My Taco zone controller shows that one valve is always calling for heat. Both red and yellow lights stay on. Why?

Hello - if you are using a power robbing T-Stat you can test the board by removing 1 wire, either the R or W and if the lights for that 1 zone go out then they it appears to be a t-stat issue. Some power robbing t-stats when not wired with a com or possibly a resistor will try to pull power off the board and in turn this keeps a closure of a switch, allowing the zone valve to stay energized and open. Should you need further assistance please feel free to contact our Customer Support team at 1-877-700-4242 as we'd be more than happy to assist!