A chiller is usually the largest single energy user in a building's HVAC system and one of the most expensive pieces of equipment to replace. It is also complex: a compressor, heat exchangers, refrigerant, oil, controls and safety devices all have to work together. Good maintenance keeps a chiller running efficiently and catches problems while they are still inexpensive to fix.
This guide covers the core maintenance practices for water-cooled and air-cooled chillers in commercial buildings, with particular attention to operating logs, heat exchanger condition and oil analysis. Exact tasks vary by chiller type and manufacturer, so always follow the manufacturer's maintenance manual alongside this general guidance.
Operating logs: the foundation
Most chiller problems show up in operating data before they cause a failure. Recording key readings regularly, either manually or through the building automation system, provides a baseline and reveals trends. Typical log points include:
- Entering and leaving chilled water temperatures and flow or pressure drop.
- Entering and leaving condenser water temperatures and flow or pressure drop.
- Evaporator and condenser refrigerant pressures and saturation temperatures.
- Compressor motor current and percentage load.
- Oil pressure, oil temperature and oil level.
- Discharge temperature and superheat.
- Purge run time and pump-out counts for low-pressure chillers.
- Alarms, trips and run hours.
Modern chiller controllers record much of this automatically. The value comes from someone reviewing it. A trend that drifts slowly over months is easy to miss without regular review.
Approach temperatures
The approach temperature is the difference between the refrigerant saturation temperature and the leaving water temperature in a heat exchanger. It is one of the best indicators of heat exchanger condition.
| Heat exchanger | Approach | What a rising value suggests |
|---|---|---|
| Evaporator | Leaving chilled water temperature minus evaporator saturation temperature | Fouled tubes, low refrigerant charge, oil logging, low water flow |
| Condenser | Condenser saturation temperature minus leaving condenser water temperature | Fouled or scaled tubes, non-condensables in the refrigerant, high water flow bypass |
Design approaches for modern chillers are small, often around 0.5 to 2 C (1 to 4 F) at full load, but the exact value depends on the chiller and load. Compare against the commissioning baseline or manufacturer data at similar load. A steadily rising approach is a signal to investigate.
Rising condenser approach is especially costly because higher condensing pressure increases compressor lift and power. Fouled condenser tubes are among the most common causes of avoidable chiller energy waste in open condenser water systems.
Heat exchanger maintenance
Tube cleaning
Water-cooled condenser tubes are exposed to open cooling tower water and gradually accumulate scale, biofilm and sediment. They are commonly brushed annually, or more often if approach monitoring or water treatment results indicate fouling. Evaporator tubes in closed chilled water loops foul more slowly and are cleaned less frequently, often based on approach trends.
Good water treatment reduces fouling. Our guide to cooling tower maintenance and Legionella control covers the water side of the condenser circuit.
Eddy current testing
Eddy current testing passes a probe through each tube to detect wall thinning, pitting and cracks. It identifies tubes at risk of failure before they leak, which matters because a tube leak can allow water into the refrigerant circuit and cause serious damage. Testing is often performed every few years on larger chillers, or more often for older machines or those with known water quality issues. Failing tubes can be plugged, within limits set by the manufacturer.
Refrigerant management
Refrigerant charge affects capacity and efficiency, and leaks are both an environmental and regulatory issue. Key practices:
- Leak testing at intervals required by regulations and manufacturer guidance, using electronic leak detectors and refrigerant monitors in the plant room.
- Refrigerant monitors and alarms in machinery rooms as required by CSA B52, with ventilation interlocks.
- Records of refrigerant added and removed. Provincial regulations on ozone-depleting substances and halocarbons often require leak testing, record keeping and certified technicians, and federal rules apply to federal facilities.
- Purge units on low-pressure chillers, which use refrigerants such as R-123 or R-1233zd(E) and operate below atmospheric pressure on the low side, remove air and moisture that leak in. Increasing purge run time is an early sign of a leak.
See CSA B52, the mechanical refrigeration code and refrigerant regulations and low-GWP options for the regulatory context.
Oil analysis
Compressor oil circulates through the bearings, gears and other moving parts, carrying evidence of what is happening inside the compressor. Laboratory oil analysis reads that evidence. It is typically performed annually, and more often for critical or older chillers.
What oil analysis measures
- Wear metals. Iron, copper, aluminum, tin, lead and other metals indicate wear of specific components such as bearings, gears, rotors or impellers.
- Moisture. Water in the oil can result from tube leaks, poor purging or contamination during service, and leads to acid formation and corrosion.
- Acidity. Rising acid number shows oil breakdown or contamination and can damage motor windings in hermetic compressors.
- Viscosity. Changes indicate refrigerant dilution, contamination or oil degradation.
- Contaminants and additives. Silicon may indicate dirt or sealant contamination, and changes in additive levels show oil depletion.
Using the results
A single result has limited meaning. Trends over several years show whether a component is wearing gradually or suddenly. The laboratory report usually flags readings as normal, caution or critical based on its database and the compressor type. A rising wear metal trend may justify vibration analysis or inspection. Sampling must be done correctly, from the right point with clean equipment, so the sample represents the oil in the system. Vibration analysis complements oil analysis by detecting mechanical faults directly; see vibration analysis for HVAC equipment.
Other routine tasks
- Inspect and test safety controls, including high and low pressure cutouts, flow switches and freeze protection.
- Check motor insulation resistance on open and hermetic motors at manufacturer intervals.
- Inspect starters, variable speed drives and electrical connections, and check for hot spots with infrared thermography.
- Verify chilled and condenser water flow against design. Low evaporator flow risks freezing, and incorrect condenser flow hurts efficiency.
- Calibrate temperature and pressure sensors used for control and logging.
- Perform seasonal start-up and shutdown procedures, including draining or freeze protection for idle chillers in Canadian winters.
Frequently asked questions
How often should chiller oil be changed?
Many manufacturers now recommend changing oil based on analysis results rather than a fixed schedule. Follow the manufacturer's guidance, and change oil when analysis shows degradation, contamination or after a significant event such as a tube leak or compressor repair.
Can I tell if my chiller is wasting energy?
Compare current efficiency, in kW per ton or coefficient of performance, at similar loads and condenser water temperatures against the baseline. Rising approach temperatures and increased power for the same load are common warning signs.
Who should perform chiller maintenance?
Annual and major tasks are usually done by the manufacturer's service organization or a qualified refrigeration contractor holding the provincial certifications required for refrigerant work. In-house staff typically handle logs and routine checks.
Key takeaways
- Consistent operating logs, reviewed regularly, reveal most chiller problems before failure.
- Evaporator and condenser approach temperatures are key indicators of heat exchanger fouling and refrigerant issues.
- Condenser tube cleaning and eddy current testing protect efficiency and prevent tube failures.
- Oil analysis tracks wear metals, moisture, acidity and viscosity, and is most useful as a trend.
- Refrigerant leak testing and records are required under CSA B52 and provincial regulations, and verified water flows from water balancing support efficient operation.
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