Liquid floodback to the compressor

Troubleshooting · Applies to: Direct expansion refrigeration system · Heat pump and air conditioning · Liquid chiller (water or brine) · Cold rooms with several evaporators

Liquid floodback step by step: migration at standstill, badly mounted bulb, too little air and an oversized or badly set valve.

Last checked: October 25, 2026 Field mode: large buttons, questions first and the screen kept on.

When liquid reaches the compressor the oil is diluted and the bearings wear, and in the worst case the valves break. The signs are low or no superheat, a cold compressor and frost far along the suction line. The expansion valve bulb must be correctly placed with good contact to the suction line for the valve to control the superheat ^[1].

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Measure superheat at the compressor suction connection: below about 5 K, frost on the compressor suction side or a knocking sound point to liquid. Check the oil sight glass: foaming oil points to refrigerant in the oil.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

Causes, most likely first

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

1
Flowchart D
The expansion valve is oversized or badly set, or the electronic valve's control is wrong.
Remedy: Set the superheat according to the manufacturer. Check the valve size, the bulb or sensors and the controller parameters.
🛠 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.
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.
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.
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.
2
Flowchart B
The bulb is badly placed or has poor contact, so the valve opens too much.
Remedy: Mount the bulb according to the manufacturer, on a clean, horizontal suction line, and insulate it.
🛠 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.
Mount the bulb and equaliser line
The TXV regulates on what its bulb feels. A bulb in the wrong place, with poor contact or without insulation gives hunting or wrong superheat, and an equaliser connected before the bulb disturbs the control.
1. Choose the position. Choose a horizontal part of the suction line just after the evaporator, before any trap or riser, and not close to a heat source or where liquid may collect.
  ✓ Check: The chosen section is horizontal, close to the evaporator outlet and free of traps.
2. Clean the pipe. Clean the pipe at the bulb position down to bare metal with a cleaning cloth or fine abrasive. Remove frost and paint. The pipe can be cold; wear gloves.
  ✓ Check: The pipe surface is bright and clean where the bulb will sit.
3. Fit the bulb at the right clock position. Clamp the bulb along the pipe with the metal clamp supplied with the valve. On smaller suction lines place it on top, on larger lines at about 4 or 8 o'clock, never at the bottom where oil runs. Follow the valve manufacturer's instructions for your pipe size.
  ✓ Check: The bulb lies flat along the pipe at the right position and cannot be turned by hand.
4. Insulate the bulb. Wrap the bulb and the pipe with closed-cell insulation tape so that ambient air and radiant heat cannot affect it.
  ✓ Check: The bulb and the clamp are fully covered and the insulation is sealed at the ends.
5. Connect the equaliser after the bulb. Connect the external equaliser line on the top of the suction line, a short distance after the bulb (in the flow direction). If it must be brazed, the circuit is opened: recover the refrigerant (F-gas certification), braze with nitrogen flowing and observe hot work rules, then pressure test with nitrogen and evacuate.
  ✓ Check: The equaliser is connected downstream of the bulb and the joint is tight in the pressure test.
6. Check the superheat. Start the system, let it run stable and measure the superheat.
  ✓ Check: The superheat is stable without hunting and at the value given for the system.
3
The evaporator fan has stopped: failed motor, capacitor, fuse or control.
Remedy: Measure voltage and current on the fan motor and check capacitor, fuse and control.
🛠 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).
Test the start/run capacitor and the start relay
Faulty capacitors and start relays are common reasons why a compressor or fan motor hums, overheats or will not start.
1. Isolate and lock off. Switch off and lock the isolator (or unplug the unit) and remove the cover of the compressor terminal box. Set the multimeter to AC volts and measure between phase and neutral, and between each phase and earth.
