New JROTC building at high school; York 480v, 3-phase condensing units with microchannel coils.
Mechanic 'R', assigned to this school has had problems with insufficient cooling in one area in this new building.
When I assisted him in troubleshooting the system we were unable to get proper subcooling (hot liquid line).
Attempting to minimally increase the refrigerant charge only succeeded in spiking the head pressure and tripping the HPS.
We recovered the charge and recharged by weight compensating by the length of the lineset according to York charging instructions.
Again, insufficient subcooling; postulating that there might be a problem with the TXV, we ordered a replacement from RE Michel, as the unit was out of the 1st year warranty.
When we picked up the new valve it was marked, unlike the valve in the AH, with the tonnage of the valve and did not match the size of the AH.
Back at the school, investigation uncovered that the contractor had installed a 3-ton valve on a 5-ton AH and a 5-ton valve on a 3-ton AH.
After correcting this f**kup, the old 3-ton valve wouldn't modulate and we wound up installing the new valve after all (in the correct unit). Problems abated.
Things went swimmingly for the remainder of the school year, but the 3-ton began suffering from insufficient cooling a couple of weeks before school ended; now mechanic 'R' is having problems again related to or corresponding with subcooling (hot liquid line) and mechanic 'C' thinks there is a problem related to the microchannel coil.
I have been doing some online research and have found anecdotes of problems related to York microchannel coils in particular and microchanel coils in general due to the minuscule internal passages of this design.
It is alleged that they are seriously susceptible to blockage due to brazing scale and numerous other installation and service practices.
In conversing with 'R' this A.M., he related that he had a problem with a York RTU at the same school earlier this week; one circuit locked out on head pressure. Long story short; HP over 500; after resetting the unit and holding the contactor in manually to bypass the thermostat time delay, thus defeating the HPS, the HP spike ended abruptly and pressures went back to normal.
I am currently postulating that, since the factory probably flows nitrogen during assembly and brazing, this plausibly eliminates brazing scale as a possible factor?
I am tending to reflect on the possibility of 'oil logging'?
'R' and I are going to experiment on the split system this coming week, first week of school permitting.
Any input of your experiences is much appreciated.