The chiller trips at 2pm on the hottest day of the year. Building occupants are already calling. The BMS is showing three alarms and the chiller controller is locked out. Your engineer is on another site and your technician is standing in the plant room not sure where to start.
This is the scenario every facilities manager dreads — and the one that tests whether your team actually understands the system they’re responsible for, or just knows how to press the reset button and hope for the best.
Chiller troubleshooting is not guesswork. There is a logical sequence to diagnosing a chiller fault — and once you understand the four main subsystems (evaporator, compressor, condenser, and chilled water circuit), most faults become predictable. The machine is telling you what’s wrong. You just need to know how to read it.
This guide covers the most common chiller faults your team will encounter, what causes them, and the step-by-step checks to diagnose and resolve them — written from real plant room experience, not a manufacturer’s theoretical service manual.
Figure 1: Chiller System — Key Components and Fault Locations
| CHILLER SYSTEM — COMPONENT OVERVIEW & FAULT LOCATIONS | |
| ❄️ Evaporator | Common faults: low refrigerant pressure, low evap temp alarm, freeze protection trip, fouled tubes causing low ΔT. |
| 🔄 Compressor | Common faults: high discharge pressure, motor overload/trip, surge (centrifugal), oil pressure fault. |
| 🌡️ Condenser | Common faults: high head pressure, fouled condenser tubes, low condenser water flow, cooling tower issue. |
| 💧 Chilled Water Circuit | Common faults: low CHW flow, high/low ΔT syndrome, air in system, pump failure or trip. |
| ⚠️ Read the fault log before touching anything. The FIRST alarm in the log is the root cause — later alarms are consequences. | |
Before You Touch Anything: The First 3 Steps in Any Chiller Fault
The most common mistake in chiller troubleshooting is jumping straight to a component — checking the compressor, feeling pipes, resetting alarms — before reading what the machine is actually saying. Do these three things first, every time:
Step 1 — Read the fault log, not just the current alarm
Every modern chiller controller stores a fault history. Open it. The first alarm in the log is the root cause. Everything after it is a consequence of that first fault. A chiller that shows “high pressure cutout” followed by “compressor motor overload” didn’t fail because of both problems — the high pressure came first and caused the motor to overload. Fix the first alarm, and the second one clears itself.
Nine out of ten times when an engineer resets alarms and the chiller trips again in 20 minutes, it’s because they cleared the symptoms without reading the fault log properly.
Step 2 — Record the operating parameters before resetting anything
Before you touch the reset button, write down every reading on the chiller controller screen: chilled water supply and return temperatures, condenser water supply and return temperatures, refrigerant suction and discharge pressures, oil pressure, compressor amps, and any active alarms. These readings tell the story of exactly what was happening when the chiller tripped.
If you reset first and record later, that information is gone. You’re now troubleshooting blind.
Step 3 — Check the external systems before blaming the chiller
A chiller that trips on “low chilled water flow” isn’t necessarily a chiller problem. Check whether the chilled water pumps are running, whether any isolation valves were recently closed, and whether there are any BMS commands restricting flow. Many reported chiller faults are actually pump faults, BMS faults, or upstream isolation issues. Confirm the chiller’s support systems are healthy before opening the chiller panel.
| 📌 For chiller preventive maintenance that prevents most of these faults, see the Chiller Low Delta T Syndrome guide on. Chiller Low Delta T Syndrome → |
Chiller Troubleshooting Guide: Common Faults and How to Diagnose Them
These are the faults your team will encounter most frequently on centrifugal and screw chillers in commercial buildings. Each one follows a recognisable pattern once you’ve seen it a few times.
Fault 1: High Condenser Pressure / High Head Pressure Alarm
This is one of the most common chiller alarms — especially in buildings where the cooling tower isn’t being maintained properly. The condenser is trying to reject heat from the refrigerant into the condenser water, but something is preventing it from doing so efficiently.
Common causes, in order of likelihood:
• Condenser water flow too low — check condenser water pump is running, flow rate is at design, and no isolating valves are partially closed
• Condenser water temperature too high — if cooling tower is not performing, the water returning from the tower is warmer than design. Target return temp: 29–32°C. Anything above 35°C will push head pressure high
• Fouled condenser tubes — scale and biological fouling on tube surfaces significantly reduces heat transfer. The temperature difference between refrigerant and condenser water will be higher than normal. Requires tube brushing or acid clean
• Non-condensable gases in refrigerant circuit — air or nitrogen in the system pushes head pressure up regardless of cooling water temperature. Requires refrigerant recovery and recharge by a certified refrigeration engineer
• Refrigerant overcharge — excess refrigerant causes elevated head pressure. Check subcooling against manufacturer spec
| 💡 Site tip: Before calling a refrigeration engineer for high head pressure, check condenser water temperature and flow first. In most cases the cause is on the cooling water side, not in the refrigerant circuit — and it’s a much cheaper fix. |
Fault 2: Low Evaporator Pressure / Low Suction Pressure Alarm
Low suction pressure means the refrigerant is boiling at too low a temperature in the evaporator. Left unchecked, this leads to the freeze protection alarm — and potential ice formation on the evaporator tubes, which can cause serious damage.