  ✓ Check: The meter shows 0 V AC on the incoming terminals.
2. Photograph, discharge and disconnect the capacitor. Photograph the wiring with your phone so that you can put it back the same way. Discharge the capacitor through a resistor of about 20 kΩ, 5 W with insulated leads, held across the terminals for a few seconds. Never short it with a screwdriver: the spark damages the terminals and the capacitor. Then pull off the cables.
  ✓ Check: The meter shows 0 V DC directly across the capacitor terminals.
3. Measure the capacitance. Set the multimeter to capacitance (µF) and measure across the terminals. Compare with the rating printed on the capacitor. Tecumseh gives the tolerances: a start capacitor −0/+20 % of its rating, a run capacitor ±10 %. A swollen or leaking capacitor, or one that reads 0 µF, is faulty.
  ✓ Check: The measured value is within the tolerance of the printed rating.
4. Check the start relay. Pull the relay off the compressor pins and look for burn marks or a smell of burnt plastic. A current relay is tested upright: L–S open, L–M closed (Tecumseh). A PTC starter must cool for about 3 minutes before a resistance reading means anything; compare it with the value in the compressor or starter data sheet (usually a few to some tens of ohms at room temperature).
  ✓ Check: The relay contacts are as described, or the PTC reads its data-sheet value; OL (open) means the part is faulty.
5. Replace the faulty part and test run. Fit a new part with exactly the same data: capacitance in µF, voltage rating and relay type. Mount the relay the right way up. Reconnect from your photo, refit the cover, switch on and start the unit. If capacitors or relays fail again, look for the cause: low supply voltage, short cycling or a relay of the wrong type.
  ✓ Check: The compressor starts smoothly, without humming and without the motor protection tripping.
4
Flowchart C
Too little load on the evaporator, so the refrigerant does not have time to evaporate.
Remedy: Correct the airflow and the defrost.
🛠 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 the defrost system
Incomplete defrost leaves ice on the evaporator, which reduces capacity and can block the airflow completely. Check heaters or hot gas, termination, interval, drip time and fan delay.
1. Check the defrost settings. Read defrost type, interval, maximum time, termination temperature, drip time and fan delay in the controller and compare with the manufacturer's recommendations. Check the controller clock if defrosts are scheduled.
  ✓ Check: The settings match the application and the clock is right.
2. Measure the heater resistance. Isolate, lock off and prove dead, then disconnect the heaters and measure each one. The expected resistance is R = U² ÷ P from the heater's voltage and power. Also measure insulation to earth with an insulation tester.
  ✓ Check: Each heater is close to the calculated value and has no earth fault; OL means a broken heater.
3. Measure the current during defrost. Restore power, start a manual defrost and measure the current in each heater supply with a clamp meter. Watch out for hot surfaces.
  ✓ Check: The current is about P ÷ U for each heater, or matches the nameplate defrost current.
4. Check hot gas defrost. On hot gas systems, check that the hot gas valve opens (warm pipe after the valve) and that the liquid or suction valves close as intended. Never exceed the system PS.
  ✓ Check: Hot gas reaches the evaporator and the coil warms evenly.
5. Check termination. Check that the termination sensor sits on the coldest part of the coil, usually where ice remains longest, and that the defrost ends on temperature rather than on maximum time.
  ✓ Check: The defrost ends on temperature and the coil is completely free of ice.
6. Check drip time and fan delay. After the defrost, watch the drip time and the evaporator fans when refrigeration restarts. Check that the drain and the drip tray heater work.
  ✓ Check: Fans stay off until the coil has reached the fan-delay setpoint, and water drains without refreezing in the tray.
Continue with: Evaporator keeps icing up →
5
Flowchart A
Refrigerant migrates to the compressor during standstill and is carried along as liquid at start-up.
Remedy: Check the crankcase heater and the pump-down. A suction accumulator may be needed.
🛠 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.
