12 m Line Set on a 2.5 HP R-32 Inverter: Charge, Vacuum and Flare Decisions
A two-person crew is commissioning a 2.5 HP inverter split-type air conditioner in a newly built 28 m² dental clinic (ceiling height 2.5 m). The unit uses R-32. The nameplate states a factory charge of 1.10 kg, sufficient for 5 m of 9.52 mm (3/8 in) liquid line, with an additional 30 g per metre beyond 5 m. The actual pipe run is 12 m, with the outdoor unit 5 m above the indoor unit; the manual allows a maximum of 20 m and a maximum elevation of 10 m. The indoor unit was brazed in place; the technician flowed nitrogen on the first joint only, then ran out of nitrogen. During evacuation the manifold gauge sat at −30 inHg for 4 minutes before the technician closed the valves and declared it "good enough". Outdoor ambient is 34 °C, indoor 27 °C dry bulb / 19 °C wet bulb. The clinic opens at 08:00 the next day, and the technician wants to tighten the flare nuts with a pipe wrench "so they definitely don't leak".
1. Compute the total system charge and explain how the crew should introduce and verify it on an inverter unit.
Read the worked answer
Additional charge = (12 − 5) m × 30 g/m = 210 g, so total = 1,100 + 210 = 1,310 g (1.31 kg).
Introduce it by weight on a refrigerant scale, not by watching the gauge. On an inverter unit, suction pressure and superheat move with compressor speed: at part load the unit will look overcharged at one moment and undercharged five minutes later, so neither is a commissioning criterion. Charge from the cylinder in liquid phase through the service port - R-32 is a single-component refrigerant, so fractionation is not a concern, but liquid charging delivers the weighed amount quickly and cleanly.
Then verify at stabilised full-load conditions (15–20 minutes, doors and windows closed, unit forced to rated capacity): check liquid-line subcooling against the commissioning sheet (typically 5–10 K) and the supply/return air temperature split. If the reading is outside the manual's range, recover and re-weigh - do not "top up until it looks right". Record the weighed amount on the unit label and in the commissioning report.
2. Is the 4-minute standing-vacuum test acceptable? Describe the correct evacuation procedure and the consequence of the brazing shortcut.
Read the worked answer
No. A 4-minute hold at −30 inHg proves nothing: a manifold gauge cannot resolve the difference between 500 microns and 5,000 microns, and the system may still be full of moist air.
Correct procedure: confirm both service valves are front-seated so the evacuation covers the indoor unit and line set; connect the pump through a manifold with a micron gauge at the port furthest from the pump; evacuate to 500 microns; isolate the pump and hold 10–15 minutes. A rise to about 1,000 microns that then stabilises is acceptable outgassing; a continued rise means a leak or moisture, and the process restarts. Use triple evacuation with dry-nitrogen breaks if moisture is suspected.
The brazing shortcut matters: joints made without nitrogen flow form copper-oxide scale inside the tubing. That scale travels to the filter-drier and capillary and becomes next month's restriction. Since several joints were made dry, replace or add a filter-drier and flow nitrogen on any rework before charging.
3. The technician plans to torque the flares with a pipe wrench and leak-test with soap for 5 minutes. Assess this and state what should happen before charging.
Read the worked answer
A pipe wrench on a brass flare nut crushes and work-hardens the cone. The joint may hold at first, then weep under vibration and thermal cycling - and soap bubbles on a 5-minute test at low pressure will not reveal it.
Use a torque wrench to the manual's table (commonly 14–18 N·m for 1/4 in, 33–42 N·m for 3/8 in, 50–62 N·m for 1/2 in) with a backing wrench on the unit side. Then pressure-test with dry nitrogen - typically 4.0 MPa for R-32 systems, per the manual - and hold long enough to distinguish a real drop from temperature-driven pressure drift.
Because R-32 is A2L (mildly flammable), keep torches and smoking away and ventilate the room. A useful sanity check: 1.31 kg in a 28 m² × 2.5 m room is about 0.019 kg/m³, far below the ~0.3 kg/m³ A2L concentration limit, so no additional mitigation is needed here - but in a small bedroom the same check would be part of the commissioning decision.