Lennox HS22 condensing unit

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.

Thanks,
Cleverpig

UPDATE
I received no replies to my original post but here is an update.

My HVAC service technician found the compressor run capacitor to be weak and replaced it. The compressor stall has not recurred but it hasn't been as hot as it was when the problem occurred. Hopefully that has resolved the problem.

The technician did check the subcooling and superheat and found them to be the same as I had measured - both 25 degrees F. He thought the 25 F subcooling was probably OK so did not add any R22. I would still like to have any input on the appropriate subcooling for this system.

Thanks in advance for any assistance.

Quote:

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Originally Posted by cleverpig

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?

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I know some Lennox do run a little higher SH, but to me that SH and SC is to high, does it have a TXV or Piston metering device?

Mr. Bill,
Thanks for your reply. This system uses an expansion valve. The first sentence of my original post gives the model numbers of all of the system components including the expansion valve.

Lennox refrigerant charge is adjusted using "approach" temperature, instructions are on the inside of the unit door.

i would not reuse a c16 coil for anything but scrap, they were the last model with metal drain pans and are starting to fail.

t527ed,
Unfortunately there are no instructions on the inside of the unit door. They may have been there at one time but aren't there now. Is there somewhere else I can learn about adjusting the charge using approach temperature?

Yes, the c16 coil has a metal drain pan. This one is currently in tact but I can imagine it could rust through at any time. I expect I will need a new coil to get an optimum match with a new condenser using one of the new refrigerants.

Thanks for your help.

I would charge it up to 10 degrees of sub cool and check the super heat

google Lennox approach, yours should be 5 degrees

Thanks for all the help!

I found the HS22 installation manual on the Lennox website. It provides the approach method procedure along with an approach temperature of 5 +/-1 degrees F for this unit. I need to retake my readings because my previous readings result in an approach temperature of only 1-2 degrees. If this were true and I understand the procedure correctly I would need to recover refrigerant to increase the approach temperature to 5 degrees. I would not expect this old system to be overcharged but I guess it is possible. It has been 8 years since it was last charged.