Test the crankcase heater
The crankcase heater keeps refrigerant from condensing in the compressor oil at standstill. A broken or loose heater leads to foaming oil and liquid slugging at start.
1. Isolate, lock off and prove dead. Switch off and lock the isolator and prove dead at the heater terminals. The heater and the compressor shell can be hot.
  ✓ Check: The voltage tester shows 0 V on the heater terminals.
2. Measure the heater resistance. Disconnect one heater lead and measure the resistance. Work out the expected value from the rating: R = V² / P (for example 230 V and 50 W gives about 1060 Ω). Self-regulating (PTC) heaters show a different value; compare with their data sheet.
  ✓ Check: The resistance is close to the calculated or data-sheet value; OL means the heater is broken.
3. Check the insulation to earth. Measure between the heater leads and the compressor body. A heater with a damaged sheath often leaks to earth and trips the earth-fault breaker.
  ✓ Check: The meter shows no connection (OL) between the leads and earth.
4. Check the position. Check that the heater sits where the compressor manual says, usually tight around the lower shell near the oil sump, with full contact and the clamp tightened.
  ✓ Check: The heater has full contact with the shell at the position in the manual.
5. Check that it is on at standstill. Reconnect, restore power and keep the compressor stopped. Measure the voltage at the heater and its current (I = P / V). After a while, check the shell temperature with a contact thermometer.
  ✓ Check: The heater has voltage and draws about its rated current at standstill, and the shell bottom is warmer than the surroundings.
6. Check the control. On units where the heater switches off while the compressor runs, check that it switches on again when the compressor stops.
  ✓ Check: The heater is on whenever the compressor is stopped.
Check the pump-down function
In pump-down the liquid solenoid valve closes and the compressor pumps the low side down until the low pressure switch stops it. If the valve leaks or the switch is set wrong, the compressor short cycles or liquid migrates to the crankcase.
1. Connect gauges. Connect gauges to the suction and discharge service ports with short hoses and minimal refrigerant loss. Note the low pressure switch cut-out and cut-in settings from the switch or the controller.
  ✓ Check: Both gauges show stable operating pressures and the switch settings are noted.
2. Start the pump-down. Make the thermostat or controller stop the cooling call (raise the setpoint or open the thermostat contact) so the liquid solenoid valve is de-energised.
  ✓ Check: The coil voltage on the solenoid valve drops to 0 V (measured with a CAT III/IV meter) and the valve clicks closed.
3. Watch the suction pressure fall. Watch the suction gauge while the compressor keeps running and pumps refrigerant into the condenser and receiver.
  ✓ Check: The suction pressure falls steadily towards the cut-out setting; it does not stop falling well above it.
4. Check the low pressure cut-out. Note the pressure at which the compressor stops and compare it with the setting.
  ✓ Check: The compressor stops at the set cut-out pressure, within the accuracy of your gauge.
5. Check that the solenoid valve holds. With the compressor stopped, watch the suction pressure for several minutes and feel or measure the pipe temperature after the solenoid valve. A slow rise as the evaporator warms up is normal; a fast rise, or a cold pipe or frost after the valve, means the valve is passing liquid.
  ✓ Check: The suction pressure rises only slowly and the pipe after the valve is not getting cold.
6. Check the restart. Restore the cooling call. The solenoid valve should open, the suction pressure rise above the cut-in and the compressor start. Without a cooling call the compressor must not restart on its own within a short time; check the anti-short-cycle timer in the controller.
  ✓ Check: The compressor restarts only when cooling is called for and does not cycle on the low pressure switch alone.
6
Overcharge on a system with a capillary tube or fixed orifice: the evaporator floods because nothing limits the feed.
Remedy: Recover refrigerant until the superheat at the evaporator outlet is right, or recover all and weigh in the charge on the nameplate. Record the quantity in the logbook.
🛠 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.
Recover refrigerant