• Low chilled water flow through evaporator — the most common cause. With insufficient water flow, the refrigerant absorbs heat too quickly and suction pressure drops. Check chilled water pumps, flow meters, and differential pressure across evaporator
• Chilled water temperature setpoint too low — if the setpoint is 5°C or below during low load conditions, suction pressure will drop towards freeze protection. Review setpoint and chilled water reset strategy
• Refrigerant undercharge — low refrigerant level reduces suction pressure. Check for leaks at joints, valve stems, and sight glass condition before recharging
• Fouled evaporator tubes — scale on chilled water side reduces heat transfer, causing the refrigerant to extract less heat and suction pressure to fall
• Expansion valve fault — a TXV or EXV that is not opening correctly starves the evaporator of refrigerant. Check superheat readings against design spec
Fault 3: Compressor Surge
Surge is specific to centrifugal chillers and is one of the most distinctive faults you’ll encounter — the chiller makes a loud banging or whooshing sound, the chiller controller shows a surge alarm, and the machine either recovers or trips. Surge happens when the compressor impeller can no longer maintain forward flow of refrigerant gas.
• Operating too far from design point — surge most commonly happens at part load conditions, especially when condenser water temperature is lower than design. The chiller’s operating point moves outside its stable range
• High lift condition — large difference between evaporator and condenser pressure pushes the compressor beyond its surge line. Usually associated with high head pressure fault combined with low load
• Inlet guide vane fault — worn or stuck IGVs prevent the compressor from modulating correctly to avoid surge. Check IGV movement and actuator
What to do: If the chiller surges and recovers, log the operating conditions at the time of surge — load percentage, chilled water temperatures, condenser water temperature. Share this data with the OEM service engineer. Repeated surge events cause bearing damage over time and should not be ignored.
Fault 4: Chiller Low Delta T Syndrome
Low ΔT syndrome is not an alarm — it’s a performance problem that slowly kills your chilling capacity and nobody notices until summer arrives and the building can’t cool. Design ΔT for most chilled water systems is 5–6°C (6°C supply, 12°C return). Low ΔT means you’re getting 2–3°C — the chiller is running but delivering only a fraction of its cooling capacity.
• FCUs and AHUs with stuck-open control valves — bypassing chilled water around the coil, mixing return water temperatures down. Walk the building and feel the chilled water pipework — unusually warm return pipes from specific zones point to stuck valves
• Oversized chiller — a chiller running at 15–20% load during shoulder season produces a very low ΔT. Consider staging or using a smaller trim chiller
• Air in the chilled water system — air pockets in risers and coils reduce effective heat transfer. Check automatic air vents on high points and bleed coils if necessary
| 📌 For a detailed guide on diagnosing and fixing low ΔT in chilled water systems, see the Chiller Low Delta T Syndrome article. Chiller Low Delta T → |
Fault 5: Compressor Motor Overload / High Motor Amps
A compressor drawing more current than nameplate rating will trip on overload. This is the chiller protecting itself — but the underlying cause needs to be found before you restart.
• High head pressure causing compressor to work harder — the most common cause. Address condenser-side issues first (see Fault 1 above)
• Low refrigerant oil pressure — inadequate lubrication causes compressor internals to run with increased friction, raising current draw. Check oil level in sight glass and oil pump operation
• Electrical supply issue — low voltage or phase imbalance forces the motor to draw higher current to maintain torque. Measure voltage at compressor terminals on all three phases. More than 5% phase imbalance warrants immediate investigation
• Motor winding fault — measure winding resistance and insulation resistance (Megger test). Insulation below 1 MΩ indicates a deteriorating winding
Chiller Fault Quick Reference Table
Use this table on-site when an alarm fires and you need a fast starting point. It covers the 10 most common chiller faults across centrifugal and screw machines:
| Symptom / Alarm | Diagnosis & Action |
| High condenser pressure | Most likely: cooling tower / CW issue. Check CW flow and return temp first. If healthy, check condenser tube fouling. |
| Low suction pressure | Most likely: low CHW flow or refrigerant undercharge. Check CHW pumps and flow rate before checking refrigerant. |
| Freeze protection trip | High risk of tube damage. Restore CHW flow immediately before restart. Never bypass freeze protection. |
| Compressor surge | Log conditions at time of surge. Check IGV operation. Engage OEM engineer — repeated surge causes bearing damage. |
| Motor overload / high amps | Check head pressure first — most common cause. Then check motor voltage balance on all 3 phases. |
| Oil pressure fault | Check oil sight glass level. Listen for oil pump operation. Low oil = do not restart without investigation. |
| Low chilled water ΔT | Check individual zone return temps. Warm return pipes = stuck-open valves. Review chiller staging at low load. |
| Refrigerant leak alarm | Check sight glass for bubbles. Low charge = gradual capacity loss over weeks. Call F-gas certified engineer. |
When to Call the OEM Engineer — and When Not To
Not every chiller fault needs an OEM service call. Knowing the difference saves you time, money, and the frustration of waiting for a specialist when the problem was something your team could have fixed.
