Installation and Commissioning
Core concepts
Installation quality determines system performance. Understand pipe sizing and layout, correct refrigerant line sizing (suction, liquid, hot gas), oil return with traps and slopes, insulation and vapour barriers, and proper support and isolation. For chilled water, know flushing, water treatment, air venting and pump alignment. Electrical work includes correct supply, earthing, protection devices and control wiring.
Commissioning is the documented process of verifying that the installed system meets design intent. Key steps: pressure testing with dry nitrogen, evacuation to a deep vacuum (typically below 500 microns, held to confirm no leaks), charging by weight or by subcooling/superheat, airflow balancing, control sequence verification, and recording as-found and as-left readings. Know the difference between commissioning and testing/balancing.
Traps in questions
- Never use oxygen or air for pressure testing refrigerant systems - use dry nitrogen.
- A vacuum that rises after isolation indicates moisture or a leak; do not charge.
- Charging by weight is the reference method; gauge-based charging is a cross-check.
- Airflow must be set before refrigerant charge is finalised on DX systems, since airflow affects superheat.
Rule of thumb
Evacuate to 500 microns or lower and hold for at least 10 minutes with the pump isolated; a rise above about 1000 microns suggests moisture or leakage.
Worked example
After evacuation to 400 microns and 15 minutes isolated, the gauge reads 1500 microns. Diagnosis: moisture or a leak. Perform a nitrogen leak test, then repeat evacuation with a fresh filter-drier; do not proceed to charge until the hold test passes.
Maintenance, Troubleshooting and Safety
Core concepts
Preventive maintenance includes coil cleaning, filter replacement, belt and bearing checks, refrigerant leak detection, electrical connection tightening, and recording operating pressures and temperatures to trend performance. Troubleshooting follows a logical sequence: confirm the complaint, gather readings, compare with design or baseline, then isolate the cause. Use superheat and subcooling together: high superheat with low suction pressure suggests undercharge or restriction; low superheat with high suction pressure suggests overcharge or a flooded evaporator.
Safety is non-negotiable. Refrigerants can displace oxygen in enclosed spaces; some are flammable (A2L, A3) and some are toxic. Know PPE, lockout/tagout, safe handling of nitrogen and oxygen-free brazing, recovery and recycling requirements, and electrical safety. Never braze on a pressurised or charged system, and never mix refrigerants.
Traps in questions
- Adding refrigerant to fix high superheat without checking for a restriction can overcharge the system.
- Ice on the suction line is not proof of correct operation - it may indicate flooding or a restriction.
- Ignoring compressor current and discharge temperature can miss mechanical faults.
- Venting refrigerant is prohibited; recovery is required.
Rule of thumb
Measure superheat and subcooling together. Normal subcooling at the condenser outlet is roughly 5–8 K; abnormally low subcooling with bubbles suggests undercharge, high subcooling suggests overcharge or a restriction.
Worked example
Suction pressure low, superheat 25 K, subcooling 2 K, sight glass bubbling. Diagnosis: undercharge. Leak-test, repair, evacuate and recharge by weight - not by simply adding gas, which would leave the leak unresolved.