Before the refrigerant circuit is opened, the refrigerant is recovered into a cylinder, never vented. Recovery requires F-gas certification (EU 2024/573 or the national equivalent; EPA 608 in the USA) and is recorded in the logbook.
1. Choose cylinder and equipment. Use a recovery cylinder that is marked for recovery, within its test date and used for one refrigerant only; never mix refrigerants. For flammable refrigerants (A2L/A3) use a recovery unit and cylinder rated for them and remove ignition sources. Wear goggles and gloves: liquid refrigerant causes frostbite.
  ✓ Check: The cylinder label shows the refrigerant, the test date and the maximum fill weight.
2. Connect and evacuate the hoses. Put the cylinder on the scales. Connect the recovery unit between the system service ports and the cylinder with short hoses. Evacuate the hoses (and an empty cylinder) with a vacuum pump before you start, so no air enters the recovered refrigerant.
  ✓ Check: The hoses hold vacuum and the scales show the starting weight.
3. Recover liquid first, then vapour. Recover liquid first if the system allows it (it goes faster), then switch to vapour recovery. Follow the recovery unit manual for the valve settings.
  ✓ Check: The system pressure falls steadily.
4. Watch the cylinder fill. Watch the scales the whole time and stop before the maximum fill weight on the cylinder (about 80 % liquid volume). Use the unit's overfill switch if the cylinder has one. Never heat the cylinder.
  ✓ Check: The cylinder weight stays below its maximum fill weight.
5. Reach the final recovery pressure. Continue until the system reaches the recovery pressure the recovery unit and national rules call for, and check that it does not rise again after a few minutes (refrigerant left in oil or liquid pockets).
  ✓ Check: The pressure stays at the final recovery pressure after the unit is stopped.
6. Record. Close the cylinder valves, clear the recovery unit as its manual describes, and record the recovered quantity (by weight), the refrigerant and the cylinder number in the logbook.
  ✓ Check: The logbook shows the recovered quantity and which cylinder it went into.
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.
Charge refrigerant by weight
The only reliable way to get the correct charge is to weigh it in. The system must be leak tight, pressure tested and evacuated first, and the work must be done by F-gas certified personnel.
1. Check that the system is ready. Check that leaks are repaired, the nitrogen pressure test is passed and the system is evacuated with a passed decay test (see those guides). Find the charge on the nameplate plus any addition for pipe length given by the manufacturer.
  ✓ Check: Records of the tests exist and you know the total charge in kg.
2. Set up cylinder and scale. Check the refrigerant on the cylinder label (not by colour). Place the cylinder on a calibrated scale on a level surface, connect evacuated or low-loss hoses and zero the scale. On A2L/A3 systems: no ignition sources, ventilation and a gas detector.
  ✓ Check: The scale reads zero and the hoses are free of air.
3. CO2 only: break the vacuum with vapour. On a CO2 system, charge vapour first until the pressure is well above the triple point (5.2 bar absolute) before any liquid goes in. Liquid into a deep vacuum turns to dry ice.
  ✓ Check: The system pressure is above 5.2 bar absolute before liquid charging starts.
4. Charge liquid into the high side. With the system off, charge liquid into the receiver or liquid line. Always charge blends (400 series) as liquid. Watch the scale and close when the target is reached.
  ✓ Check: The scale shows the weight that has left the cylinder.
5. Add the rest carefully. If not all of the charge goes in, start the system and add the remainder slowly into the suction side through a throttled valve, so that liquid does not reach the compressor. Never exceed the nameplate charge.
  ✓ Check: The total on the scale equals the target charge.
6. Check operation. Run until stable and measure superheat and subcooling.
  ✓ Check: Both are within the manufacturer's targets.
7. Disconnect and record. Close the cylinder, remove the hoses with minimal loss, refit the caps and leak test the ports. Record type and quantity charged in the logbook.
  ✓ Check: The caps are on, no leak is found and the logbook shows the charged quantity.
7
The solenoid valve before the expansion valve does not close tightly at stop, so the evaporator fills during standstill.
