The ASI Master Chiller Technician Training Guide is built for HVAC technicians, service mechanics, building engineers, controls technicians, commissioning teams, and facility operators who need to understand chillers at the system level.

A chiller does not operate by itself. It is part of a complete mechanical plant that includes the refrigeration circuit, chilled water loop, condenser water loop, pumps, cooling towers, dry coolers, valves, sensors, controls, building automation, safeties, and the building load.

A strong chiller technician must be able to read the full system, not just the alarm on the screen.

ASI training focuses on helping technicians understand:

How the chiller works.

 - What the readings mean.

 - Why the system is failing.

 - How the plant should respond.

 - How to find the root cause.

A chiller removes heat from water and rejects that heat somewhere else.

Warm return water comes back from the building or process load and enters the evaporator. Refrigerant inside the evaporator absorbs heat from that water. The water leaves the chiller colder and is pumped back to the building, air handlers, process equipment, data center cooling equipment, or other connected loads.

The compressor moves refrigerant through the machine and raises the refrigerant pressure and temperature. The condenser then rejects that heat.

An air-cooled chiller rejects heat through condenser coils and fans.

A water-cooled chiller rejects heat into condenser water, which is then sent to a cooling tower, dry cooler, or heat exchanger.

The basic refrigeration cycle is:

Evaporator absorbs heat → Compressor raises pressure → Condenser rejects heat → Expansion device meters refrigerant → Cycle repeats

A technician must understand both sides of the machine:

Water side: flow, delta-T, pumps, strainers, valves, air, glycol, water treatment, tower operation, heat exchangers, and building load.

Refrigerant side: suction pressure, discharge pressure, saturation temperature, superheat, subcooling, approach, oil, compressor loading, expansion valve position, VFD speed, and safeties.

Chiller troubleshooting starts with readings. The machine is always telling a story. The technician’s job is to know which readings matter and how they connect.

Entering chilled water temperature is the warmer return water coming back from the building.

Leaving chilled water temperature is the colder supply water leaving the chiller.

The difference between the two is chilled water delta-T.

Example:

56°F entering water – 44°F leaving water = 12°F delta-T

A normal delta-T depends on the system design, load, flow, and equipment type. The key is not just the number. The key is whether the temperature difference makes sense for the load and flow condition.

Low delta-T usually means the system is moving too much water for the amount of heat being removed.

Common causes include:

• Low building load 
• Excessive chilled water flow 
• Control valves not closing properly 
• Three-way valve bypassing 
• Poor coil performance 
• Dirty coils 
• Incorrect valve sizing 
• Poor airside control 
• Bad sensor readings 
• Poor control tuning 
• Improper chilled water reset 
• Building load not matching plant operation 

Low delta-T is often not a chiller failure. It is usually a plant, controls, airside, or building-load problem.

High delta-T usually means the water is picking up a lot of heat or the flow is restricted.

Common causes include:

• Low chilled water flow 
• Plugged strainers 
• Closed or partially closed valves 
• Pump problems 
• Air in the loop 
• Dirty evaporator tubes 
• Glycol or water quality issues 
• High building load 
• Improper balancing 
• Failed flow control 

High delta-T can lead to poor comfort, low suction pressure, freeze protection alarms, unstable leaving water temperature, and nuisance trips.

For water-cooled chillers, technicians must understand both chilled water and condenser water.

Entering condenser water temperature is the water coming from the cooling tower or heat rejection system into the chiller condenser.

Leaving condenser water temperature is the warmer water leaving the condenser and going back to the tower or heat rejection system.

If condenser water temperature rises, refrigerant head pressure usually rises. Higher head pressure increases compressor lift, reduces efficiency, and can cause high-pressure alarms, surge, unloading problems, or capacity loss.

Common condenser water problems include:

• Dirty condenser tubes 
• Low condenser water flow 
• Plugged strainers 
• Failed condenser water pump 
• Poor cooling tower performance 
• Failed tower fan 
• Dirty tower fill 
• Low tower basin level 
• Bad tower bypass control 
• Incorrect condenser water setpoint 
• Poor water treatment 
• Non-condensables in the refrigerant circuit 

Refrigeration pressures are not just gauge readings. They represent refrigerant saturation temperatures.

A technician must compare:

Evaporator pressure → evaporator saturation temperature → leaving chilled water temperature

Condenser pressure → condenser saturation temperature → condenser water temperature or outdoor air temperature

This comparison shows whether the chiller is transferring heat correctly.

Low suction pressure does not automatically mean low refrigerant.

