Compressor runs but does not cool

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 system is empty of refrigerant.

   Remedy: Leak test, repair, pressure test, evacuate and charge according to 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.

   Pressure test with nitrogen

   After repair or installation the circuit is strength and tightness tested with dry nitrogen before evacuation. The test pressures follow from the system's PS according to EN 378-2. Certified personnel only.

   1. Find the test pressure. Find the system's PS (nameplate or documentation) and the strength and tightness test pressures EN 378-2 or the plant documentation give for it. Never use oxygen or compressed air.

      ✓ Check: You have the test pressures in writing and they do not exceed what every component in the test section tolerates.

   2. Isolate sensitive components. Isolate or remove components with a lower PS or that must not see the test pressure: safety valves, transducers, sight glasses, some controls.

      ✓ Check: Every component in the test section is rated for the test pressure.

   3. Connect nitrogen through a regulator. Connect dry oxygen-free nitrogen through a pressure regulator and a calibrated gauge. Never connect a cylinder without a regulator. Ventilate small rooms.

      ✓ Check: The regulator is fitted and its outlet is set below the test pressure.

   4. Raise the pressure in steps. Raise the pressure in steps to the strength test pressure, keep it for the time given and keep people away from the circuit while it is at strength pressure.

      ✓ Check: The pressure holds and no joint gives way.

   5. Lower to tightness pressure and search for leaks. Lower the pressure to the tightness test pressure. Apply leak spray to all joints made or disturbed.

      ✓ Check: No bubbles form on any joint.

   6. Hold and check the pressure. Hold the pressure for the time required, note the pressure and the ambient temperature at start and end, and correct for temperature.

      ✓ Check: The temperature-corrected pressure is unchanged.

   7. Release and record. Release the nitrogen slowly to the outside, refit the isolated components and record test pressures, time and result.

      ✓ Check: The circuit is at 0 bar(g), the components are back and the test is recorded.

   Evacuate a refrigeration system

   Evacuation removes air and moisture from the circuit before charging. A good vacuum and a decay test show that the system is dry and tight; moisture and air left in the circuit give acid, ice in the valve and high pressures.

   1. Check before you start. Evacuate only after the pressure test with nitrogen has passed. Opening the circuit requires F-gas certification (EPA 608 in the USA). For flammable refrigerants (A2L/A3) use a vacuum pump rated for them and remove ignition sources.

      ✓ Check: The pressure test is passed and documented, and the pump suits the refrigerant.

   2. Connect for a fast vacuum. Connect the vacuum pump to both the high and low side with short, large-bore hoses. Remove the valve cores with a core tool if you can. Fit the micron gauge on the system side, as far from the pump as practical, so that it can be isolated from the pump.

      ✓ Check: All connections are tight and the micron gauge reads the system, not the pump.

   3. Pull the vacuum. Check the pump oil, open the valves and start the pump. Never start the compressor or megger it while the system is in vacuum.

      ✓ Check: The micron gauge reading falls steadily.

   4. Reach the target vacuum. Continue until the gauge reaches the target in the equipment or compressor manufacturer's instructions; many use 500 microns (about 0.67 mbar). Large or wet systems take longer; warming the system helps moisture to boil off.

      ✓ Check: The gauge shows the target vacuum.

   5. Do the decay test. Isolate the pump from the system and watch the micron gauge, for example for 15 minutes or the time in the instructions. A reading that rises and levels off means moisture; one that keeps rising means a leak.

      ✓ Check: The reading rises only a little and levels off within the limit in the instructions.

   6. Charge or repeat. If the decay test fails: for moisture, evacuate longer; for a leak, find and repair it and pressure test again. When it passes, refit the valve cores and charge the refrigerant by weight directly from the vacuum.

      ✓ Check: The system is charged from vacuum, and the vacuum and decay results are recorded.

   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.

   Leak check of refrigeration systems: F-gas intervals and steps →

2. 2
   Flowchart D

   Wrong rotation: the scroll compressor runs backwards.

   Remedy: Switch off immediately and swap two phases. Check the phase sequence relay. A scroll tolerates only a short time backwards.

   🛠 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 and correct phase sequence

   Scroll and screw compressors and many fans must run in the right direction. A scroll compressor running backwards is noisy, does not pump and is damaged quickly.

   1. Measure the phase sequence. With the unit stopped, connect a phase rotation meter to L1, L2 and L3 at the unit's supply terminals (live measurement: CAT III/IV, insulated probes). If the unit has a phase monitoring relay, read its indicator too.

      ✓ Check: The meter shows right-hand (L1-L2-L3) rotation, or you know that it is reversed.

   2. Decide where to swap. If the whole unit is reversed, swap at the unit's supply terminals after the isolator. If only one motor runs backwards, swap at that motor. For a motor on an inverter, the input sequence does not matter; change direction with a parameter or at the drive output.

      ✓ Check: You know which two conductors to swap and where.

   3. Isolate, lock off and prove dead. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all phases and to earth.

      ✓ Check: The tester shows no voltage on any phase.