Remedy: Check that the coil is de-energised at stop and that the valve closes tightly. Replace the valve or its insert.
🛠 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 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 the pump-down function
In pump-down the liquid solenoid valve closes and the compressor pumps the low side down until the low pressure switch stops it. If the valve leaks or the switch is set wrong, the compressor short cycles or liquid migrates to the crankcase.
1. Connect gauges. Connect gauges to the suction and discharge service ports with short hoses and minimal refrigerant loss. Note the low pressure switch cut-out and cut-in settings from the switch or the controller.
  ✓ Check: Both gauges show stable operating pressures and the switch settings are noted.
2. Start the pump-down. Make the thermostat or controller stop the cooling call (raise the setpoint or open the thermostat contact) so the liquid solenoid valve is de-energised.
  ✓ Check: The coil voltage on the solenoid valve drops to 0 V (measured with a CAT III/IV meter) and the valve clicks closed.
3. Watch the suction pressure fall. Watch the suction gauge while the compressor keeps running and pumps refrigerant into the condenser and receiver.
  ✓ Check: The suction pressure falls steadily towards the cut-out setting; it does not stop falling well above it.
4. Check the low pressure cut-out. Note the pressure at which the compressor stops and compare it with the setting.
  ✓ Check: The compressor stops at the set cut-out pressure, within the accuracy of your gauge.
5. Check that the solenoid valve holds. With the compressor stopped, watch the suction pressure for several minutes and feel or measure the pipe temperature after the solenoid valve. A slow rise as the evaporator warms up is normal; a fast rise, or a cold pipe or frost after the valve, means the valve is passing liquid.
  ✓ Check: The suction pressure rises only slowly and the pipe after the valve is not getting cold.
6. Check the restart. Restore the cooling call. The solenoid valve should open, the suction pressure rise above the cut-in and the compressor start. Without a cooling call the compressor must not restart on its own within a short time; check the anti-short-cycle timer in the controller.
  ✓ Check: The compressor restarts only when cooling is called for and does not cycle on the low pressure switch alone.
8
The crankcase heater is faulty or not connected.
Remedy: Measure the heater resistance and voltage. Replace a faulty heater.
🛠 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.
Test the crankcase heater
The crankcase heater keeps refrigerant from condensing in the compressor oil at standstill. A broken or loose heater leads to foaming oil and liquid slugging at start.
1. Isolate, lock off and prove dead. Switch off and lock the isolator and prove dead at the heater terminals. The heater and the compressor shell can be hot.
  ✓ Check: The voltage tester shows 0 V on the heater terminals.
2. Measure the heater resistance. Disconnect one heater lead and measure the resistance. Work out the expected value from the rating: R = V² / P (for example 230 V and 50 W gives about 1060 Ω). Self-regulating (PTC) heaters show a different value; compare with their data sheet.
  ✓ Check: The resistance is close to the calculated or data-sheet value; OL means the heater is broken.
3. Check the insulation to earth. Measure between the heater leads and the compressor body. A heater with a damaged sheath often leaks to earth and trips the earth-fault breaker.
  ✓ Check: The meter shows no connection (OL) between the leads and earth.
4. Check the position. Check that the heater sits where the compressor manual says, usually tight around the lower shell near the oil sump, with full contact and the clamp tightened.
  ✓ Check: The heater has full contact with the shell at the position in the manual.
5. Check that it is on at standstill. Reconnect, restore power and keep the compressor stopped. Measure the voltage at the heater and its current (I = P / V). After a while, check the shell temperature with a contact thermometer.
  ✓ Check: The heater has voltage and draws about its rated current at standstill, and the shell bottom is warmer than the surroundings.
6. Check the control. On units where the heater switches off while the compressor runs, check that it switches on again when the compressor stops.
  ✓ Check: The heater is on whenever the compressor is stopped.

