Commissioning a New System With a Suspect Vacuum
You are commissioning a new 8 kW R-32 split system with a 15 m pipe run. After pressure-testing with OFN at the required test pressure and finding no decay, you evacuate with a two-stage pump and a micron gauge at the pump, not at the system. The gauge reads 400 microns after 40 minutes, but when you isolate the pump the reading rises to 1200 microns within five minutes and holds there. The installer is keen to charge and leave. The manufacturer's manual specifies evacuation to below 500 microns with a standing vacuum test.
1. Interpret the vacuum readings and decide whether to charge.
Read the worked answer
The initial 400 microns looks acceptable, but the standing vacuum test fails: on isolating the pump the pressure rises to 1200 microns and holds. That indicates either a leak (nitrogen test may have missed a small leak, or a joint/valve is passing), moisture in the system boiling off, or the gauge being in the wrong location giving a false reading.
I would not charge. Steps:
- Move the micron gauge to the system (far from the pump) to get a true reading - a gauge at the pump can read low while the system is still wet or leaking.
- Repeat the standing vacuum test; a rise that then stabilises can indicate moisture, a continuous rise indicates a leak.
- If moisture is suspected, use a triple evacuation with nitrogen breaks, or replace the drier, and re-evacuate.
- If a leak is suspected, re-pressure-test with OFN and locate it.
Only charge once the system holds below the manufacturer's specified vacuum (e.g. <500 microns) on a proper standing test.
2. Why does the location of the micron gauge matter, and what records should commissioning produce?
Read the worked answer
A micron gauge at the pump measures the pressure at the pump, not in the system. Restrictions (Schrader valves, hoses, driers) mean the system can be at a higher pressure - wetter or leaking - than the pump-side gauge shows. Placing the gauge on the system, isolated from the pump during the standing test, gives the true condition.
Commissioning records:
- Pressure test: medium (OFN), test pressure, duration, result.
- Evacuation: final vacuum achieved and standing test result.
- Refrigerant: type (R-32), charge added by weight, and CO2 equivalent (charge × GWP).
- Pipe run and any adjustments to charge.
- Date, engineer, and equipment details.
R-32 is flammable (A2L), so the charge limit for the room volume and any ventilation/ignition-source requirements must be checked against the standard before charging.