Hey guys, after a few months, the refrigerant in our mini split unit reduces and the air gets warm. From what I understand, the only cause of this is a leak. We've had 3 different AC repair guys come, they do a routine clean and add refrigerant for around ฿500. Each time we ask them to check for a leak and each time, they plug in a pressure gauge, confirm the refrigerant is low, take a cursory look at the copper tubing, then inform me that there isn't a leak. The last one even charged us ฿200 for this 2 minute facade assessment. They aren't charging the system to 100+PSI and measuring input and return pressure over time. They are not spraying the tubing with soapy water. They claim that a dirty filter, blocked exhaust, etc., is the culprit, but I understand these wouldn't typically cause a drop in refrigerant pressure.
So my questions are these: Is my (admittedly limited) understanding about HVAC misinterpreting the situation(s)? Could they deliberately be facilitating a slow leak for repeat business? Or are they lazy and/or incompetent (3 for 3 though, surely this is unlikely)?
And finally, anyone ever test for leaks themselves? If so, any areas I should check first other than the brazed joints throughout the line? General estimate on cost of tools? Can I just use a friend's air compressor and spare gauges, or will I need nitrogen specifically? I'd probably want an empty refrigerant tank to collect the refrigerant, then a full one to replace the refrigerant in the purged system right? Thanks, appreciate any advice anyone may have!
The unit in question is probably about 4-5 years old and is in use every day. At this use rate, it needs servicing every 6 months(ish). This is an ongoing issue with all of our AC units depending on frequency of use since they were first installed.