Taco 571 zone valve stays open w/ no t-stat wires connected until term 1 wire is discon
I have 3-zone heating (w/ Taco 571 zone valves installed near the output of the gas furnace), and 3 times in the last 4 years the second zone (main floor) all of a sudden stays open and heat never shuts off to this one particular zone. The other 2 zones always work fine. I disconnect the thermostat wires for that zone it still stays open heats and heats. I have to disconnect terminal 1 on this valve to get the valve to close and stop heating this zone. When I connect back up terminal 1 the valve opens and heats again. When this issue occurs the temp goes from 68 (normal) to as high as 78 until I disconnect terminal 1, and wait till the temp comes down and then hook up terminal 1 again and it starts heating up again. What's most weird is eventually the valve closes on its own, sometimes 12 or 16 hours later than it should and the heat is really warm (I am in New England and the temp outside is in the 20s and 30s). This problem just happened 3 days ago (first time in 2 years) and fixed itself 2 days ago, and now it is back again so I am now having the problem again right now. Is this an intermittent stuck zone valve? It looks like all 3 taco 571 valves for my 3 valves have removable heads. Should I try to swap removeable heads to another valve to see if it is something on the valve head? Any ideas how to troubleshoot/resolve?