Main Chiller Failures: Causes, Diagnostics, and Prevention
Introduction
The reliability of a chiller is an indicator of the quality of the entire refrigeration system. Unlike most engineered systems, a chiller integrates mechanical, electrical and thermal components, each operating under high loads and thermodynamic stress. A failure of any one of these components doesn’t just reduce efficiency – it can shut down an entire process or air conditioning system.
Modern chillers are equipped with a self-diagnostic system, but it only records symptoms – high pressure, overheating, temperature deviations – without indicating the real cause. The engineer’s experience in interpreting this data remains the key factor. Proper diagnosis does not start with replacing components, but with understanding the interrelationship of processes: how pressure changes affect heat exchangers, how hydraulics affect evaporator stability, and why electrics are often just a manifestation of an underlying thermal problem.
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Overheating and thermal degradation of compressor oil
When a compressor overheats, becomes wet or the refrigerant decomposes (especially in systems with POE oil), organic acids form in the oil. They destroy winding insulation, corrode copper tubes and bearings, and clog filter-driers with reaction products. The main causes are contamination of the condenser, insufficient blowing, excessive refrigerant, low mains voltage.
Signs: darkening of the oil, burning odour, increase in current and case temperature.
Control and prevention: control of discharge temperature, oil acidity, condition of filter-drier, regular condenser washing, condensing pressure control, oil change in case of acidity > 1 mg KOH/g.
With the help of laboratory tests we can determine the acidity value of the oil. The acidity value or simply acid number is the amount of alkali in milligrams (usually potassium hydroxide KOH) required to neutralise the acids contained in 1 gram of oil. This number depends on the total amount of acid products contained in the oil and is expressed as TAN (Total Acid Number) or mg KOH/g.
Figure 1. Acid level test for mineral oils
Interpretation of KOH/g
| Acid value | Oil condition | Recommendations |
| < 0.1 mg KOH/g | Norm | Oil in order |
| 0.1-0.5 mg KOH/g | Beginning of degradation | Control after 3-6 months |
| 0.5-1.0 mg KOH/g | Accelerated ageing | Check overheating, replace filter drier |
| > 1.0 mg KOH/g | Critical condition | Change oil and filter drier, flush circuit |
Increased oil entrainment and/or non-return
Modern chillers often operate with modulated cooling capacity (inverter or step control). At low loads – the gas velocity in the discharge line is reduced, oil entrainment and return from the evaporator is reduced, the pressure drop is insufficient for a stable oil flow and as a result – some oil remains in the circuit, especially in the evaporator. Often occurs in screw and scroll compressors where oil return is sensitive to flow rate.
Signs: increased vibration, metallic noise, increased bearing temperature.
Control and prevention: level measurement by sight glass, pressure drop between crankcase and discharge, oil separator serviceability, oil selection according to refrigerant type (POE / PVE).
Fig 2. Visual inspection of the compressor oil level
Hydraulic shock
Liquid refrigerant enters the cylinder (or scroll/scroll chamber). Since the liquid is incompressible, a sudden pressure surge occurs and the valves or scrolls are destroyed. On cooling units with shell-and-tube evaporator it is practically uncommon, on units with FST there is a possibility of occurrence.
The main reason is insufficient suction overheating (< 5 °C) due to improper operation of the TRV. It is also possible that the solenoid valve jammed in the open position, which in combination with a leaky closed TRV gives flooding of the evaporator, from where the compressor will pull up the liquid (in the form of foam emulsion) at the first start-up. If the chiller design provides for a liquid separator – there is a probability of its contamination, which led to disturbance of normal operation.
Fig. 3 – Effects of hydraulic shock on scroll compressor
Faults in the electrical part of the compressor
Winding burnout, phase misalignment and unbalanced currents lead to instantaneous failures. A compressor pre-emergency condition can often be noticed hours or even days before the windings burn out. In this case, the current of one of the phases increases by 10-15 % relative to the others, which causes localised overheating of the windings and accelerated ageing of the insulation. Even a short-term misalignment in the network can trigger thermal protection or current tripping. The main indicators are winding overheating, phase current imbalance and increased insulation resistance, which can be detected before the fault occurs.
- Current rise relative to rating.
If the operating current exceeds the rated current by 10-15 % at normal condensing pressure – the compressor is overloaded. At further increase up to 20 % thermal ageing of insulation starts (hourly heating above 120 °C halves winding life). - Imbalance of currents between phases.
A difference of more than 10 % between phases causes localised overheating of the windings, especially at the coil connection points. The cause is phase misalignment, bad contacts, dirty terminals. - Insulation resistance drop.
