Chiller Systems (Commercial) Reference

Why this matters

Commercial chillers are central air conditioning's bigger sibling - instead of cooling air directly, they cool water (chilled water at 42-45 °F supply) that's then circulated to air handlers throughout the building. They scale from 10 tons (small office) to 1000+ tons (high-rise). The diagnostic approach differs from split residential systems - chillers have more sub-systems (chilled water loop, condenser water loop, refrigerant circuit, controls, often multiple compressors), and a chiller failure can shut down an entire building's cooling.

Chiller architecture

Refrigerant circuit:

• Compressor (centrifugal / screw / scroll / reciprocating, varies by tonnage)
• Condenser (water-cooled or air-cooled)
• Expansion valve (TXV or EEV)
• Evaporator (cools the chilled water loop)

Chilled water loop:

• Pumps circulate water through evaporator (where it's cooled)
• Distributed to AHUs (air handler units) throughout the building
• Returns warm to the chiller for re-cooling

Condenser water loop (water-cooled chillers only):

• Pumps circulate water through condenser (where it picks up heat)
• Routed to cooling tower (rejects heat to atmosphere via evaporation)
• Returns to condenser

Air-cooled chillers skip the water condenser; instead, large condenser coils with fans reject heat directly to outdoor air. Simpler but less efficient and noisier.

Compressor types by tonnage

Key performance metrics

• Tonnage: 1 ton of cooling = 12,000 BTU/hr (the energy to melt 1 ton of ice in 24 hours)
• kW/ton: electrical input per ton of cooling. Lower is better. Modern chillers: 0.45-0.55 kW/ton at full load; 0.35-0.50 at part-load IPLV.
• IPLV (Integrated Part-Load Value): weighted-average efficiency across load profile; meaningful for buildings that run mostly part-load.
• COP (Coefficient of Performance): cooling out / electricity in. Modern chillers: 5-7+ at full load.
• ΔT chilled water: typical 10-12 °F (55 °F return - 44 °F supply, for example)
• ΔT condenser water: typical 10 °F (85 °F supply - 95 °F return, for example)

Refrigerants in chillers

• R-134a: common in centrifugal chillers; being phased down under HFC regulations
• R-1234ze, R-513A: low-GWP replacements becoming common
• R-410A: scroll chillers, packaged units
• R-32: newer scroll units
• Ammonia (R-717): industrial / very large; not residential
• Always check the refrigerant type before service; EPA Section 608 cert required

Common failure modes

Chiller won't start / locks out on safety:

• Low chilled-water flow → flow switch open
• High discharge pressure (condenser water issue, cooling tower problem, refrigerant overcharge)
• Low suction pressure (refrigerant undercharge, low chilled-water temperature)
• Compressor overload
• Microprocessor fault (read fault codes)

Chiller runs but doesn't cool to setpoint:

• Refrigerant low → recover, leak-check, recharge
• Condenser tubes scaled / dirty → reduced heat rejection → high discharge
• Evaporator tubes scaled / dirty → reduced heat absorption → low chilled-water capacity
• Compressor mechanical degradation → low compression ratio
• Excess load → building demand exceeds chiller capacity

Chiller cycles rapidly:

• Chilled water flow inadequate (short cycling on flow switch)
• Setpoint too tight
• Pump issues

Cooling tower issues (water-cooled):

• Fan failure → no heat rejection
• Spray nozzles plugged → low water flow over fill
• Bleed-off valve clogged → high TDS in water → scale formation
• Belt drive worn → fan slow

Maintenance schedules (industry-standard)

Daily / weekly:

• Read pressures, temperatures, water flows
• Log chiller status
• Check cooling tower water quality (TDS, pH)
• Verify normal operation

Monthly:

• Clean cooling tower strainer
• Check chilled water and condenser water pH, biocide levels
• Inspect for visible leaks
• Verify control sequence

Quarterly:

