Suction pressure too high

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

   High load: the pressure drops once the heat has been removed.

   Remedy: Follow the pressure for a while. If it does not drop, check doors and insulation and that the load matches the system design.

   🛠 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 doors, seals and the cold-room envelope

   Warm, moist air that leaks into a cold room gives ice on the evaporator, frost on the walls and a higher load. Doors and penetrations are the usual weak points.

   1. Test the inside release first. Before you work inside, check from inside that the door opens with the emergency release, even when it is locked from outside. Test the person-trapped alarm button if there is one.

      ✓ Check: The door opens from inside at once, and the alarm sounds where it should.

   2. Inspect the gaskets. Look for tears, flattened or hard sections and ice on the gaskets. Close the door on a strip of paper at several points around the frame and pull it out.

      ✓ Check: The paper is held firmly all the way around; places where it slides out easily need a new or adjusted gasket.

   3. Check closing and door alarm. Open the door and let it go: check that the closer or self-closing hinges shut and latch it. Leave it open and check that the door-open alarm sounds after its set delay.

      ✓ Check: The door closes and latches by itself, and the alarm works.

   4. Check frame and relief port heating. Feel the door frame and, on freezer rooms, the pressure relief port: they should be warm and free of ice and condensation. If not, test the heater circuit (isolate, lock off and prove dead before you touch its terminals; see the guide on heater cables).

      ✓ Check: Frame and relief port are warm and dry, and the relief port flap moves freely.

   5. Check strip curtains. Check that the strips overlap, hang down to the floor and are not missing, torn or tied aside.

      ✓ Check: The curtain closes the whole opening.

   6. Inspect panel joints and penetrations. Look for frost or ice on the inside and damp on the outside at panel joints and pipe or cable penetrations; an IR camera makes this quicker. Check that penetrations are sealed and that the vapour barrier on the warm side is intact.

      ✓ Check: No frost spots or damp patches, and every penetration is sealed.

   Compare cooling load with system capacity

   If a room never reaches its temperature, or the plant cycles all the time, compare the actual load with what the plant can deliver at the conditions you measure.

   1. Record conditions. Measure room temperature and humidity, the surrounding temperature outside the room, and the temperature of products as they come in. Note evaporating and condensing temperatures.

      ✓ Check: You have a set of values measured at the same time.

   2. Collect room and use data. Note room size, insulation type and thickness, product quantity per day and its entry temperature, door openings, people, lighting, fans and defrost.

      ✓ Check: Every load source has a value or a reasoned estimate.

   3. Calculate the load. Calculate the load per 24 hours with the Kylbehov app or a recognised method (for example the cooling load chapter in the ASHRAE Refrigeration Handbook), and divide by the planned run time per day, which leaves time for defrost.

      ✓ Check: You have the required capacity in kW at the room's design conditions.

   4. Find the actual capacity. Read evaporator capacity at the actual temperature difference between room air and evaporation, and compressor capacity at the measured evaporating and condensing temperatures, from the manufacturers' data. Note that rated capacities apply to standard conditions.

      ✓ Check: You have evaporator and compressor capacity at the real conditions; the lower one limits the system.

   5. Compare with run time. Read compressor run time and starts per hour from the controller log. Compare required and actual capacity.

      ✓ Check: Capacity below the load, or near-continuous running, shows undersizing or excess load; very short cycles and many starts show oversizing.

   6. Decide on measures. Reduce the load first (doors, curtains, product temperature, envelope), then consider more capacity or a different control strategy. Write down the comparison.

      ✓ Check: A written comparison of load and capacity with the chosen measure.

2. 2
   Flowchart B

   The expansion valve overfeeds: valve too large, superheat set too low or a badly mounted bulb.

   Remedy: Check the valve size against the evaporator, set the superheat and fasten the bulb properly on the suction line ^[1].

   🛠 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.

   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.

   Continue with: Liquid floodback to the compressor →

3. 3

   A hot gas valve, defrost valve or capacity regulator is open or leaking.

   Remedy: Check that the valves close (temperature before and after), the coil voltage and the regulator setting.

   🛠 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 a pressure regulator

   Evaporator, crankcase and condensing pressure regulators keep a pressure within limits. A wrong setting or a sticking valve gives wrong evaporating temperature, an overloaded compressor or low head pressure in winter.