Measured with a megger at 500 V. Normal value is not less than 1 MOhm. If it drops to 0.5 megohms, the insulation starts to conduct leakage current and the winding is punctured during the first thermal start-up.
Fig. 4 – Checking winding integrity and insulation resistance
- Increase in casing temperature.
If the winding temperature (or the casing temperature of a hermetically sealed compressor) consistently exceeds 110-115 °C, the lacquer insulation begins to lose strength. In open compressors, a winding temperature of 130 °C is considered critical. - Changes in noise and vibration.
Before breakdown there is often a “hum” – leakage currents create an uneven magnetic field, the rotor loses balance, vibrations increase up to 5-6 mm/sec. - Darkening or odour of heat on the terminal box.
This is an indicator of localised overheating of contacts – in case of poor connection, heating occurs not in the winding but in the terminal, but the effect is the same: gradual destruction of insulation.
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Fouling and fouling of heat exchangers
Heat exchangers are the most vulnerable elements of the cycle. They determine the efficiency and pressure stability both on the refrigerant side and on the water (or air) side. A heat transfer loss of only 20 % already reduces the EER by 10-12 %.
Even a thin layer of scale on the heat transfer surface or a 0.2-0.3 mm layer of dust on the lamellae increases the heat transfer resistance by 25-40 %. The main causes are usually water without chemical treatment, poor filtration, long absence of washing.
Signs:
- increase of condensation pressure by 0,3-0,5 bar;
- drop of water ΔT by 1-2 °C;
- increase of the time to reach the mode.
Control and prevention: measure ΔT of water inlet/outlet and pressure drop, chemical washing 1-2 times a year, filters ≤ 200 µm, washing of air units every 6 months.
Fig . 3 – Contamination of heat exchangers
Malfunctions of automatics
Modern controllers (Carel, Siemens, Climatix, Danfoss) control compressors, fans, pumps. Failure of any sensor or communication failure can cause a false alarm.
Possible causes: degradation of sensors, modules unsynchronisation after power failure, PID control setting errors, wear and tear of control network elements.
Check: compare readings with a reference, check sensor resistance and general condition, analyse the error log.
Fig.4 – Replacing/cleaning the temperature sensor whose poor connection caused the control system error
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Refrigerant leaks and charging accuracy
The most likely places for leaks are soldering areas, heat exchanger tube ends, condenser stools, as well as service valves and areas subject to vibration. Oil marks or discolouration of the insulation are indirect signs of Freon leakage, as oil always leaves with Freon. Electronic leak detectors, nitrogen pressure testing or vacuum pressure drop monitoring for 12-24 hours are used to confirm the leak.
Lack of refrigerant disrupts the oil circulation – the compressor loses lubrication, the temperature in the windings rises, and when the oil heats up above 130 °C, it starts thermal decomposition and the formation of acids. These acids interact with the copper, destroying the insulation of the wires and forming characteristic black deposits.
Refrigerant overcharging is equally dangerous: excessive liquid phase in the condenser reduces the active heat exchange area, the condensing pressure increases and the compressor is overcurrented. In addition, if the compressor is overcharged, liquid Freon may enter the compressor during shutdown, resulting in a hydrostatic shock during the next start-up.
For accurate charging, it is necessary to refer not only to the mass stated in the data sheet, but also to the thermodynamic parameters of the cycle. Superheating at suction 5-7 °C, subcooling at condenser outlet 1-2 °C.
Diagnostic table, reference values
As a result of the possible causes of faults described above, we can draw up a diagnostic table as well as a table of monitored parameters.
