New install: crew brazed the lineset without nitrogen, pulled a vacuum to 500 microns, charged by weight, and it ran great.

What happened:

The system has insufficient refrigerant charge or refrigerant leak. Here's why:

1. No nitrogen during brazing = oxidation scale/debris inside lines
2. Frosting back to compressor = liquid refrigerant returning (floodback)
3. 45 psi + coil icing = low charge causing pressure drop and temperature drop

The most likely scenario: refrigerant leak developed after initial charge, OR debris from improper brazing is restricting metering device flow, causing liquid floodback and subsequent icing.

The fix:

1. Recover refrigerant and check actual charge vs. nameplate
2. Install a filter/drier (if not present) - replace existing one
3. Clean/flush lineset to remove brazing debris
4. Leak check entire system with nitrogen pressure test (minimum 200 psi)
5. Evacuate properly with nitrogen purge, then vacuum to 500 microns or lower
6. Recharge by weight to manufacturer specs
7. Verify superheat/subcooling at design conditions

Critical code note: Per NFPA 99 and ASHRAE 15, systems must be installed with proper evacuation and charging procedures. The lack of nitrogen during brazing violates best practices and likely caused internal contamination.

If this is a hermetic system, you may need to replace the receiver drier or accumulator as well.

Bottom line: Debris from improper brazing is likely causing metering device malfunction, leading to floodback and icing. Clean the system, install proper filtration, and verify charge.

What happened: Brazing without nitrogen created heavy copper oxide scale inside the lineset. The 500-micron vacuum removed air/moisture but cannot remove solid oxide flakes. Refrigerant flow has since swept those flakes into the TXV orifice or distributor screen, creating a partial restriction. The valve now underfeeds the coil → superheat drops → suction line frosts → evaporator ices → suction pressure collapses to 45 psi.

Proof:

• Frost back to the compressor = liquid floodback from an iced coil, not a charge issue.
• Charge-by-weight was correct at startup; no leak reported.
• Time delay (3 weeks) matches debris migration.

Fix (requires recovery & nitrogen):

1. Recover charge.
2. Remove TXV power head; pull valve body or at least the inlet screen. You will find black/green oxide sludge. Clean screen/orifice with solvent and compressed air.
3. If the TXV body is clean but restriction persists, the distributor tubes or evaporator header are plugged—pull the distributor nozzle and blow out each circuit with nitrogen.
4. Reassemble, pressure-test with nitrogen (300 psi), pull < 250 microns vacuum, recharge by weight.
5. Verify: 10–12 °F target superheat, steady suction pressure, no frost past the evaporator outlet.

Code/Standard:

• IMC 1107.2 / IRC M1411.3 – linesets must be purged with nitrogen during brazing.
• AHRI Guideline K – vacuum ≤ 250 microns for POE systems.

Bottom line: Crew skipped nitrogen. You’re now paying for it with a teardown. Bill the callback to the installer.