I have a Lennox HS22-461 condensing unit coupled with a C16-4655 coil using a LB53081 expansion valve and a L10-65-50 line set. The system was installed in 1990 in a home in Columbia, SC (hot and humid summers!) While we are preparing to do a major renovation on the home and this system may get replaced I have developed a problem that I would like to resolve. The unit starts and runs OK and does provide some cooling. With our recent 100+ temps the compressor, after running for a while, will suddenly make a knocking racket and either restart immediately or the stall and the internal protection trips and then resets to restart. I started troubleshooting and discovered several things. First, the plastic cap holding high temperature thermostat on the top of the Copeland Scroll had broken and come off and the thermostat was loose. Second, the contactor coil was making a loud buzzing noise and the contacts were seriously eroded. Not finding a way to buy a replacement plastic cap I used high temp RTV to hold it and the thermostat in place. I replaced the contactor. Using two voltmeters I monitored the 24v contactor coil voltage and the 240v supply to the compressor. I restarted the system and after about 20 minutes the compressor made the loud noise but immediately restarted. The power to the compressor was never interrupted. I let it continue to run and after another 5 minutes it happened again but this time the compressor tripped out on overload and I opened the disconnect. Again the power to the compressor was never interrupted.
I then got out my gauges and thermometers to check the charge (R22). With an ambient dry bulb of 88 and indoor dry bulb of 85 (I don't have a psychrometer but the RH was probably 60-65%) I ran the unit for 15 minutes and made the following readings. Low side 79 psig, high side 243 psig, high side cond discharge temp 90 degrees, suction temp 71 degrees and compressor outlet temp 170 degrees. ( I did not have my clamp-on ammeter with me so couldn't measure the compressor current. By now the ambient had cooled off compared to my earlier troubleshooting and the compressor did not act up during the 30 minute period I took these readings.) By my calculations this results in about 25 degrees subcooling and 25 degrees superheat. Based on the subcooling I expect I am low on refrigerant but here are my questions.
What is the proper subcooling for this system?
With this coil and line set does anyone know the proper charge of R22?
From my research this compressor (ZR40K1-PFV-230) has some sort of internal pressure/temperature relief valve to protect the compressor. Could it be this protection actuating that is causing the compressor to suddenly make a racket and sometimes trip the internal electric overload if it doesn't restart immediately? Could a low charge condition cause this to happen on a really hot day?
Any other ideas or suggestions would be appreciated.
One last question. When I do replace this condensing unit I would like to retain the current coil because of the way it is integrated with the relatively new condensing gas furnace. Is this advisable and if so, any suggestions on a 40,000 Btuh capacity unit to use? In recent years I have become partial to Trane equipment but this is not a must.