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TXV Valve Diagnosed Inoperative - AC Still Cools Well

Posted: Fri Jul 01, 2011 9:56 pm
by gstaley
Hi I had a technician come out today after my AC line iced up yesterday. He checked a few things then did a pressure check - Too Low and Too High - however after sweltering in 100 degree heat for the day, i figured could it hurt to just try it for a few minutes (yes risking my compressor) however to my suprise the AC Pumped out cold air which cooled the house from about 85 to 70 in the span of 15 minutes - does that sound like the TXV Valve might not be damaged - line is not frosting up yet.

Thanks - Gary

- TXV Valve Diagnosed Inoperative - AC Still Cools Well

Posted: Sat Jul 02, 2011 8:55 am
by nomadpeo
a txv (thermostatic expansion valve) is a modulating valve with a thermal feedback sensor that controls the volume of refrigerant flow into the evaporator. the two points in the refrigerant loop where a restriction can occur are the txv and the liquid line filter/drier. either one can become restricted from excessive debris or water in the system. the filter/drier is there to catch debris and water and not allow it to reach the valve. its size determines the capacity of water it can hold and still allow refrigerant flow. a partial restriction at either of these two points will cause low suction pressure and high discharge pressure. when the suction pressure drops below a pressure corresponding to 32 degrees, condensation on the low side freezes on the uninsulated portion of the pipe and the surface of the evaporator. once turned off, the ice melts. turning it back on could dislodge the restriction and it may work normally. if the restriction was caused by ice crystals forming in the valve, that could explain why it works now. the problem may present itself again. it is possible the valve is the culprit, but i doubt it. the filter/drier itself can also become partially restricted and produce the same symptoms, but typically would have to be replaced before the problem goes away.

when the technician sees the system started working again, he should realize the valve may not be the problem. if the condensing unit has a liquid line service valve, he will probably pump down the system (temporarily storing the refrigerant in the condenser coil), replace the liquid line filter, pull a deep vacuum and return the system to service. this will rule out restriction in the filter and ensure water and trash gets trapped there in the future. then if the valve fails again, the other variables would be gone.

look at the smaller of the two lines (outside). there should be a filter. it could be located on the outside of the unit or it may be inside the unit. there should not be two. if it is on the outside and you can get your hands on a strap on thermometer, you can measure the temperature of the line before and after the filter while the system is running. if there is any difference, it is partially restricted. if there is not a filter, then the valve is probably being affected by a floating restriction of either debris or water.