   1. Identify the regulator and its setting. Find out which type it is (evaporator pressure regulator holds the upstream pressure, crankcase pressure regulator limits the downstream pressure, condensing pressure regulator holds the upstream pressure), and the set value in the plant documentation or on the valve label.

      ✓ Check: You know which side the regulator controls and what pressure it should hold.

   2. Connect gauges before and after. Connect calibrated gauges to the service ports before and after the regulator, with minimal refrigerant loss. Wear goggles and gloves.

      ✓ Check: Both gauges show stable readings that agree with the controller's transducers.

   3. Compare with the setting in stable operation. Let the plant run at steady load for at least 10–15 minutes. Read the controlled pressure and the pressure drop across the valve and compare with the set value.

      ✓ Check: The controlled pressure is close to the set value; a large difference or no pressure drop at all means a wrong setting or a valve that sticks.

   4. Adjust in small steps. Remove the cap and turn the adjusting spindle with the valve key a little at a time, in the direction the manufacturer gives. Wait for stable pressures after each step.

      ✓ Check: The pressure follows each adjustment smoothly and settles at the set value.

   5. Check that it closes and does not hunt. Watch the gauges over a full load change (or a compressor stage switching). An evaporator pressure regulator must close when the pressure falls below its setting; a crankcase pressure regulator must throttle at start-up.

      ✓ Check: The pressure stays at the limit without creeping or swinging; the compressor current stays within the nameplate value at start-up.

   6. Replace a faulty valve. If the valve does not respond or leaks through, it must be replaced. This opens the refrigerant circuit: only certified personnel (F-gas / EPA 608), recover the refrigerant, braze with nitrogen, pressure test, evacuate and record the work in the logbook.

      ✓ Check: The new valve holds the set value and the work is recorded.

4. 4
   Flowchart D

   The system is overcharged.

   Remedy: Recover the excess with a recovery unit until subcooling and superheat are normal. 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 subcooling

   Subcooling shows how much the liquid is cooled below its condensing temperature. Together with superheat it helps judge the charge and the condenser.

   1. Run until stable. Let the system run at steady load for at least 15 minutes, not during or just after a defrost. Check that the condenser is clean and its fans run.

      ✓ Check: Pressures and temperatures are steady.

   2. Measure the liquid pressure. Connect a gauge to the port at the condenser outlet or on the liquid line (see the guide on connecting gauges). Wear goggles and gloves.

      ✓ Check: You have a steady liquid pressure.

   3. Measure the liquid temperature. Clamp a thermometer on clean pipe at the same place as the pressure reading and insulate the probe.

      ✓ Check: You have a steady pipe temperature from the same place.

   4. Convert and calculate. Convert the pressure to saturation temperature for the refrigerant (bubble point for zeotropic blends). Subcooling = saturation temperature − measured liquid temperature.

      ✓ Check: You have a subcooling value in K.

   5. Compare with the target. Compare with the manufacturer's target for the system. In systems with a receiver, subcooling at the receiver outlet is normally small and says little about the charge. On transcritical CO2 there is no subcooling on the high side; use the gas cooler outlet temperature and pressure instead.

      ✓ Check: You know whether the value is low, normal or high for this system type.

   6. Interpret with care. Low subcooling with high superheat can mean too little refrigerant or a restriction; high subcooling with high condensing pressure can mean overcharge or non-condensables. Never add refrigerant to try: leak test first, repair, recover and weigh in the correct charge (F-gas certified personnel).

      ✓ Check: The conclusion is supported by superheat and pressures, not by subcooling alone.

   Record work in the F-gas logbook

   Leak checks, repairs and every kilogram charged or recovered must be recorded. In the EU this follows Regulation (EU) 2024/573; other countries have their own rules (USA: EPA, 40 CFR Part 82 and 84).

   1. Find the right record. Open the logbook or digital register for the specific system and check that the plant ID, location and refrigerant match the nameplate.

      ✓ Check: The record belongs to the system you are working on.

   2. Record who and when. Enter the date, your company's certificate number, your own F-gas certificate number and your name.

      ✓ Check: Date and both certificate numbers are entered.

   3. Describe the work. Enter the type of work (leak check, repair, maintenance, installation, decommissioning), the result of any leak check, where a leak was found and how it was repaired.

      ✓ Check: Someone else can understand from the entry what was done and found.

   4. Record the quantities. Enter the refrigerant type and the quantity added and recovered in kg, read from the scale. For recovered refrigerant, note whether it is to be recycled, reclaimed or destroyed and where it goes. The CO2 equivalent can be added.

