant.

      ✓ Check: The gauge shows a stable pressure, and you know where it was measured.

   3. Measure the pipe temperature. Clamp a pipe thermometer on clean bare pipe next to the expansion valve bulb (evaporator superheat) and on the suction line about 15–30 cm before the compressor (compressor superheat), insulated from the surrounding air.

      ✓ Check: Both temperatures are stable.

   4. Calculate superheat. Convert the pressure to the saturation temperature; for zeotropic blends use the dew point. Superheat = measured pipe temperature − saturation temperature.

      ✓ Check: The result is positive and is compared with the manufacturer's target; for a TXV at the evaporator outlet it is often around 5–8 K.

   5. Interpret before adjusting. High superheat together with bubbles in the sight glass or low subcooling points to low charge or a restriction, not a valve setting. Very low compressor superheat means a risk of liquid floodback. Check charge, sight glass and filter drier before touching the valve.

      ✓ Check: The cause is supported by at least one other reading.

   6. Adjust only when the cause is the valve. Adjust a TXV in small steps (about a quarter turn) and wait 15–20 minutes between steps; on an electronic valve change the setpoint in the controller. Record the values before and after.

      ✓ Check: Superheat settles at the target and stays stable during a full cycle.

   Adjust superheat and size the expansion valve

   A thermostatic expansion valve (TXV) is adjusted only after other causes are ruled out. Small steps and patience give stable superheat without liquid reaching the compressor.

   1. Rule out other causes first. Check charge, sight glass, liquid line restriction, bulb mounting and equaliser line (see those guides). Do not use the adjustment screw to hide another fault.

      ✓ Check: Other causes are ruled out or fixed.

   2. Measure superheat. In stable operation, measure suction pressure at the evaporator outlet and the pipe temperature at the bulb. Superheat = pipe temperature − saturation temperature (dew point for blends). Find the target in the evaporator or valve maker's data.

      ✓ Check: You have a steady superheat value and a target to compare with.

   3. Adjust in small steps. Remove the cap and turn the spindle a quarter to half a turn. On most valves clockwise increases superheat; check the maker's marking. Note how many turns you have made.

      ✓ Check: The spindle is turned a known amount in the right direction.

   4. Wait for stable operation. Wait at least 15 minutes before you measure again; the valve and evaporator react slowly.

      ✓ Check: Superheat stays steady over several readings.

   5. Repeat and protect the compressor. Repeat until the target is reached. Check suction superheat at the compressor as well and look for frost on the compressor; never set superheat so low that liquid reaches the compressor.

      ✓ Check: Evaporator superheat is at the target and there is no sign of liquid at the compressor.

   6. Judge the valve size. Hunting with large swings that will not settle can mean the valve or orifice is too large; high superheat with the valve fully open can mean it is too small. Compare the valve capacity at the actual evaporating temperature, liquid temperature and pressure drop with the evaporator load in the maker's selection data. Changing the orifice opens the circuit and needs F-gas certified personnel.

      ✓ Check: Valve capacity matches the load, or you have a documented reason to change it.

   7. Refit the cap and record. Refit the cap with its seal, leak test it and write down the final setting and superheat.

      ✓ Check: The cap is tight and the setting is recorded.

   Continue with: System trips on low pressure →

2. 2
   Flowchart B

   A high compression ratio gives a higher temperature after compression.

   Remedy: Lower the condensing pressure (see high pressure) or raise the suction pressure where possible.

   Continue with: System trips on high pressure →

3. 3

   The compressor cooling fan or liquid injection (where fitted) does not work.

   Remedy: Check the fan and the liquid injection valve, coil and sensor.

   🛠 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 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).

   Check a solenoid valve (liquid line or hot gas)

   A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.

   1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.

      ✓ Check: You know whether the valve should be open or closed at this moment.

   2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.

      ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.

   3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.

      ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.

   4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.

      ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.

   5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.

      ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.

   6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.

      ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.

   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.

4. 4
   Flowchart C

   Electrical fault: phase imbalance, low voltage or a winding fault that heats the motor.

   Remedy: Measure voltage and current on all phases. Check the contactor and connections.

   🛠 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.