   4. Swap two phases. Swap two phase conductors (for example L1 and L2). Never move the earth or neutral conductor. Tighten the terminals to the specified torque.

      ✓ Check: Two phases are swapped and the terminals are tight.

   5. Check the sequence again. Remove your lock, switch on and measure the phase sequence again; check that the phase monitoring relay is satisfied.

      ✓ Check: The meter shows right-hand rotation.

   6. Start and confirm direction. Start the compressor and watch the gauges. Stop at once if it sounds wrong or the pressures do not separate.

      ✓ Check: Within seconds suction pressure falls and discharge pressure rises, the current is normal and the compressor runs quietly.

   Safe isolation and lock-off

   Before you open a panel or touch electrical parts, isolate every supply, lock it off and prove that the circuit is dead. Electrical work is for qualified personnel only.

   1. Identify all supplies. Use the wiring diagram to find every supply: main feed, separate control supply, heaters, alarm and UPS supplies, and inverters that can feed back.

      ✓ Check: You know every isolation point for the equipment you will work on.

   2. Switch off, lock and tag. Switch each isolator to OFF, fit your own padlock and attach a tag with your name and date. Keep the key on you.

      ✓ Check: Every isolator is locked in OFF and cannot be switched on by someone else.

   3. Prove the tester. Check the two-pole voltage tester (EN 61243-3) on a proving unit or a known live source.

      ✓ Check: The tester shows the voltage of the known source.

   4. Prove dead. Measure phase to phase, each phase to neutral and each phase to earth at the point of work, on both line and load side.

      ✓ Check: The tester shows 0 V on every combination.

   5. Prove the tester again. Check the tester on the proving unit once more, to be sure it did not fail during the measurement.

      ✓ Check: The tester again shows the voltage of the known source.

   6. Deal with stored energy. On an inverter, wait the discharge time printed on the drive and measure the DC bus. Discharge start and run capacitors through a resistor of about 20 kΩ, 5 W with insulated leads, never with a screwdriver.

      ✓ Check: 0 V DC on the DC bus and across every capacitor.

3. 3
   Flowchart C

   The four-way valve is stuck halfway and lets discharge gas through to the suction side.

   Remedy: Troubleshoot the four-way valve.

   🛠 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 four-way (reversing) valve

   A heat pump that will not change mode, or loses capacity, often has a reversing valve that does not get a signal, does not shift or leaks internally. Temperatures on the pipes tell you which.

   1. Find out the valve logic. Check in the wiring diagram or manual in which mode the coil is energised. This differs between makers: some energise in cooling, others in heating. Note the mode the controller asks for.

      ✓ Check: You know whether the coil should be on or off in the current mode.

   2. Check the coil and its signal. With a CAT III/IV meter, measure the voltage at the coil when the mode changes. Then isolate, lock off, prove dead, and measure the coil resistance. Check that the coil voltage marking matches the supply and that the coil sits fully on the valve stem.

      ✓ Check: The coil gets its rated voltage in the right mode and its resistance matches the data sheet; OL means a broken coil.

   3. Measure the pipe temperatures. With the compressor running, measure the four main pipes with clamp thermometers: discharge into the valve (hot), suction from the valve to the compressor (cold), and the two coil connections. Beware of the hot discharge pipe. The valve needs enough pressure difference to shift, so check with the compressor running.

      ✓ Check: The hot side follows the mode: to the outdoor coil in cooling, to the indoor coil in heating.

   4. Check the pilot tubes at switch-over. Change the mode and feel or measure the small pilot capillaries on the valve during the switch. Their temperatures should change as the pilot valve moves.

      ✓ Check: The pilot tube temperatures change at switch-over; if they do but the main pipes do not, the main slide is stuck.

   5. Look for internal leakage. Compare the temperature of the suction pipe between valve and compressor with the pipe coming from the coil that is working as evaporator. A clearly warmer pipe after the valve means hot gas is leaking across the slide.

      ✓ Check: Both pipes have nearly the same temperature; a large difference, together with low capacity, points to internal leakage.

   6. Try to free it, otherwise replace. Switch the mode several times with the compressor running and full pressure difference. If the valve still sticks or leaks, it must be replaced by F-gas certified personnel: recover the refrigerant, braze with nitrogen flowing and a heat sink or wet cloth on the valve body, pressure test, evacuate and weigh in the charge. With R32 or R290 follow the rules for flammable refrigerants (no ignition sources, ventilation, gas detector).

      ✓ Check: After repair the valve shifts in both directions and the pipe temperatures follow the mode.

   Continue with: Heat pump does not switch between heating and cooling →

4. 4

   The compressor is not running although the contactor has pulled in: the motor thermal protector has opened or a phase is missing.

   Remedy: Measure the voltage after the contactor and the resistance through the windings. Let an overheated motor cool down and find out why it got hot.

   🛠 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.

