Commercial Chiller Troubleshooting: How to Diagnose Centrifugal and Screw Chiller Faults in the Field

It's 2 PM in July. A 120-ton Carrier centrifugal just tripped on high discharge pressure. The building facility manager is standing next to you. You have the controls panel and your phone. In 2026, commercial chiller troubleshooting is still where a lot of techs freeze up — not because the machine is impossible to read, but because they haven't built the mental model to work it systematically. Fault codes tell you what tripped. They don't tell you why. This post gives you the diagnostic structure to go from fault code to root cause without guessing.

Whether you're working a centrifugal chiller fault, a screw chiller high head pressure call, or a scroll-based air-cooled unit that's not cooling, the framework is the same: establish what the machine is doing physically, isolate the water side from the refrigerant circuit, and use the controls screen as your diagnostic instrument — not just a fault display. If you want a deeper look at how this fits into a broader systematic troubleshooting approach, that post covers the logic foundation.

Chiller Types and Why the Troubleshooting Logic Differs

The machine type tells you where the failure modes live before you touch anything.

• Centrifugal chillers use dynamic compression — an impeller accelerates refrigerant to high velocity, then converts that velocity to pressure in the diffuser. The oil system is load-bearing: without adequate oil pressure and temperature to the bearings and shaft seal, the compressor destroys itself quickly. Surge is the failure mode that kills centrifugal compressors. Surge occurs when the compressor can't maintain stable flow — the refrigerant backflows through the impeller in rapid pulses, creating a sound like a jet engine hiccupping. It mechanically damages thrust bearings and guide vane actuators on every event.
• Screw (rotary) chillers are positive displacement — twin helical rotors trap refrigerant and physically compress it regardless of pressure ratio. A slide valve controls capacity (unloads the compressor by bypassing suction). Surge isn't possible. The weakness in screw chiller troubleshooting is the oil system: the oil separator, oil return lines, and oil cooler are where failures accumulate. Fouled oil separator = oil carryover into the refrigerant circuit = slugging and bearing starvation.
• Scroll-based air-cooled commercial chillers follow the same logic as residential scroll systems but with staging controls and electronic expansion valves (EXVs) per circuit. Most faults are staging logic errors — one circuit locked out while the other is overloaded — or EXV hunting caused by a faulty thermistor or loss of superheat signal.

The reason fault codes alone aren't enough: a "high pressure" fault on a centrifugal could be a condenser water pump off, a cooling tower approach problem, fouled condenser tubes, or refrigerant overcharge. The fault code tells you the machine shut down on high pressure. It doesn't tell you which of those four causes you're dealing with. You need to know what the machine was physically doing at the moment of the fault.

The 5 Most Common Chiller Faults

1. High Condenser Pressure / Chiller High Head Pressure

Start with the water side — not the refrigerant. High condenser pressure is a water-side problem 80% of the time. Check condenser water flow first: design delta-T across the condenser is 10°F (entering 85°F, leaving 95°F under design conditions). If your delta-T is 15°F or higher, flow is restricted — pump VFD, strainer, valve position, or flow switch bypassed. If delta-T is below 8°F, you either have too much flow or the tower isn't rejecting heat.

After flow, check cooling tower approach temperature: the difference between leaving tower water temperature and wet bulb ambient. Design approach is typically 5–7°F. An approach of 12°F or more means the tower is undersized for current conditions, the fill is fouled, or basin level is low. Only after ruling out water flow and tower performance do you look at refrigerant — and when you get there, check for overcharge (subcooling too high) or non-condensable gases before you start pulling charge.

2. Low Suction Pressure / Chiller Not Cooling

Again, start with the water side. Low suction pressure with a chiller not cooling is a chilled water flow problem until proven otherwise. Design delta-T on the chilled water side is 8–12°F (entering 54°F, leaving 44°F). If delta-T is running 15°F+, flow is low — pump, valve, strainer, or air in the loop. If delta-T is 4°F or less with low suction, the machine is oversized for the current load or there's refrigerant circuit starvation.

After confirming chilled water flow and load, then go to refrigerant. Low suction on a properly flowing, loaded chiller points to low charge or a restriction (TXV/EXV not opening, filter-drier plugging). Check superheat before assuming low charge — high superheat with low suction = restriction or low charge. Normal superheat with low suction = flow problem still, not refrigerant.

3. Compressor Surge (Centrifugal Only)

You'll know surge when you hear it — a rapid, rhythmic banging or pulsing from the compressor, like a jet engine that can't hold its flame. It's caused by three main conditions: low load plus low lift (the compressor is running at minimum capacity with not enough pressure differential to maintain stable flow), guide vane hunting (the guide vane actuator is overshooting set point and creating flow instability), and refrigerant migration to the oil sump during off-cycle (diluted oil causes loss of oil film, which causes the machine to start under abnormal conditions).

Every surge event damages thrust bearings. If you're seeing surge codes in the fault history, the machine needs a factory tech for bearing inspection — this is not a controls tweak. For prevention: ensure the chiller doesn't start under low load conditions, verify guide vane calibration, and check crankcase heat operation before long off-cycle periods.

4. Oil Pressure Low / Oil Temperature High

This is a drop-everything fault on any chiller type. Low oil pressure differential — below 15 psid on a machine spec'd for 20–30 psid — means the bearings are running at reduced lubrication. The common causes are a plugged oil filter bypassing (you'll see normal differential but high oil temperature), oil separator fouling reducing oil return to the sump, loss of subcooling to the oil cooler (check the liquid line valve to the oil cooler), and bearing wear reducing clearances enough to cause abnormal oil flow.