Checks from the manufacturers’ manuals (27)

Values and checks the manufacturers give for their own products; they apply to the product named.

Carel EVD ice/mini: if superheat stays below about 2 K with the valve shown at 0 steps, close it fully in manual; if superheat is still low, check the stator wiring and suspect a valve stuck open.^[2]
Copeland EazyCool ZX: alarm E50 means discharge temperature stayed too close to mid-coil condensing temperature for too long, which points to liquid returning to the compressor or too much injection.^[3]
Heatcraft Beacon II: check the EEV at the EXV TEST pins (0 V DC = closed, 5 V DC = fully open); each valve winding should see about 20–22 V AC while the valve moves.^[4]
Dixell XEV12D, XM669K/XM679K: a low superheat alarm (LSH) closes the valve, so repeated LSH points to a misplaced or loose suction probe, a wrong refrigerant setting or a too low LSH limit.^[5]
Danfoss ETS 175L–500L: lost steps (noise from power cables, long thin cables, weak supply) can make the valve leak internally; the controller must overdrive it past closed now and then to re-find zero.^[6]
Danfoss EKE 1: liquid hammer or a frosted suction line is often a loose or uninsulated S2 sensor, a too low minimum superheat, or a wrong refrigerant or transmitter setting in the controller.^[7]
Temperzone UC7 units; Goodman inverter condensing units; Mitsubishi SUZ-KA: Measure discharge superheat (discharge line temperature minus condensing temperature). Below about 10 K for a long time means liquid is reaching the compressor; above about 38 K means too little refrigerant or an underfeeding valve. Mitsubishi inverter units also flag a discharge temperature at or below 50 °C for 20 minutes.^[8]
Copeland refrigeration scrolls: in steady operation the oil sump (bottom centre of the shell, insulated sensor) should be at least 27 K warmer than the suction line; during defrost it may drop below this for at most 2 minutes and never below 14 K. Suction superheat should be at least 11 K, measured 150 mm from the suction port.^[9]
Danfoss condensing units and MT/MTZ/NTZ; Tecumseh semi-hermetic: The crankcase heater protects only if it keeps the oil at least 10 K above the saturation temperature of the refrigerant; check this at low ambient, switch the heater on at least 12 hours before starting after a long stop, and keep the compressor the warmest part of the circuit even at standstill. If that cannot be achieved, use pump-down.^[10]
Danfoss condensing units, MT/MTZ; Tecumseh semi-hermetic: Recommended suction superheat ranges: Danfoss condensing units 5–12 K at the expansion valve bulb; Tecumseh semi-hermetic compressors 3–20 K in all seasons and loads. A suction accumulator should hold at least 50 % of the system charge (not recommended with zeotropic blends such as R407C), and MT/MTZ need an accumulator or pump-down above about 2.5 kg charge per cylinder.^[11]
Carel EVD evolution with multiplexed cabinets: Multiplexed cabinets on a rack: many cabinets defrost at the same time, the delay before the electronic valve takes over after defrost is too short, or the valve opening at start-up is set too high on many cabinets, so liquid returns to the rack after defrost. Stagger defrost starts or raise superheat and the low-superheat limit by at least 2 K on those cabinets.^[12]
Carel EVD evolution with E2V valves; Danfoss ETS 175L–500L: Electronic expansion valve: put the driver in manual and open the valve fully; superheat must fall, and closing must raise it. If it is the other way round the stator is wired in reverse. Also measure each stator winding (Carel E2V about 36 Ω, Danfoss ETS 175L–500L about 52 Ω at 20 °C) and check that the valve type, steps, phase current and direction set in the controller match the valve.^[13]
Hillphoenix Advansor; Hussmann transcritical CO2 racks: CO2 booster MT suction: keep at least about 8 K (14 °F) superheat at the compressors and a suction temperature not below about −15 °C (Hillphoenix); Hussmann gives 11–22 K as the normal MT range. If superheat is lower, the MT hot gas dump or liquid injection control is not working.^[14]
Carel EVD evolution twin with E*V valves: Check that an electronic valve really moves the right way: in manual mode, opening it fully must lower the superheat and closing must raise it. If it is the other way round the motor wires are swapped. Carel E*V stator windings should each read about 36 Ω.^[15]
Danfoss EKE 1A–1D superheat controllers: Electronic superheat control: a suction pressure transmitter mounted far from the evaporator outlet (for example at the compressor) on a line with a large pressure drop reads too low, so the controller calculates too high a superheat and overfeeds. Mount transmitter and S2 sensor close together near the evaporator outlet.^[16]
