A2L Commissioning: Nitrogen, Vacuum and the Shortcut
You are commissioning a replacement split system in a first-floor office measuring 4.5 m × 3.5 m × 2.7 m. The unit is factory-charged with 2.4 kg of R-32 (A2L) and the interconnecting pipework, which you have just brazed, adds no extra charge. Your van carries an oxygen-free nitrogen cylinder and regulator, a two-stage vacuum pump, a digital micron gauge, a recovery machine and a standard manifold gauge set that has been used on R-410A and R-134a. A colleague suggests pressure-testing 'with a squirt of R-32 and the sniffer' to save an hour, and wants to braze one remaining joint after the system has been part-charged.
1. How should the new pipework be pressure tested, and why is your colleague's suggestion unacceptable?
Read the worked answer
Pressure test with oxygen-free nitrogen only, never with refrigerant and never with oxygen.
- Refrigerant must not be used as a test gas: it is a deliberate release of an F-gas, it is illegal, it tells you little about a small leak at test pressure, and it contaminates the circuit if the joint is then opened.
- Oxygen is an absolute prohibition - with POE oil present it creates an explosion and fire risk.
- Use a regulator and a calibrated gauge, raise the pressure in stages to the test pressure specified by the manufacturer, isolate, and observe for a pressure drop. Check every brazed and flared joint with leak-detection fluid or an appropriate detector.
- Remember the factory charge is already in the outdoor unit: follow the manufacturer's procedure to keep the nitrogen test confined to the new pipework and not pressurise the factory-charged section.
On the brazing: never braze a joint on a charged system. The refrigerant must be recovered, the section isolated/purged with nitrogen, and the joint made before any charging.
2. What evacuation standard should you work to, and how would you know the system is dry and tight?
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Evacuate to 500 microns (about 0.67 mbar) or lower, measured at the system with a digital micron gauge - not inferred from a manifold gauge, which cannot resolve that range.
Then do a standing vacuum (decay) test: isolate the pump and watch the gauge. A vacuum that holds steady is acceptable. A rise that plateaus in the low thousands of microns suggests moisture boiling off; a rise that keeps climbing towards atmospheric suggests a leak or a bad connection. Where moisture is suspected, use triple evacuation - break the vacuum with oxygen-free nitrogen between pulls - and change the filter drier as needed.
Never use the system's own compressor to pull the vacuum: it cannot reach the required level, it will not remove moisture, and it risks running the compressor under conditions it was not built for. Pulling a vacuum is also not a substitute for a pressure test - do both.
3. What additional precautions does the R-32 charge require here, including anything specific to this room?
Read the worked answer
R-32 is A2L (lower flammability), so:
- No ignition sources, no smoking or naked flame; do the hot work before charging, on an uncharged purged system.
- Use a flammable-gas detector suitable for A2L refrigerants, not an old halogen sniffer.
- Ventilate the space and avoid trapping gas in the plant area; charge as a liquid, by weight, from a dedicated cylinder.
- Keep the gauge set and hoses clean and free of other oils; label the unit with its A2L marking and refrigerant.
- Check the room: 4.5 × 3.5 × 2.7 = 42.5 m³. A 2.4 kg charge gives about 0.056 kg/m³, roughly 18 % of R-32's lower flammability limit of around 0.306 kg/m³ - reassuring for a whole-room release, but confirm against the minimum room area stated in the installation manual and EN 378, because that is the governing number and it changes for smaller rooms, basements or ceiling voids. If the room falls short, the charge or the installation must be changed, not ignored.