(The content of this post is intended for consideration by trained service personnel only)
The illustration below is a fairly typical cross section of a thermostatic expansion valve.
..The valves open or close, as a result of the pressures P1, P2 and P3. P1 is the sensing bulb pressure, P2 the evaporator (suction) pressure and P3 the spring pressure. The sensing bulb pressure varies with sensing bulb temperature, which is, more or less, the suction line temperature. If the suction line warms, the sensing bulb warms, the refrigerant inside the bulb warms and expands, so the pressure increases…vice versa for when the suction line cools.
The spring and suction pressures have a closing effect on the valve. The bulb pressure has an opening effect on the valve. When a system is at steady state operation, the refrigerant flow is constant, so P1=P2+P3. When the heat load on the evaporator coil changes, the rate of refrigerant evaporation or boiling inside the coil, will change. This affects both the evaporator (suction) pressure and suction line (sensing bulb) temperature, so P1 isn’t equal to P2+P3. At that point the valve will open or close a little, trying to get the pressure formula back to an equality…
Whether or not we can fully comprehend all that is pretty much academic, relative to diagnostics. You just need to remember the purpose of expansion valves in the first place: to control or maintain a relatively constant system superheat…
So, if you’re trying to troubleshoot a valve, all you really have to do is decide whether or not it’s doing what it’s supposed to do…to do that, you need to measure the system vital signs: low pressure, high pressure, superheat and subcooling. With those four values, you can diagnose a problem with the TXV or something else.
But first of all, if you measure the superheat and get, say 10-15 degrees, regardless the other numbers, the TXV isn’t faulty, because it’s doing what it was designed to do…
If you measure 30 degrees superheat or 2 degrees superheat, there’s a problem somewhere that could be due to a faulty valve. You have to analyze the other system vital signs before deciding the valve is suspect.
You can see a more in depth explanation of TXV operation and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” and “Troubleshooting TXV’s” rental videos:
Troubleshooting Heat Pump Refrigerant Systems
(The content of this post is intended for consideration by trained service personnel only)
WayneShirley.com 