Troubleshooting a Taco Zone Valve: Checking the Voltages
Channel: jimbo88
Date: 2011-12-02

Some background: I have a forced hot water system with five zones. One of the zones wasn't working and I had to debug the system without having much knowledge of plumbing & heating. I took the wires from a working zone valve and put them on the non-working valve and the valve worked, so that meant the trouble was with the electric circuit in the zone. A little research helped me find out that when the thermostat calls for heat, 24 volts is sent to the zone valve and can be measured using terminals 1 + 2 on the valve head. If you don't get 24 volts, then there's a break in the system that could be the thermostat, the wire, or the transformer and I wasn't getting any voltage. I measured and got 25 volts at the transformer so that was okay. That left the thermostat or the two-wire cable from the thermostat to the furnace. I tried another thermostat and that didn't make a difference, so it was something to do with the cable. I had to trace the wiring through a spaghetti-like maze of wires and found that the red wire that goes from the thermostat at the RH terminal to the transformer at the R terminal was loose at the transformer. I tightened it up and turned up the thermostat to call for heat, measured the voltage and there it was.