Test before anything is closed up
Pressure testing happens before the pipework is plastered over, before shafts are closed and before ceilings go up. Get the order wrong and every leak found means demolition. The pressure must hold for the full duration and that duration must be recorded — "we looked and it wasn't dripping" is not a test record.
Temperature affects pressure. A test started in the morning reads differently in the afternoon even with no leak. That is why ambient temperature goes on the record too; otherwise a normal fluctuation gets read as a leak.
Commissioning is the proof the system performs as designed
Pipework can be sound and still run wrong: the pump on the wrong speed, a valve throttled, controls left at factory settings, no balancing done. Commissioning is where those settings are made and the measured values compared against the design.
A building declared "working" without measurements will show problems at the first heavy use or the first change of season. By then it is no longer easy to tell whether the fault is in the pipework or in the settings.
The handover file is part of the work
When test and commissioning figures are not written down, the building loses its history. Five years later, the first thing to look at when investigating a fault is what values the system started life with.
The handover file should hold the test records, the commissioning measurements, the as-built drawings, the equipment manuals and the warranties. This file cannot be assembled afterwards — try, and half of it will be missing.
Which fluid to test with
Testing with water is far safer than testing with air. Water does not compress, so when a joint lets go the pressure drops instantly. Compressed air stores energy, and the same failure becomes an explosive one.
Air or nitrogen is still used where there is a freezing risk, or where the line must stay dry. In that case the test pressure is kept low, the area is cordoned off, and nobody stands near the line while it is under test.