Sunday, September 21, 2014

Thermostatic Expansion Valve Failures

There have been a run of TXV problems in new units stemming from a chemical used as a rust inhibitor on some of the compressor bearings in Copeland Scrolls. A reaction between the rust inhibitor and the POE oil used in the compressor causes the POE to coagulate and collect in the TXV. This appears to be an industry wide problem, not limited to a single equipment manufacturer. The chemical causes the valves to get gunked up and stick, usually resulting in a restriction at the TXV. The symptoms can look very similar to an undercharged system – very low suction pressure, low head pressure, and high superheat. Normally, the way I differentiate low charge from a restriction is by subcooling. A restriction typically has a normal to high subcooling. However, this is often not the case if the compressor in the system is a newer Copeland scroll. The Copeland scroll compressors designed for R-410A unload when the suction pressure drops below 55 psig. This is to keep them from operating at pressures which will damage the compressor. If the restriction is severe enough, the suction pressure on an R-410a system can drop below 55 psig. The compressor unloads, limiting how much refrigerant is pumped and holding down the head pressure.  You have low suction pressure, low head pressure, high superheat, and low subcooling: classic undercharge symptoms. However, adding charge does nothing unless you really go whole hog and grossly overcharge the system. Then you can build some head pressure, but it won’t fix the restriction or make the system cool. If the compressor is unloading, you can often hear it and even feel it in the piping. If you have what looks like an undercharged system with no detectable leak and adding refrigerant has no effect on the pressures, I bet you have a restriction. These days the restriction is often in the TXV. Several equipment manufacturers have service bulletins out regarding this issue. Below are links to a bulletin from Nordyne and a link to a YouTube video showing the bulleting from Emerson.

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