The Triac (V) outputs on the TAC Xenta 120-FC device goes ON, but never OFF. In this case, it can open the valve, but the device could not close the valve.
TAC Xenta 121-FC
TAC Xenta 121-HP
If the device is powered with 24 VDC, the Triac will not work properly.
According to the datasheet, the TAC Xenta 121-device should always be powered by AC, not DC.
Triac is an acronym for Triode for Alternating Current, and could only be working with AC power supply.
Correct the power supply to the device to use AC, not DC.