For proper Thermal Expansion Valve (TXV) operation, there needs to be a solid column of liquid refrigerant of feeding the TXV. If there is no subcooled refrigerant, TXV operation maybe erratic and super heat will be high.
To confirm the expansion valve is opening and closing there are a few methods. One of the easiest is to “exercise” the TXV.
The first thing that needs to be done is an inspection of the TXV. Ensure that the sensing bulb is securely attached the suction line and the bulb is insulated. Make sure there are no kinks or breaks on the sensing bulb tube or equalization tube. Look for frosting before the body of the expansion valve, this could be a sign of the screen before the TXV is restricted.
SAFETY FIRST! Be aware you are working around moving part and with refrigerant.
To start, baseline readings of the refrigerant pressures, superheat and subcooling needs to be recorded. Once these baselines have been recorded, it will be necessary to remove the TXV sensing bulb from the suction line. Place the bulb into hot water. This should cause the expansion valve to open allowing more refrigerant to pass through the TXV into the coil. Suction pressure should rise, and super heat should drop. Opening of the valve should be relatively quick. Even holding he bulb in your hand should provide enough heat to open the valve.
Next, place the TXV bulb into cold water. This should cause the expansion valve to close reducing the amount of refrigerant to pass thought the TXV into the coil. Suction pressure should drop, and superheat should rise.
If TXV does not respond to the cold or hot water, keep the bulb in the water for a minimum of one minute at a time. This may force the expansion valve to open or close accordingly. Continue to switch the bulb from the hot water to the cold water. You may be able to free up the TXV by doing this. If the TXV is non-responsive the TXV will need to be replaced.
In some instances where the TXV is stuck, a lubricant additive like A/C ReNew may be added to the refrigerant system to help free the valve. Once the additive is added, it is still necessary to exercise the TXV by placing the bulb into hot and cold water several times. Improvement may be seen in only a couple hot and cold-water cycles but may take up to 48 hours of normal run time of the air-conditioning system. Check with manufacture before added any additives to the refrigerant system to make sure warranty will not be voided.