99% sure that this is a "go to Home Depot and buy a valve and swap" but wanted to verify: 4 zones on my boiler for home heating. One zone quit. With thermostat turned down, zero volts on terminals 1&2. Turn thermostat up to call for heat, have 22V on 1&2. Wait 2 minutes, nothing. I measure 26V across terminals 2&3. The manual lever is very stiff and after 2 minutes, will not easily go to the open spot. I twisted the head off. Here's where I go off the rails: terminals 2&3 go to zero volts and the heat kicks on. I took a pair of pliers and pushed down on the valve, and got hot water to circulate through. Twist the head on and the furnace kicks out. I haven't come across this in the various links I've hit up so I'm confused. I think it just means the zone head is bad and that is just three wires (kill power and swap) but I'm not sure why the switch wouldn't work installed. I think it means that whatever wax pellet it has inside is not melting with enough force to push the valve, but with enough to close the switch when it's removed.