Tech Challenge – March 2013


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A rooftop unit utilizing R-410A with a TXV feed devise has a customer complaint of insufficient cooling. The symptoms confirm inadequate cooling and the compressor appears to be running all of the time. What are the “possible causes” (Note: There is only one problem intended) with the following measured conditions on this field service check sheet?

Field Service Check Sheet

Compressor Discharge Temp. 202 deg. F.
Condensing Press./Temp. 302 psig / 102 deg. F.
Condensing Outlet Temp. 89 deg. F.
Condenser Sub cooling 13 deg. F.
Condenser Split 14 deg. F.
Entering Feed Device Temp. 50 deg. F.
Evaporator Press./Temp. 104 psig / 33 deg. F.
Evaporator Outlet Temp. 68 deg. F.
Evaporator Superheat 35 deg. F.
Compressor Inlet Temp. 78 deg. F.
Total S. H. 45 Deg. F.
Ambient Temp. 88 deg. F.
Room/Box Temp. 78 deg. F.
Compressor Volts 240 V.
Compressor Amp. Draw Low

Share this challenge with your technicians to find out if they can figure out the problem. You can also share your thoughts here by posting them in the comments section below. Check back after May 1 for the correct solution.

And the answer is: 

The problem is a system restriction ahead of the feed device. The temperature entering the feed device is lower (50 deg. F.) than the ambient temperature of 88 deg. F., caused by the restriction boiling of refrigerant (evaporation). The likely problem is a partially blocked filter/drier. The low side of the system is starved and is not picking up heat in the evaporator so the high side conditions are low also with low pressures and splits. The superheat is high causing a high discharge temperature and the condensing subcooling is with-in one degree of ambient and appears to be normal because of the low refrigerant flow.


Posted In: ACCA Now, Tech Challenge

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