Just to update this thread, enquired with a couple of those aircon installers offerign their services on Shopee Lazada etc. Reviews are full of praise from customers, but guess what? all are talking about neat wiring, piping, kept the place clean while install fast service etc.

When I ask them "do you pull vacuum before releasing the refrigerant", they all say no!

I wonder then...how are these guys purging air and moisture from the lineset? do they even purge? at least release some air from the fittings la....
these guys charge about 300 for a 1.5hp install... so I guess if want a professional one that vacuum the lineset its going to be 500?

Release gas at the fitting?

I mean after connecting outdoor unit to indoor unit but before releasing the prefilled refrigerantt in the compressor into the line set,  they purge the air by charging the lineset only with some gas and then loosen the nut fitting slightly to push out the air in the lineset). Or is this step not even done?

  In the service manual they are supposed to vacuum the line set before opening the service valves that will release the prefilled gas. They will always be some air in the lineset since they are open to the atmosphere before connecting the outdoor and indoor unit.
You seem like a tech in hvac? If so can tell me why most tech here seem to not even vacuum the lineset and don’t have issues with moisture and air in the system after a while?