High head pressure does not automatically mean a bad condenser.

The reading must be compared against water temperature, flow, load, valve position, compressor loading, and system condition.

Approach is one of the most important chiller diagnostic readings.

Approach compares the water temperature to the refrigerant saturation temperature.

Evaporator approach helps show how well heat is transferring from the chilled water into the refrigerant.

A high evaporator approach can point to:

• Fouled evaporator tubes 
• Low refrigerant charge 
• Oil logging 
• Low water flow 
• Poor heat transfer 
• Incorrect readings 
• Sensor problems 
• Internal refrigerant distribution issues 

Condenser approach helps show how well heat is being rejected.

A high condenser approach can point to:

• Dirty condenser tubes 
• Scale 
• Low condenser water flow 
• Non-condensables 
• Overcharge 
• High condenser entering water temperature 
• Poor tower performance 
• Airflow problems on air-cooled equipment 

Approach connects the water side to the refrigerant side. That is why it matters.

Cascading has two important meanings in chiller work.

As load increases, the plant may cascade equipment on.

Example:

Building load rises → valves open → pump speed increases → chiller loads up → next chiller starts → tower fans increase

As load decreases, the plant may cascade equipment off.

Example:

Building load drops → chiller unloads → pump speed reduces → tower fans slow down → lead/lag rotation changes

This is normal plant sequencing.

Cascading can also mean one problem creates another problem.

Example:

Dirty tower → high condenser water temperature → high head pressure → high compressor lift → reduced capacity → surge or high-pressure trip

The alarm may say high pressure, but the root cause may be tower airflow, dirty condenser tubes, failed tower fan, bad bypass valve, poor water treatment, or incorrect controls.

ASI training teaches technicians to troubleshoot the full system, not just the final alarm.

Chiller problems usually fall into several major categories.

Common issues include:

• Low refrigerant charge 
• Overcharge 
• Restrictions 
• Moisture or contamination 
• Non-condensables 
• Expansion valve problems 
• Failed pressure transducers 
• Low suction pressure 
• High head pressure 
• Low superheat 
• High superheat 
• Low subcooling 
• High approach 
• Poor heat transfer 

Common compressor issues include:

• Bearing wear 
• Oil loss 
• Poor oil return 
• Oil pump problems 
• Oil heater failure 
• Motor winding issues 
• VFD faults 
• Failed contactors 
• Bad unloaders 
• Slide valve problems 
• Surge 
• High lift operation 
• Short cycling 
• Overheating 
• Compressor lockout 

Different compressor types fail differently. Scroll, reciprocating, screw, centrifugal, and magnetic bearing compressors each have their own operating logic, failure patterns, and service approach.

Common water-side issues include:

• Plugged strainers 
• Dirty tubes 
• Closed valves 
• Air in the loop 
• Low flow 
• Excessive flow 
• Poor balancing 
• Failed pumps 
• Bad check valves 
• Glycol problems 
• Expansion tank issues 
• Water treatment problems 
• Incorrect bypass piping 
• Poor differential pressure control 

Water-side problems often show up as refrigeration alarms.

Air-cooled chillers can have:

• Dirty condenser coils 
• Blocked airflow 
• Failed condenser fans 
• Fan VFD faults 
• Hot air recirculation 
• Poor installation clearance 
• High ambient operation 
• Incorrect head pressure control 

Water-cooled chillers can have:

• Dirty condenser tubes 
• Poor cooling tower operation 
• Failed tower fans 
• Plugged strainers 
• Low condenser water flow 
• Poor water treatment 
• Tower bypass problems 
• High condenser entering water temperature 

Common controls issues include:

• Bad temperature sensors 
• Bad pressure transducers 
• Wrong BAS point mapping 
• Reversed command/status logic 
• Incorrect setpoints 
• Poor PID tuning 
• Bad staging timers 
• Disabled safeties 
• Failed proof points 
• Incorrect lead/lag logic 
• Communication loss 
• Local chiller control fighting BAS control 
• Incorrect reset schedules 
• Poor trend setup 

A controls problem can look like a mechanical problem if the technician does not verify the sequence.

Common electrical issues include:

• Loose terminations 
• Failed contactors 
• Overload trips 
• Phase loss 
• Voltage imbalance 
• Ground faults 
• Weak control transformers 
• VFD faults 
• SCR / IGBT failures 
• Motor insulation breakdown 
• Bad wiring 
• Failed relays 
• Control circuit faults 
• 
  

A technician must know when the problem is refrigeration, hydronic, electrical, or controls-related.