   Test a contactor

   Worn, welded or loosely connected contactor contacts cause single-phasing, overheating and motor damage. Check the contacts, the coil and the voltage drop, and replace a worn contactor. Electrical work is for qualified personnel only.

   1. Isolate and lock off. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all line and load terminals, also the coil supply, which can come from a separate control circuit.

      ✓ Check: 0 V on all poles, the coil terminals and to earth.

   2. Inspect the contacts and the mechanism. Remove the arc cover if the design allows. Look for heavily pitted, burnt or welded contacts, and discoloured terminals or melted plastic. Press the armature by hand (power off) and check that it moves freely and springs back.

      ✓ Check: Contacts are without heavy pitting or welds, and the armature moves freely.

   3. Check the coil. Disconnect one coil lead and measure the coil resistance. Compare with the manufacturer's data or a new contactor of the same type.

      ✓ Check: The coil is neither open (OL) nor near 0 Ω.

   4. Check the coil voltage and pull-in. Reconnect, remove your lock, restore power and use the CAT III/IV meter to measure the voltage across the coil when the controller calls for operation.

      ✓ Check: The coil voltage matches the rating printed on the coil, and the contactor pulls in firmly without humming or chattering.

   5. Measure the voltage drop across each pole. With the motor running, measure from line to load terminal across each closed pole.

      ✓ Check: The drop is close to 0 V and about equal on all poles; a pole with clearly higher drop has worn contacts.

   6. Tighten or replace. Isolate and lock off again. Tighten all terminals to the manufacturer's torque. Replace a worn contactor with one of the same coil voltage and at least the same motor rating (AC-3), then restart and measure the current on each phase.

      ✓ Check: Terminals are tight, and the motor current is about equal on all phases after restart.

5. 5
   Flowchart D

   Internal leakage in the compressor (broken valves or gasket), so the same gas is compressed several times.

   Remedy: Check the compressor according to the manufacturer's instructions and replace it if needed. After a burnout: acid test and a new filter drier.

   🛠 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.

   Replace a compressor and clean up after burnout

   After a motor burnout the system holds acid, moisture and carbon. A new compressor fitted without a proper clean-up fails again. The work opens the refrigerant circuit and requires F-gas certification.

   1. Confirm the fault. Confirm that the compressor is faulty: measure winding and insulation resistance (with the power isolated, locked off and proven dead), and test the oil for acid to find out whether there was a burnout.

      ✓ Check: The fault is confirmed and you know whether the oil is acidic.

   2. Recover the refrigerant. Recover the refrigerant into a marked recovery cylinder (gloves and goggles; burnt oil is acidic). For flammable refrigerants (A2L/A3), remove ignition sources and ventilate. Record the quantity.

      ✓ Check: The system is at the final recovery pressure and the quantity is recorded.

   3. Remove the old compressor. Get a hot work permit and have an extinguisher at hand. Cut or unbraze the connections with nitrogen flowing. Lift with lifting gear. Keep the old oil for inspection and seal the pipe ends.

      ✓ Check: The compressor is out and the open pipes are sealed.

   4. Fit the new compressor and filters. Check that the new compressor has the right model and oil type and the correct oil quantity. Fit it with new mountings, a burnout filter drier in the liquid line and, after a severe burnout, a suction filter. Braze with nitrogen flowing gently.

      ✓ Check: The compressor and filters are fitted and all joints are brazed with nitrogen flowing.

   5. Pressure test and evacuate. Pressure test with dry nitrogen, leak test, then evacuate to the target vacuum and do a decay test.

      ✓ Check: The system holds the test pressure and passes the decay test.

   6. Charge, start and check. Charge the nameplate quantity by weight. Check the wiring, contactor and motor protection setting before starting. Run the system and check superheat, subcooling and current.

      ✓ Check: Superheat, subcooling and current agree with the design data.

   7. Follow up the clean-up. After some operating time, test the oil for acid again and check the pressure drop over the suction filter. Change filter drier and oil until the acid test passes; remove or replace the suction filter as its manufacturer says. Record everything in the logbook.

      ✓ Check: The acid test passes and the logbook shows the work, the quantities and the follow-up.

   Test the oil for acid

   Acid and moisture in the oil follow a motor burnout, overheating or a wet system, and they destroy a new compressor quickly. An acid test tells you whether the oil and drier must be changed and when the clean-up is finished.