      ✓ Check: Quantities in kg match the scale readings.

   5. Plan the follow-up. After a leak repair, the system must be checked again by certified personnel within one month. Note that date and the next periodic leak check.

      ✓ Check: Both dates are written in the logbook.

   6. Sign and keep. Sign the entry and give the operator a copy if the logbook is not kept on site. The operator keeps the records for at least five years.

      ✓ Check: The entry is signed and the operator has access to it.

   Continue with: System trips on high pressure →

5. 5
   Flowchart C

   The compressor pumps poorly: leaking valves or an internal bypass.

   Remedy: First check that no hot-gas, defrost or bypass valve leaks (the pipe after a closed valve must not be hot). Then troubleshoot the compressor that runs but does not cool.

   🛠 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.

   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.

   Continue with: Compressor runs but does not cool →

6. 6
   Flowchart E

   The compressor is too small for the load, or the capacity control (speed, unloading) is stuck at low capacity.

   Remedy: Check the controls, the inverter and the unloader valves. Compare the compressor capacity with the load.

   🛠 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 capacity control

   Capacity control makes the plant follow the load. If it does not work, suction pressure swings, compressors short cycle and wear out.

   1. Read the controller settings. Note the suction pressure (or temperature) setpoint, neutral zone, number of steps, speed range and the minimum run and off timers. Compare the timers with the compressor maker's limit for starts per hour.

      ✓ Check: Settings are noted and the timers respect the maker's start limit.

   2. Check the control sensor. Compare the suction pressure transducer that the controller uses with a calibrated gauge at the same point.

      ✓ Check: The controller reading agrees with the gauge.

   3. Follow the control over time. Log suction pressure, capacity steps or speed and compressor starts over at least 30 minutes, or read the controller's trend log.

      ✓ Check: Suction pressure stays within the neutral zone and steps change in order without rapid on-off.

   4. Test each step in manual mode. In the controller's manual or test mode, switch each unloader or capacity solenoid. Measure coil voltage with a CAT III/IV meter and watch current and suction pressure. Check in the manufacturer's data whether an energised coil loads or unloads.

      ✓ Check: Each step gives a clear change in current and suction pressure in the expected direction.

   5. Check speed and digital control. For an inverter compressor, check that the speed follows the controller signal. For a digital scroll, check the modulation signal and the solenoid.

      ✓ Check: Speed or modulation follows the demand over the whole range.

   6. Count starts. Count compressor starts per hour from the log and compare with the manufacturer's maximum.

      ✓ Check: Starts per hour are below the manufacturer's limit.

   Check an inverter (VFD) and read its alarms

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Compare cooling load with system capacity

   If a room never reaches its temperature, or the plant cycles all the time, compare the actual load with what the plant can deliver at the conditions you measure.

   1. Record conditions. Measure room temperature and humidity, the surrounding temperature outside the room, and the temperature of products as they come in. Note evaporating and condensing temperatures.

      ✓ Check: You have a set of values measured at the same time.

   2. Collect room and use data. Note room size, insulation type and thickness, product quantity per day and its entry temperature, door openings, people, lighting, fans and defrost.

      ✓ Check: Every load source has a value or a reasoned estimate.

   3. Calculate the load. Calculate the load per 24 hours with the Kylbehov app or a recognised method (for example the cooling load chapter in the ASHRAE Refrigeration Handbook), and divide by the planned run time per day, which leaves time for defrost.

      ✓ Check: You have the required capacity in kW at the room's design conditions.

   4. Find the actual capacity. Read evaporator capacity at the actual temperature difference between room air and evaporation, and compressor capacity at the measured evaporating and condensing temperatures, from the manufacturers' data. Note that rated capacities apply to standard conditions.

      ✓ Check: You have evaporator and compressor capacity at the real conditions; the lower one limits the system.

   5. Compare with run time. Read compressor run time and starts per hour from the controller log. Compare required and actual capacity.

      ✓ Check: Capacity below the load, or near-continuous running, shows undersizing or excess load; very short cycles and many starts show oversizing.

   6. Decide on measures. Reduce the load first (doors, curtains, product temperature, envelope), then consider more capacity or a different control strategy. Write down the comparison.

      ✓ Check: A written comparison of load and capacity with the chosen measure.

7. 7

   A pressure sensor reads wrong, so the controller sees the wrong suction pressure.

   Remedy: Compare the sensor reading with a calibrated gauge 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.