Don't restart a machine that shut on low oil pressure without investigating. An oil sample will tell you if there's bearing metal in the oil. That's your decision point for whether this is a filter change or a bearing replacement call.

5. Motor / Compressor Overcurrent

High amps with the machine loaded to setpoint: start with the electrical supply. Check all three phases — voltage imbalance above 2% causes current imbalance that reads as overcurrent on one phase. High ambient raising condensing pressure raises compressor power draw. A machine running at design conditions in 100°F ambient will pull more amps than nameplate rating assumed (usually spec'd at 95°F ambient). Beyond electrical, overcurrent points to mechanical restriction — bearings beginning to fail, liquid refrigerant slugging on startup, or a shaft seal dragging.

Reading the Controls Screen Like a Technician

The controls screen is your real-time instrument panel. Five numbers to pull immediately on any fault call:

• Chilled water supply temp vs. setpoint. Is the machine actually running and producing? If CHWS is at setpoint, the problem may have cleared — look at fault history, not current conditions.
• Condenser water entering and leaving temps. Calculate your delta-T and compare to design. Is the tower doing its job?
• Refrigerant pressures converted to saturation temps. Use a PT chart or the controls screen's built-in sat temp display. Calculate approach temperature: leaving condenser water temperature minus condensing saturation temperature. Design approach is 2–5°F. Approach above 8°F means fouled condenser tubes, not enough water flow, or a refrigerant problem.
• Compressor amps vs. full load amps (FLA). Is the machine loaded? A compressor running at 40% FLA on a peak cooling day with a building complaint tells you the machine is unloaded, not failing — investigate the controls, not the compressor.
• Oil pressure differential. Spec is typically 20–30 psid. Below 15 psid = alarm. Below 10 psid = the machine should not be running.

3-Step Field Triage Before You Call Tech Support

Step 1: Establish baselines. Is the machine actually in design conditions? What are the actual chilled water and condenser water flow rates right now? What's the building load — is the cooling tower wet bulb ambient at design? A machine tripping at 5 AM on a 65°F ambient night is a completely different diagnosis than the same fault at 2 PM in July. Don't start with fault codes — start with what conditions the machine is operating in.

Step 2: Isolate refrigerant circuit vs. water side. Before you touch a refrigerant valve or run a leak check, confirm the water side is functioning correctly. Verify flow rates by checking pump amps and comparing to design, check strainer differential pressure, and verify the cooling tower is running at full capacity. Eighty percent of commercial chiller failures that dispatch a technician are water-side problems — flow restrictions, tower performance, fouled tubes. Eliminate those before you open the refrigerant circuit.

Step 3: Pull the complete fault history log. The fault that tripped the machine is the last event — often not the root cause. What happened in the 10 minutes before the trip? A machine that shows "high pressure" preceded by "low oil pressure" is a completely different diagnosis than one that shows "high pressure" preceded by "condenser water flow fault." The fault log is your timeline. Read it backwards from the trip.

Preventive Items That Prevent Most Emergency Calls

• Annual tube brushing on both condenser and evaporator barrels. Scale and biofilm on condenser tubes increases approach temperature by 3–5°F, which directly raises head pressure and compressor power consumption. You can calculate the efficiency loss — every 1°F of excess approach temp costs roughly 1.5–2% in chiller efficiency.
• Oil analysis and filter change. Annual oil analysis catches bearing wear (metal particles), refrigerant contamination (acid number, moisture), and oxidation before any of them cause a failure. Change the filter regardless of analysis results — it's a $40 part with an outsized failure prevention value.
• Refrigerant leak check and charge verification. A machine that's 5% undercharged runs hotter, loads the compressor harder, and may surge at low load conditions. Verify charge by comparing subcooling to the manufacturer's spec — don't eyeball suction pressure.
• Vibration analysis on compressor bearings. Trending vibration signatures over multiple annual measurements catches bearing wear 3–6 months before failure. A vibration spike that appears between annual checks is always actionable.
• Eddy current testing every 3–5 years. Eddy current inspection of the condenser and evaporator tubes finds wall thinning and pitting before a tube fails and floods the refrigerant circuit. A tube failure in a 120-ton chiller is a six-figure repair. Eddy current testing is a few thousand dollars. Run the math.

When to Call the Factory vs. Handle It Yourself

Call the OEM for motor winding diagnostics and rewinding, surge diagnosis and guide vane recalibration, compressor bearing replacement, and any refrigerant circuit issue that requires factory tooling (impeller clearance adjustment, shaft seal replacement on centrifugals).

Handle in the field: water-side cleaning and flow verification, cooling tower servicing, controls programming and setpoint adjustment, oil system maintenance (filter, oil change, oil cooler cleaning), refrigerant leak testing and charge correction, and tube brushing. These are all field-serviceable with the right documentation and test equipment.

Take the Guesswork Off the Job

The Commercial Chiller Systems Troubleshooting Guide ($29.99) has everything above organized as a structured fault matrix — symptoms, root causes, field tests, and resolution steps for centrifugal, screw, and scroll chillers. Built for commercial techs who get dispatched without a service manual. When you're standing in front of a tripped machine with a facility manager asking questions, you want the decision tree in your hands, not in your head. Get it at hvacproguide.com/products.

Posted by the Promptly team — AI tools and field guides built for HVAC professionals.