Diagnosing Faulty TXV
I was working on a walk-in cooler today with the journeyman that I am apprenticing under. The unit was not keeping temperature. The refrigerant was 134a. The unit had a TXV and a reciever. The sight glass was showing a full head of liquid. Suction 18, head 110. 45 degrees superheat at the compressor. The coil was clear and both evap fans were operating properly.
We attempted to adjust the TXV. It was a 1/3 ton Danfoss TXV. We spent about 2-3 hours trying to adjust it properly. It would temporarily begin to feed better as indicated by suction pressure and SH, but SH would then again rise suction pressure would drop...I believe that we were allowing adequate time for things to settle in on each adjustment (5-10mins) but it just would not seem to stay consistent.
In the end, we ended up changing out the TXV and this fixed the problem.
This took a long time to diagnose and then more time to change. I am wonderning if you would share some experience with me in regards to diagnosing TXV's? Because I don't know how to PROVE a faulty TXV in a time efficient manner. The entire time that we were tweaking the original TXV, I suspected that it was the problem, but I didn't know how to prove it. Even in the end, we just sort of conceded to the fact that "oh well, I guess it must be a bad TX valve". And I guess I'm just asking you guys if I can do better than that on my diagnosis?
I 2nd that question if there is a way to speed it up. Only thing I've learned is refrigerant work is slow.
5-10 minutes really probably wasn't adequate unless the TXV was external to the unit cause with opening and closing a door and you guys being in there the load probably hadn't stabalized. Anyway just my opinion/take on the matter. Read my post at
as bulb location and position become far more critical in refrigeration applications (med and low temp) vs. a/c applications (high temp). Really read the the installation instructions and become familair how a TXV works and why it works the way it does.
TXV's can be a pain - a blown powerhead due to a leak in at the cap. tube or bulb is easier, but a restriction at txv can really be a pain. More often than not I find TXV's that are not installed correctly and that is the only problem. One other thing is make sure the bulb is insulated (as stated in the installation instructions.
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First thing with TXV's...
Clean the sensor bulb and contact surface. It's like magic.
I have had txv wax up or gum up on inside and to check this i poor hot water over valve. AlsoManufacturers recommend 15m minimum between adjustments.
Nice trick with hot H2O, bama! ...but why is there wax in the system? No, I know, I know...that's why I started flaring and valving my driers.
But what is 15m txv adjustments? Do you mean 15 degrees? I'm going to assume so...but when adjusting I always just start with one full turn in one direction or another, just to see if anything at all is going to happen anyway.
Remember; cost is the only possible reason why txv's aren't on every possible system.
And also remember; there's a spot especially for you in hell if you expose the system and don't install a drier.
15 minutes settle in time on TXV adjustments, also if your valve is hunting (suction pressure goes way up then down then up again and down) odds are it's either a faulty power head or you're overcharged. Power heads are easier to change than a complete TXV. Clear sight glass really doesn't mean much of anything, should always check the subcooling.
Don't let your ego get in the way of a good decision
you need to be looking at subcooling and superheat to correctly adjust txvs. refrigeration systems usually will need no more than 2 to 5 degrees subcooling. subcooler rows are not added to refrigeration condensers.
This is a very helpful PDF file to read. Good info about TXVs!
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