Diagnostic table
| Symptom | Possible cause | Check / Action |
| Compressor overheating at normal pressure | Phase misalignment >2 %, low voltage, loose contacts | Measure phase currents (<10 % difference), check voltage ±2 %, tighten terminals |
| Compressor does not start, “Phase loss” protection | Phase sequence fault or power failure | Check phase control relays, L1-L2-L3 connection order, fuse integrity |
| Current protection tripped without mechanical cause | Insulation degradation, partial winding breakdown | Measure insulation resistance with megohmmeter (≥1 megohm), enclosure temperature <110 °C |
| Frequent shutdowns or unstable operation | VFD overheating (>70 °C), failure of enclosure fans | Check heatsink temperature, fan rotation, clean filters, measure currents |
| False alarms and trips | Sensor error, power cable inrush, controller failure | Check resistance of temperature sensors, cable shielding, controller event log |
| Uneven operation of compressors in cascade | Load logic broken / failure of one pressure sensor | Check sensor calibration, setpoint settings, synchronisation in controller |
| Compressor does not start with normal signal | Contactor or starter defective | Check coil resistance, contact condition, winding voltage |
| Compressor overheating or humming after installation of frequency converter | No sine filter, voltage harmonics | Check waveform with oscilloscope, install LC-filter, check grounding |
| Protection tripping at start-up | Undervoltage of mains, unstable make-up voltage | Check minimum start-up voltage, estimate power supply capacity, compensate losses |
| Vibration and hum at normal pressure | Incorrect phase sequence (reverse rotation) | Check direction of compressor rotation, correct phases |
| Suction pressure drop, overheating increase | Refrigerant leakage, incomplete charge | Check superheat (normal 5-7 K), suction pressure, perform leak detection or nitrogen pressure test |
| Condensing pressure increase, high discharge temperature | Refrigerant recharge, excess liquid phase in condenser | Measure subcooling (3-5 K is normal), reduce refrigerant mass to design value, control compressor current |
Control parameters and intervals
| Parameter | Norm / permissible deviation | Control interval | Inspection method / comment |
| Suction pressure | Within nameplate value, ±0.3 bar | Weekly | Manometric check at steady load |
| Condensing pressure | Within nameplate value, not more than 1 bar from design value | Weekly | Compare with outside air or condenser water temperature |
| Overheating (suction) | 5-7 °С | Weekly | Temperature probes / sensors at compressor inlet |
| Subcooling (liquid line) | 1-2 °С | Weekly | Temperature after condenser minus condensing temperature |
| Phase voltage | ±2 % of rating | Weekly | Terminal measurement, phase control relay test |
| Winding insulation resistance | ≥ 1 megohm | Quarterly | 500 V megohmmeter, measure with compressor off |
| Oil acidity (acid test) | ≤ 1 mg KOH/g | Annually or after repair | Test kits for synthetic oils |
| Oil colour and clarity | No turbidity | Quarterly | Visual inspection, change if darkened |
| Oil level in crankcase | Within sight glass | Weekly | Check with compressor stopped |
| Pressure drop across filter drier | ≤ 0.2 bar | Quarterly | Pressure gauges before and after filter |
| Circuit tightness | No pressure drop >0.1 bar/24 h | Quarterly | Vacuum test or nitrogen pressure test |
| Condition of heat exchangers | Visually after disassembly | Seasonally | Visually / thermochamber / ΔT measurement |
| Air temperature in control cabinet / VFD | ≤ 45 °C (Soft Start), ≤ 70 °C (VFD) | Monthly | Integrated sensors or IR pyrometer |
| Operation of cabinet fans / EC fans | No vibration or noise | Monthly | Visual and acoustic monitoring |
| Make-up pressure in the hydraulic circuit | 1,5-2,0 bar | Weekly | Pressure gauge on the supply line, make-up if necessary |
| Inlet/outlet water temperature | ΔT within 4-6 °C | Continuously | Comparison of sensor readings, flow stability control |
Main alarm scenarios and diagnostic algorithm
To summarise, most chiller failures can be reduced to a limited set of recurring scenarios. The engineer’s main task is to determine the root cause by a combination of parameters: pressure, superheat, ΔT water and compressor current.
1. Increased condensing pressure
A 1-2 bar increase in discharge pressure above the design pressure is accompanied by an increase in discharge temperature and compressor current consumption. The cause is a deterioration of the heat dissipation. The most frequent sources are contamination of the condenser fins, fan stoppage or excessive refrigerant charge, which increases the liquid phase volume in the condenser and reduces the active heat transfer area. The condensing pressure increases, the compressor works with increased load and the oil temperature increases. It is necessary to flush the condenser, check the rotation and performance of fans, correct the refrigerant mass according to the subcooling indicator (3-5 °C).
2. Low suction pressure
Falling evaporating pressure leads to evaporator frosting and frequent low pressure shutdowns. The main cause is a lack of heat flow to the evaporator. This can be caused by refrigerant leakage, reduced water flow or incorrect TRV adjustment. When the flow rate decreases, water does not have time to warm up the heat exchange surface, the refrigerant boils partially, the temperature drops below the freezing point. It is necessary to measure overheating (norm 5-7 °C), compare ΔT of water at inlet and outlet and check the pressure on both sides of the evaporator.
3. Compressor overheating
Oil overheating above 120 °C indicates a disturbance in the compressor heat balance. Under normal conditions, the heat generated during compression should be efficiently dissipated through the condenser and oil circuit. If heat dissipation is impaired due to condenser fouling, undervoltage or insufficient oil, winding and oil temperatures rise, viscosity decreases and accelerated bearing wear begins. To restore the mode it is necessary to flush the heat exchanger, check the phase voltage, oil level and ΔP in the lubrication system.
