See Vlad, I knew Ray would be able to explain it a little better. The “DC offset,” can occur if you have 2 ground points at differing potentials or more basically ‘voltages.’ At any ground point in the system you can use a basic multimeter or voltmeter to measure across to another ground point and you should read 0 volts, if not this can ‘latch’ the triac and hold the valve open. Like Ray said, divide and conquer… isolate the issue, find a potential cause, fix it, then reintegrate the portion you isolated and test again.