   Measure compressor windings and insulation

   Winding resistance shows whether a motor winding is open, shorted or unbalanced, and insulation resistance shows whether it leaks to earth. Together they tell you whether the compressor motor is faulty before you replace it.

   1. Isolate, lock off and prove dead. Switch off and lock the isolator, put up a tag and prove dead with a two-pole tester between all phases and between each phase and earth at the compressor terminals.

      ✓ Check: The tester shows 0 V on every combination.

   2. Discharge capacitors and disconnect. Discharge any start or run capacitor through a resistor (about 20 kΩ, 5 W, insulated leads), never with a screwdriver. Photograph the wiring, then remove all cables from the compressor terminals. Disconnect any inverter and electronic protection module.

      ✓ Check: The meter shows 0 V DC across the capacitor terminals and the compressor terminals are free.

   3. Let a hot compressor cool. If the compressor is hot, an internal protector may be open. Measure the shell temperature; let it cool (it can take an hour or more) before judging the windings.

      ✓ Check: The shell is close to room temperature, or you have noted its temperature.

   4. Measure the winding resistance. Use the low-ohm range. Single-phase: measure C–R, C–S and R–S; C–R plus C–S should equal R–S. Three-phase: measure between the three terminal pairs; the values should be nearly equal. Compare with the compressor data sheet at the measured temperature. Open between C and both R and S while R–S reads normal points to an open internal protector.

      ✓ Check: The readings agree with the data sheet and with each other; OL means an open winding or protector.

   5. Measure the insulation resistance. Never megger under vacuum or with electronics connected. Connect the insulation tester (500 V DC, or the voltage the manufacturer gives) between each terminal and a clean earth point on the compressor body. Liquid refrigerant in the shell lowers the reading; run the crankcase heater and measure again before condemning the motor.

      ✓ Check: The reading is above the minimum in the compressor manufacturer's data; a reading near zero means the winding is earthed.

   6. Reconnect or replace. If the motor is good, reconnect from your photo, refit the cover and test run while measuring the current. If it is faulty, plan a compressor replacement and test the oil for acid.

      ✓ Check: The compressor runs at nameplate current, or the fault is recorded and a replacement planned.

5. 5
   Flowchart E

   Internal leak in the compressor (broken valve plates, an open internal bypass, a damaged scroll) or an external hot gas or unloader valve that leaks.

   Remedy: Check that hot gas and unloader valves close. A compressor that pumps poorly often draws low current. Contact the manufacturer and replace the compressor if it has an internal fault ^[2].

   🛠 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 whether the compressor pumps

   Low pressure difference, low current and poor capacity can mean a worn compressor, but often the cause is a bypass elsewhere. Compare measurements with the manufacturer's data and rule out other causes before you condemn the compressor.

   1. Measure in stable operation. Connect gauges to suction and discharge, and clamp thermometers on the suction line and on the discharge line close to the compressor, where the manufacturer specifies. Let the system run steadily for at least 15 minutes. Beware of the hot discharge pipe.

      ✓ Check: Pressures and temperatures are steady and noted together with the saturation temperatures.

   2. Measure current and voltage. Measure the supply voltage and the current in each phase with a true-RMS clamp meter (CAT III/IV, insulated probes, no loose clothing).

      ✓ Check: You have voltage and current for all phases; the phases are balanced.

   3. Compare with the compressor data. Enter the measured evaporating and condensing temperatures in the manufacturer's selection software or data sheet and read the expected current, capacity and discharge temperature.

      ✓ Check: Measured current and pressure difference are close to the data; clearly lower current at a low pressure difference suggests the compressor is not pumping.

   4. Rule out bypasses. Check the hot gas bypass valve, unloaders or capacity control solenoids, a digital scroll valve and any reversing valve: a pipe that is hot where it should be cold shows gas leaking back to suction.

      ✓ Check: All bypass paths are closed when they should be; otherwise repair them first and measure again.

   5. Check the internal relief valve. Many hermetic and scroll compressors have an internal pressure relief valve (IPR) between discharge and suction. If it has opened, suction pressure is high, discharge low, the shell is hot and the motor protection may trip. Find out why it opened (high pressure, blocked condenser) before restart.

      ✓ Check: The cause of an opened relief valve is found and removed, or the relief valve is ruled out.

   6. Check the valves after stop (reciprocating). On a reciprocating compressor, watch the gauges right after it stops. Discharge pressure falling fast while suction rises points to leaking discharge valves. Do not run scroll compressors into deep vacuum to test them; it damages them.

      ✓ Check: Pressures equalise slowly; fast equalisation confirms leaking valves.

   7. Decide. If the compressor still pumps too little with all other causes ruled out, plan a compressor replacement (see the guide on replacing a compressor). Record the measurements.

      ✓ Check: The decision is based on measurements compared with the manufacturer's data, and they are written down.

   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.

   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.

6. 6
   Flowchart B

   The compressor pumps, but the capacity is not enough.

   Remedy: Troubleshoot low capacity.

   Continue with: System does not reach temperature →