4. Frequent compressor starts
Cyclic starts at intervals of 1-3 minutes lead to thermal stresses and destruction of winding insulation. This is most often the result of the low inertia of the hydraulic system: the buffer volume is insufficient, the temperature of the coolant fluctuates rapidly and the automatic system prematurely signals the compressor to switch on. Additional causes are refrigerant leakage that prevents the compressor from reaching the setpoint or a too narrow hysteresis of the temperature controller. To stabilise the cycle, increase the buffer capacity (approx. 10 l/kW cold), optimise the hysteresis (2-3 °C) and check the correctness of the temperature sensor signal.
5. Insufficient cooling capacity
If the chiller does not reach the design temperature when the compressor is running continuously, it is an indication of reduced heat transfer efficiency or compressor degradation. Evaporator fouling, refrigerant leakage or valve wear will result in reduced Freon mass flow rate and lower EER. It is necessary to compare the actual EER with the nameplate EER, measure ΔT between water inlet and outlet, superheat and subcooling. In case of deviations of more than 10 %, cleaning and leak test should be carried out.
6. Vibrations and compressor noise
Increased casing vibration and metallic hum indicate hydraulic or mechanical shocks inside the compressor. The main cause is liquid refrigerant ingress (liquid shock) with insufficient suction superheating. Rotor unbalance, weak compressor mounting or resonance with pipework are also possible. Vibrations are additionally increased if the anti-vibration mountings are not properly secured and if cavitation phenomena occur in the pumps. It is recommended to check TRV setting, evaporator flow rate, unit mounting and condition of elastic inserts.
7. Increased suction pressure
If the suction pressure is higher than normal and the coolant temperature does not decrease, this indicates a throttling fault or hot gas leakage into the evaporator. The most common causes are leaking compressor valves, improper adjustment of the TRV or loose closing of the bypass at the heat pump. In this case, the compressor operates with a reduced pressure drop, the capacity drops and the discharge temperature remains elevated. Diagnostics: measure superheat (should be 5-7 °C), check condensing pressure and valve tightness, check that the bypass closes completely.
8. Fluctuations in condensing pressure
Pressure pulsations on the discharge line are a sign of unstable operation of the ventilation or throttling device. If the condenser fins are dirty, the fans periodically go to maximum speed, cooling becomes cyclic and the pressure “jumps” within 1-2 bar. In systems with electronic TRV, pressure spikes are possible when the signal from the temperature sensor is delayed. This causes fluctuations in freon consumption and unstable operation of the compressor. It is recommended to check the cleanliness of the condenser, the EC fans, the pressure sensor and the PID settings of the controller.
9. Unstable operation of the electronic RTD
If the evaporator is alternating between freezing and overheating, the cause is an unstable opening of the TRV or EEV. Electronic valves are sensitive to contamination and humidity of Freon: even microscopic oil or ice particles can interfere with stem movement. The compressor operates with variable overheating, the suction pressure fluctuates and the water temperature is unstable. It is necessary to check the filter-drier, dry the circuit, check the stability of the temperature sensor signal and update the valve settings via the controller (zero position, step test).
10. Drop in EER / increase in energy consumption for no apparent reason
When a chiller is outwardly stable but the actual energy efficiency drops by 10-15%, it is almost always due to a gradual deterioration in heat transfer. Even a thin layer of scale in the water condenser (0.3 mm) or dust on the fins of the air block reduces the heat transfer coefficient by 25-30 %. The compressor compensates for this by increasing pressure and current consumption. As a result, the EER is reduced for the same cooling output. To check this, compare the condensing pressure with the outside air temperature, measure the ΔT of the water and the discharge temperature. If the pressure is more than 1 bar above the design pressure, the condenser must be cleaned and the hydraulic circuit flushed.
Conclusion
A chiller is a complex system in which the failure of one component is almost always caused by the failure of another. Compressor overheating starts with a dirty condenser, temperature instability starts with a clogged hydraulic circuit, and false automation failures start with a bad contact in the power supply circuit. In most cases, failure is not a sudden event, but the result of an accumulation of small abnormalities that could have been detected during regular diagnostics.
The engineer’s key tasks during operation are control, cleanliness and documentation. Control – temperature, pressure, current and flow. Cleanliness – of heat exchangers, oil and electrical connections. Documentation – keeping a log of parameters to track the dynamics of degradation of components.
Prevention is cheaper than any accident, and a reliable refrigeration machine is not a successful model, but the result of disciplined operation.
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Author of the article:
Andrey Kohan, refrigeration equipment engineer
23.10.2025