I think that the primary difficulty with all this, is that Tom Richardson (or the powers that be) had insisted that they needed a 6,000btuh capacity condensing unit. I think it is obvious (since the 3/4 horse is now in) that the actual required capacity is nearer to 3,500btuh @ 20degF SST. But all of us at this forumn assumed that there was some science behind the 6K figure... NOT!
The High head disappeared because the three quarter got put in. NOT becuase the 3/4 is a larger capacity unit, but because it is rated at a max. evap temp of 30F. the 1/2 horse was rated at a max evap temp of 25F.. which is why it couldn't handle the pulldown at a 35F evap temp.
I am beginning to think that a proper 1/2 horse of a true medium temp rating (the one they are using is rated med/low) might do the job, meaning perhaps a unit with a max evap temp rating of at least 30F... 35F would be preferrable, and would help assure long and prosperous compressor life (this is NOT a low temp application). If my assumptions are correct regarding the "real" capacity requirement, I might recommend a:
FTAH-A050 R-134A whose max evap temp is rated 45F (you'll never overload that sucker.. 3,300btuh @ 20F SST
or
F3AH-A050 R-22 whose max evap temp is 45F and 3,570btuh @ 20F SST
Also, the massive Bohn 430 coil that is in that box seems inapprpriate. Do we need all that CFM produced by it? A Bohn TL-21 or any evap near a 200btuh/degFT T.D. would seem appropriate. but if the airflow is a requirement, it is not an issue (it's more expensive), and the result is merely a low TD (ahem, higher SST referencing failure of 1/2 horse that cannot stand SST over 25F)
Of course that is assuming that the current 3/4 hp is as unloaded as it appears to be.
I don't think anyone in Tom Richardson's presence on this project is able to set the TEV (superheat), much less know what superheat actually is. Maybe he needs some direct instructions in how to perform the task (PS: always put the cap back onto the stem when monitoring the effects of an adjustment :))
But I swear!!!! If someone doesn't reset that d*mn cold control to a 24deg cut-out, I'm going to hop a plane, and go do it myself!!!!
Tom, The refrigerant that is boiling in the evaporator is approximately 19degrees, according to the information that you have provided. The coil must always be colder than the temperature of the space that you are cooling. Typically a refrigeration system is designed for the coil to be 10 degrees colder than the design space temperature. At a 35F space, the coil will be 25F. Because of system design T.D. (Temperature Difference between coil and refrigerated space) we can control space temperature by monitoring coil temperature (which is 10 degrees colder) and insure coil deforst if things go awry and the space is not at design, but the coil gets too cold (freezes).
Got it? ;)