Heatcraft Beacon II with EEV: On a newly started system with an electronic expansion valve, a few stops on a low superheat alarm in the first minutes can be normal while the valve finds its position (Beacon II: off after 2 minutes, then restarts). Let it run a few cycles before changing settings; if it continues, check sensor, transducer and refrigerant setting.^[17]
Hussmann CO2 racks with case controllers and stepper EEVs: A stepper-motor expansion valve stays where it was when the power fails, so the evaporator can fill and flood the compressors when the plant restarts. Hussmann recommends a liquid line solenoid valve ahead of every stepper valve for this reason.^[18]
Hillphoenix CO2One condensing unit: Frosted or wet suction lines on a CO2 condensing unit: set the controller so that suction superheat stays at least 15 K, and clean or replace an expansion valve that sticks open.^[19]
Heatcraft Beacon II raises a low superheat alarm when superheat stays at 0 K for 2 min. First check that the refrigerant set in the controller matches the system (a wrong setting gives a wrong superheat), then ice on the coil, then whether the EEV closes: pump down and see if the LP switch opens; if the pump-down times out, the EEV leaks or the LP switch is set wrong.^[20]
Temperzone UC8 units; Goodman/Daikin DX20VC: Low discharge superheat is a reliable floodback sign: below about 10–11 K (Temperzone alarms at under 10 K for 15 min at full capacity; Goodman flags under 20 °F), and close to 0 K in real floodback because the liquid evaporating in the compressor absorbs the heat of compression.^[21]
HTPG EcoNet; Carel EVD mini: Test whether the electronic valve closes: command it shut (manual mode or forced pump-down). The suction pressure must fall and the compressor pump down within a couple of minutes; superheat below about 2 K with the valve at 0 steps means the valve leaks.^[22]
Carel EVD mini: The start opening of the electronic valve is set too high, so liquid reaches the compressor for a short time at every start after a stop.^[23]
HTPG EcoNet; Carel EVD mini: Check that the refrigerant selected in the superheat controller matches the system and that the pressure transducer type is set right; otherwise the calculated superheat is wrong and the valve overfeeds or underfeeds.^[22]
Danfoss Optyma Plus, Heatcraft MAC, Hussmann CO2 racks, Toshiba and Actron Air outdoor units: After installation or a long power cut, switch on the crankcase heater 12–24 hours before the compressor is started, otherwise refrigerant dissolved in the oil causes liquid slugging and oil foaming at start.^[24]
Copeland EXD-SH1/2 with EX/CV valves: The stepper valve is wired with a coil pair swapped, so it opens when the controller commands it to close and vice versa.^[25]
Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: In heating the VRV outdoor expansion valve controls superheat at the outdoor coil outlet as suction pipe thermistor Ts minus the saturation temperature Te from the low pressure sensor, target normally 5 K (9 °F), with a range of 0–2000 pulses for R410A units. If liquid returns to the compressor in heating, compare Ts and the low pressure reading with your own gauges and thermometer before suspecting the valve.^[26]
Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II crankcase heater: it is switched on 30 min after all compressors have stopped, and at power-on, and off when a compressor starts; it does not follow temperature. If the compressor stops soon after start with liquid slugging, check whether the low pressure is too high: an indoor EEV opened too far, or the EEV of a stopped indoor unit left open.^[27]

📝 Fill in a service report

Terms in this article

Liquid slugging Superheat Expansion valve

Liquid floodback to the compressor

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Adjust superheat and size the expansion valve

Measure superheat

Check an electronic expansion valve

Mount the bulb and equaliser line

Check a condenser or evaporator fan motor

Test the start/run capacitor and the start relay

Check a condenser or evaporator fan motor

Check the defrost system

Test the crankcase heater

Check the pump-down function

Recover refrigerant

Measure superheat

Charge refrigerant by weight

Check a solenoid valve (liquid line or hot gas)

Check the pump-down function

Test the crankcase heater

Checks from the manufacturers’ manuals (27)

Service report

Terms in this article

Sources

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Adjust superheat and size the expansion valve

Measure superheat

Check an electronic expansion valve

Mount the bulb and equaliser line

Check a condenser or evaporator fan motor

Test the start/run capacitor and the start relay

Check a condenser or evaporator fan motor

Check the defrost system

Test the crankcase heater

Check the pump-down function

Recover refrigerant

Measure superheat

Charge refrigerant by weight

Check a solenoid valve (liquid line or hot gas)

Check the pump-down function

Test the crankcase heater

Checks from the manufacturers’ manuals (27)

Service report

Terms in this article

Sources

Read also