   1. Choose the right kit. Use an acid test kit made for the oil in the compressor (POE, PAG, mineral or PAO; see the compressor nameplate). Check the expiry date. Wear goggles and gloves: burnt oil is acidic and the reagent is a chemical; follow its safety data sheet.

      ✓ Check: The kit matches the oil type and is within its expiry date.

   2. Take the oil sample. Take the sample from the compressor's oil drain or sampling valve, opening it slowly with low pressure in the crankcase and the sample bottle held firmly. On a hermetic compressor without a valve, the refrigerant must be recovered first and the oil drained from a removed compressor; that work requires F-gas certification (EPA 608 in the USA). Let the first few drops run to waste.

      ✓ Check: The clean bottle holds the amount the kit asks for, without refrigerant foaming over.

   3. Look at and smell the oil. Before testing, judge the oil by eye: clean oil is clear and light. Dark or black oil, a sharp burnt smell, sludge or metal particles point to a burnout or mechanical wear.

      ✓ Check: Colour, smell and any particles are noted.

   4. Run the test. Mix the oil and the reagent exactly as the kit instructions say (amounts, shaking time, waiting time).

      ✓ Check: The mixture has stood for the time stated in the kit instructions.

   5. Read the result against the kit chart. Compare the colour with the chart supplied with the kit. The colours differ between makes, so use only that kit's chart, never colours remembered from another brand.

      ✓ Check: The result is read as 'acceptable' or 'acid' according to the kit's own chart.

   6. Decide and record. If the oil is acidic: change the oil, fit a burnout filter drier (suction and liquid line) and repeat the test after some operating time until it passes. Record the result, the action and the date in the logbook.

      ✓ Check: The logbook shows the test result, the action taken and when the next test is due.

   Replace a filter drier

   Replace the filter drier after moisture, a compressor burnout, a restriction across the drier or whenever the circuit has been opened. Work on the refrigerant circuit requires F-gas certification; brazing is hot work.

   1. Isolate or recover the refrigerant. Pump down or close valves to isolate the drier section where the system allows, otherwise recover the refrigerant with a recovery unit into a marked recovery cylinder. Break the vacuum with dry nitrogen to just above atmospheric pressure before opening. With A2L/A3 refrigerants purge with nitrogen and check with a gas detector.

      ✓ Check: The section reads slightly above 0 bar(g) on nitrogen and the pressure does not rise (no refrigerant left).

   2. Choose the right drier. Use a drier of the same size or larger, with connections and capacity for the refrigerant. After a burnout or acid in the oil use a burnout (acid-removing) type and plan to replace it again after the clean-up; after moisture choose one with ample water capacity.

      ✓ Check: The new drier matches the refrigerant, size and situation, and its arrow follows the flow direction.

   3. Cut out the old drier. Cut the pipes with a pipe cutter (never a hacksaw, which leaves filings) and deburr with the pipe end held downward. Keep the new drier capped until just before fitting.

      ✓ Check: Pipe ends are round, clean and burr-free.

   4. Braze with nitrogen flowing. Get a fire permit, have an extinguisher at hand and remove flammable material. Purge a low flow of dry nitrogen through a regulator while brazing, and cool the drier body with a wet cloth so the desiccant is not overheated. Never use oxygen or compressed air. Flare driers are fitted with torque spanners instead.

      ✓ Check: Clean joints with full penetration, and no black oxide scale inside the pipe ends.

   5. Pressure test with nitrogen. Pressurise with dry nitrogen through a regulator to the test pressure for the system, never above PS, and leak test the new joints.

      ✓ Check: The pressure holds and no leak is found at the joints.

   6. Evacuate, charge and record. Evacuate with a micron gauge to the manufacturer's level (commonly below 500 micron/0.67 mbar), close off the pump and check that the vacuum holds. Charge the specified amount by weight and record the work and refrigerant amounts in the logbook.

      ✓ Check: The vacuum holds, the charge is weighed in and the logbook entry is complete.

6. 6

   A faulty discharge sensor or thermostat that reads too high.

   Remedy: Compare with a calibrated thermometer and replace a faulty sensor.

   🛠 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.