• Check refrigerant charge (subcool / superheat) - see Superheat Subcooling Method Reference
• Test safety controls (high-pressure cutout, low-pressure cutout, freeze stat)
• Inspect electrical connections in starter

Annually:

• Eddy-current testing of chiller tubes (every 3-5 years)
• Condenser water chemistry full analysis
• Refrigerant leak test (per EPA 608)
• Compressor oil analysis
• Bearing lubrication / replacement per manufacturer

Water quality (water-cooled chillers)

Cooling tower water is exposed to air and concentrates impurities as water evaporates. Without treatment:

• Scale formation: calcium carbonate deposits on condenser tubes; thermal resistance climbs; chiller efficiency drops
• Biological growth: algae, bacteria (including Legionella) thrive in warm, oxygenated water
• Corrosion: dissolved oxygen + dissolved minerals attack metal components

Treatment program:

• Conductivity-based bleed-off (controls TDS / cycles of concentration; typical 3-5 cycles)
• Corrosion inhibitor
• Scale inhibitor / dispersant
• Biocide (oxidizing + non-oxidizing rotation)
• Routine testing (weekly or biweekly)
• Annual Legionella testing per ASHRAE 188 in some applications

Brand-specific notes

• Trane CenTraVac (centrifugal): premium, common large commercial; Magnetic Bearing chillers (no oil) on newer Series E units
• Carrier 19XR / 23XR (centrifugal): premium large
• York Millennium / OptiView (screw): common
• Daikin McQuay: screw and scroll across range
• Tecnoclima / specialty European: premium efficiency, limited US distribution

Common service mistakes

• Working the starter or VFD cabinet without an energized-work assessment. Chiller starters carry serious available fault current. De-energize and verify absence of voltage before opening; if the task genuinely requires it live, it needs an arc-flash risk assessment and rated PPE, not a flashlight and a screwdriver.
• Chasing a low-suction lockout with refrigerant. On a chiller, low suction is far more often a chilled-water flow problem (fouled evaporator, closed balance valve, failed pump, air bound loop) than a low charge. Add gas to a flow problem and you set up a freeze event that splits evaporator tubes. Verify flow and delta-T across the barrel first.
• Trusting the panel readouts without verifying with your own instruments. Building-mounted sensors drift. Confirm chilled water supply and return, condenser water in and out, and refrigerant pressures with calibrated gauges and a thermometer before you conclude anything.
• Reading approach temperature without reading it against the log. Approach (refrigerant saturation minus leaving water) is the fouling indicator, but it only means something compared to the same machine's clean baseline at similar load. No baseline, no diagnosis.
• Ignoring the purge run-time on a low-pressure machine. A low-pressure chiller running below atmospheric pulls air and moisture in through any leak. Rising purge run time is the leak alarm, and moisture in the refrigerant makes acid. Log it every visit.
• Skipping oil analysis because the oil looks fine. Acid number, moisture, and wear metals do not show up by eye. On a large compressor the analysis is trivial next to the rebuild it prevents.
• Pulling a vacuum without an isolation-valve check. Leaving a service valve or the purge isolated the wrong way gives a false vacuum reading and a system that never actually got dehydrated.
• Treating the cooling tower as somebody else's equipment. Tower fouling, a stuck bleed valve, or a failed treatment feed shows up first as chiller high head. If the tower and water treatment are not in scope, say so in writing rather than letting the customer believe the chiller was the problem.
• Restoring the machine without documenting the log sheet. The next tech diagnoses from your numbers. Pressures, temperatures, amps, water flows, oil level, and purge time, every visit.

References

• ASHRAE Handbook - HVAC Systems and Equipment (chillers chapter)
• ASHRAE 90.1 (energy standard for buildings; chiller efficiency requirements)
• ASHRAE 188 (Legionellosis: Risk Management for Building Water Systems)
• AHRI 550/590 (chiller performance ratings)
• EPA Section 608 (refrigerant handling)
• Manufacturer service manuals (Trane, Carrier, York, Daikin McQuay)