2 Ton AC + AHU coil passes pressure test, lost all charge after running for ~72 hours. Pressure tested again for a period of 3 days, no issues. What happened to the charge?

I have a background of nearly a decade doing mechanical HVAC design for large commercial buildings. I welcome your technical response.

My residential system went to shit and was recently fixed. The building is a city apartment with individual compressors on the roof and AHUs in utility closets that are adjacent to the apartment. The coil in the AHU was replaced and a new 2 ton unit was installed on the roof. The tech pressure tested the system with nitrogen using a fixed gauge (so no hookup losses). Over a period of 24 hours it held pressure. The system was charged and fired up, this was on Thursday afternoon. On Sunday the system was working fine until the afternoon, when it went out over a period of about an hour or less. I noticed the apartment was heating up, so I checked the AHU coil, and sure enough the liquid and suction line were ambient while the system was running. Dammit.

I called the guy back Monday morning and he came out. The system was completely empty, zero pressure. The two of us have been trying to figure out wtf happened. Over the last few days he has pressure tested the system multiple times, checking it with refrigerant and nitrogen, leak checked, etc. Today, it was recharged, and it's blowing cold once again.

This may or may not be related, but on the first charge the first discharge vent in my apartment was so cold that the vent was condensing water. Today, a week later and on the second charge, it is blowing cold air but there is no water condensing. It is not a humidity difference as my gauge shows it is consistently 50% +/- 5% RH.

So my question to you folks: WTF happened, why did the system lose its entire charge?

submitted by /u/bennytehcat
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