TXV Troubles

The expansion valve can have several problems that can look like refrigerant charge problems. This makes diagnosing an expansion valve a little tricky.
Before you dive into trouble-shooting the TXV, make sure you don’t have conditions the valve is just re-acting to.
·        Un-insulated sensing bulb.
·        Loose sensing bulb.
·        Incorrectly mounted sensing bulb. Usually should be in the 2 or 10 o’clock position.
·        Power head is blown. Check this by removing the bulb and warm it in your hands while watching the suction pressure, then return it to the line. You should see the suction pressure change as the valve tries to keep up.
·        While you’re looking at the power head, make sure it’s the correct one for the refrigerant in the system.
If you have verified the above the above to be correct, you have eliminated many problems that can occur with a TXV valve. The remaining problems, flooding, starving, and hunting only have a few causes.
Flooding or refrigerant over-feed. If you’re sensing bulb is mounted right and working, then there are only three reasons the TXV will over-feed the coil. The valve is stuck open, the valve is too big, or the valve is adjusted for too low of a superheat.
If the valve is stuck open, the liquid flood back conditions will not respond to any adjustment, charge or setting, superheat will remain essentially zero.
If the valve is too large, there’s usually a load condition that it will work okay, then trips the oil switch at night or low load. The valve will also try to change with the evaporator load, fluctuating the suction pressure, this is called “hunting”.
Superheat setting.
A TXV comes factory set to some value, I’ve seen 10 degrees on about everyone I’ve picked up for A/C service. That setting is particular to the construction of the valve and power assembly. The setting is independent of the system. So, unless you, or someone else, tinkered with the setting it should be correct.
Note: when installing a new TXV, put a dab of red nail polish where the cap for the adjust meets the body of the valve, that way, you’ll be able to tell if it’s been tampered with.
If the setting is not correct, an adjustment of no more than one turn at a time, with a waiting period of 15 minutes, should be attempted. Do Not force the screw, there is an end of travel in most valves that is easily broken off, you pass it, and the internal wheel falls off the thread, its game over. Caution- if you are working on a valve that runs below freezing -10 or less, the brass may ice seize and you will twist off the adjustment. If it’s hard to turn, something is wrong.
Note on fixed metering devices, only times they over-feed is from over-charge, or, unusually high head pressure.
Starving the evaporator.
Stuck, wrong valve, set wrong, or inlet is plugged. If it’s your TXV valve causing the problem. (Assuming you checked the power assembly and/or the liquid line filter)
If the valve is stuck closed, partially or completely, no changes are going to make that high suction superheat go away. You’ll also have low compressor power, compressor runs hot, and a high supply temperature 60-70 F.
If the valve is too small, there may be times, possibly at night, the system keeps up. You can drop the blower speed a couple steps, and if the system begins too cool at a moderate load, the valve might be too small or stuck.
Once again, unless you know the setting has been tampered with, it’s a last resort.
Plugged? most TXV valves come with an inlet strainer, poor installation/service practices see too it these plug off. You’ll may only know this for sure once you’ve pulled the valve out and looked in the strainer.
Note on fixed metering devices, low head pressure will starve the evaporator.
Hunting. Hunting shows up as a fluctuating suction pressure is caused by either a problem with the sensing bulb, or, the valve is too big for the system and overshoots the process needs continuously. If the valve is too big, you’ll have flood back conditions, if it’s a sensing problem, everything will be moderately okay, just low superheat and poor cooling performance.