Field Rules · Core Field Rules
The Field Rules
The cross-cutting rules that apply across the whole trade — from evidence, equipment integrity, temperature, isolation, pressure paths, to oil-return balance.
These are the rules experienced technicians carry from job to job. Each one exists because ignoring it has cost a callback — or worse. They are cross-referenced from the technical chapters where the detail lives.
Diagnose from evidence
Don't troubleshoot from habit. Troubleshoot from evidence. A tripped safety is information, not merely something to reset. A complaint is a clue, not a diagnosis.
- Complaint → Observation → Logical start → Test → Prove → Follow evidence → Root cause → Verify
Clean up your leaks & "Big Blue"
Refrigerant leaks carry oil; oil holds dirt. An oil-stained area is evidence of history, not the exact leak point — oil migrates along pipework, fittings and insulation. Clean the area, inspect, leak-test, identify the actual point, repair, retest, clean again, reinspect for fresh oil.
- A dirty leak is evidence of history. A clean joint gives you evidence of what is happening now.
- The dirtiest spot is not automatically the leak.
Test equipment integrity
Gauges, manifolds, hoses, adaptors, Schrader depressors and service fittings are all leak points. Prove your equipment is sound before you trust a result. Don't create a leak while looking for a leak.
- If you haven't proven your test equipment is sound, you haven't proven your test result.
- Inspect access points before introducing test equipment, and recheck them after reinstating.
Nitrogen and temperature
Nitrogen is inert, but its pressure changes with temperature. A lower gauge reading does not automatically prove a leak. Record pressure and temperature, allow stabilisation, and interpret against temperature before chasing a leak.
- Nitrogen doesn't leak just because the gauge moved. Check the temperature.
- There is no universal allowable pressure drop.
Ghost leaks
Don't chase a ghost leak. Prove where the pressure is going — temperature, equipment, access points, isolation, stabilisation, then actual leakage. A gauge reading tells you what happened, not why.
Ball valves and isolation
A closed ball valve is an assumption, not proof of a pressure boundary. A valve can pass internally. Know your test boundary before you introduce nitrogen, and confirm what is on each side of every valve.
- A closed valve is an isolation point — not proof that everything beyond it is protected.
- A closed ball valve is an assumption. A proven isolation is evidence.
Test pressure — time and place
Higher pressure is not automatically better. Choose the test pressure for the job — the boundary, the components inside it, and the purpose. Never use 100 psi or 600 psi as a universal setting.
Evacuation and the micron gauge
A micron gauge tells you what is happening, not why. Refrigerant boil-off, retained oil, moisture and temperature all move the gauge. Tell them apart before you call it a leak — and it is not "refrigerant glide".
Pressure paths
A pressure switch can only respond to the pressure it can actually see. Trace and prove the pressure path; don't just ask whether the discharge is open. Never set controls from memory.
Oil return balance
Balance oil-carrying velocity against pressure drop and capacity. Smaller pipe is not always better. An oil separator does not fix poor piping. Floodback is not poor oil return — diagnose them separately.
Field Rules
- 01
"Don't troubleshoot from habit. Troubleshoot from evidence."
- 02
"Don't create a leak while looking for a leak."
- 03
"A gauge reading tells you what happened. It doesn't tell you why."
- 04
"A closed ball valve is an assumption. A proven isolation is evidence."
- 05
"Never set controls from memory."
