# RTU Refrigeration Problem



## evergreen (Sep 15, 2015)

I am a service manager for a company in Seattle, WA. We have been having issues with an RTU (Carrier 48SD-036C90311) and these issues aren't adding up. Any ideas you all may have would be appreciated. Here is the history:

Jul 2015: Replaced failed compressor. After replacing found bad aftermarket TXV which was replaced. System worked fine for a few days after. A different technician went out and said the system was low on charge but could not find any leaks. We recharged and the system worked again for a few days. Sent original tech back out and determined the new TXV had failed so we replaced that. Unit acted as if it still had a restriction. We flushed the unit with ProFlush, pressurized, evacuated, etc... and recharged with new R22. System worked fine for a couple of days. 

Aug 2015: We replaced evaporator coil. System worked fine for a couple of days. Returned and removed aftermarket TXV and installed OEM piston. System worked fine for a couple of weeks.

Sept 2015: Returned to find odd pressures that indicated it was low on R22. Weighed out and it still had the same factory charge that we had put in to it. Performed leak search, evacuation, and recharge with nothing out of ordinary noted. Worked for a few days and stopped maintaining temp again.

I went out today, 9/15/15, with my technician to get my eyes on it. We treated it as if we had never worked on it before. The conditions weren't ideal for running AC checks but this is what we had on our arrival:

121 head / 52 suction
55 outdoor ambient
66 return air
44 supply air
21 superheat
The pressures were right on with the factory charts but the superheat was slightly low. Slight frosting on the upper portion of slab coil.

We added .7 lbs of R22. No changes noted. Weighed out the charge and recovered 7.25 lbs. Factory charge is 7.2 lbs which means we had lost a little bit since out there last. We had removed a T-fitting previously thinking that created more possibility of a leak. We knew the valve was leaking but it didn't appear to be leaking out of the cap.

We devised a way to get a filter drier in (limited space) and fixed the schrader leak. We pressurized to 400psi and held for 30 minutes. We soap bubbled every field and factory joint with no signs of leaks. Evacuated to 350 micron and it held. Recharged with 7.2 lbs R22 and inserted AC Leak Seal. These are corrected numbers:

131 head / 57 suction
64 outdoor ambient
64 return air
41 supply air
14 superheat

Again pressures right on with factory charts but low superheat. No frosting of the coils. After the unit ran for about 40 minutes we started to see the temp on the suction line move up and down. It was as if the system had a TXV that was hunting for the right position but this is fixed orifice. The suction line entering the compressor seemed to be jumping around 45-51 degrees further away from the compressor and jumping around 33-38 at the compressor. This doesn't make sense to me.

Does anyone have any thoughts on what could be going on here?


----------



## beenthere (May 30, 2009)

Probably moving oil through the system from opening/recovering the system so many times.


----------



## heatingrepairchicago (Nov 8, 2014)

try putting a vacuum on it and then recharging. leave it on there for awhile.


----------



## heatingrepairchicago (Nov 8, 2014)

I need a self sufficient tech in Chicago. Must have own truck and tools. Call 3124870416


----------



## rdjr1964 (Jan 29, 2012)

I had an issue one time with a heat pump that would eventually trip the head pressure switch, traced it back to a supply duct that was to close to the return. Just a thought but maybe you have a similar situation. Doesn't hurt to double check the duct work.


----------