| Your team can handle this | Call a specialist engineer |
| Cooling tower fault causing high head pressure | Refrigerant leak — requires F-gas certification |
| Chilled water pump tripped — restart and investigate | Compressor mechanical failure or surge damage |
| BMS setpoint incorrect — adjust and monitor | Refrigerant recharge or recovery |
| Strainer blocked — clean and restore flow | Expansion valve replacement (electronic EXV) |
| Air in CHW system — bleed and vent | Oil system faults — oil pump, oil cooler, separator |
| Isolation valve incorrectly closed — open | Persistent surge that IGV adjustment doesn’t resolve |
One important rule: if you’re not sure whether a fault is within your team’s competence — don’t guess. A chiller compressor that costs £50,000–£150,000 to replace is not worth damaging to save a service call fee. When in doubt, isolate the machine safely and get the right engineer on site.
Preventive Checks That Stop Most Chiller Faults Before They Happen
The chiller faults in this guide are almost all preventable. Here are the monthly and quarterly checks that eliminate 80% of reactive callouts:
• Monthly — cooling tower inspection: check fan operation, basin water level, drift eliminator condition, and water treatment dosing. A poorly maintained cooling tower is the single biggest cause of high head pressure faults
• Monthly — chilled water pump checks: confirm duty/standby changeover, check motor current, listen for bearing noise, confirm differential pressure is within design range
• Monthly — log all operating parameters: chilled water supply/return temps, condenser water supply/return temps, refrigerant pressures, compressor amps. Trending these numbers over months reveals developing problems before they cause a trip
• Quarterly — condenser tube inspection: check approach temperature (difference between refrigerant condensing temperature and leaving condenser water temp). Approach temp rising above 2–3°C above design indicates tube fouling
• Quarterly — refrigerant leak check: use an electronic refrigerant detector at all valve stems, joints, and the sight glass. A small leak caught early costs far less than an emergency recharge in midsummer
• Annual — full service by OEM or specialist: vibration analysis, oil sample analysis, eddy current tube testing (on older machines), controls calibration, safety device testing
Frequently Asked Questions
Why does my chiller keep tripping on high pressure?
High pressure trips are almost always caused by a problem on the condenser water side, not inside the chiller itself. Start by checking the cooling tower — are all fans running, is the basin level correct, is the water treatment programme active? Then check the condenser water flow rate and return temperature. If the return temperature is above 35°C, the chiller is fighting a heat rejection problem. Only if the condenser water system is healthy and verified should you look at refrigerant-side causes such as tube fouling or non-condensable gases.
What does chiller surge sound like and is it dangerous?
Chiller surge on a centrifugal machine sounds like a loud bang, whoosh, or repetitive thumping — sometimes described as the sound of a washing machine with an unbalanced load. It’s unmistakable once you’ve heard it. Occasional surge events are not immediately dangerous, but repeated surge causes bearing wear, impeller damage, and compressor housing stress over time. If your chiller is surging regularly, log the conditions — load percentage, lift, condenser water temperature — and share them with the OEM engineer. It’s not a problem to reset and ignore.
How do I know if my chiller has a refrigerant leak?
The signs of a refrigerant leak on a chiller are: refrigerant sight glass showing bubbles (which indicates low charge), gradual decline in cooling capacity over weeks or months, rising suction pressure over time compared to historical data, and refrigerant leak detector alarms if the plant room has fixed detection. A small leak can take months to show symptoms. The best way to catch it early is to trend your suction pressure monthly — a consistent downward trend is the earliest indicator of refrigerant loss.
Can I restart a chiller that has tripped on freeze protection?
Not without checking first. A freeze protection trip means the evaporator temperature dropped close to 0°C. Before restarting, confirm the chilled water flow rate is normal — a restart with low flow will trip freeze protection again immediately, and repeated rapid freeze-thaw cycles can crack the evaporator tubes. Restore chilled water flow fully, verify the pump is running at design flow, then restart the chiller and monitor evaporator temperature for the first 5–10 minutes of operation.
What is the correct chilled water delta T and why does it matter?
Design ΔT for most commercial chilled water systems is 5–6°C — chilled water supplied at 6–7°C and returned at 11–13°C. This ΔT directly determines how much cooling the chiller can deliver: a chiller rated at 1,000 kW at design ΔT will only deliver 400–500 kW if the ΔT drops to 2–3°C, because the same flow rate is carrying less heat. Low ΔT syndrome is one of the most costly and under-diagnosed problems in commercial HVAC — it causes chillers to run longer hours for the same cooling output, dramatically increasing energy consumption and wear.
Conclusion
Most chiller faults follow a recognisable pattern. Read the fault log first, record the parameters before resetting, check the external systems before blaming the chiller, and work through the likely causes in order. That logical sequence resolves the majority of chiller problems without an emergency call-out.
The faults that do need a specialist — refrigerant work, compressor issues, persistent surge — are much easier to diagnose and fix when your team has already ruled out the simpler causes and has a clear record of what happened. For preventive maintenance that stops most of these faults from developing, see the FCU Maintenance Checklist → and the PPM Checklist Generator → to build a chiller-specific maintenance schedule for your site.