Mistery Zone valve

Norm Harvey

Sounds like a difficult problem to diagnose here, but I'll give you a few thoughts.

The 24v tot he thermostat should be comming right out of the transformer, so it it goes blank, I would test the output of the transformer at that time.

The taco 3 wire zone valves require a heat anticipator setting on the thermostat to be set at .9 amps else the zone valve may "hunt" or open and close, open and close. If your aware of how to check what the valve is calling for by doing the 1 coils of wire trick, you may want to check that out. I am unfamiliar with your peticular thermostat, do you have a model number?

Is the oil burner running all the time? even when no other zones are calling for heat. Is the boiler up on high limit.
If the valve body itself is stuck it can cause a constant call for heat.

Without vbeing there with a good multi-meter thats a tough one to diagnose.

Good luck

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Tony Viz

I have a 3/4 Taco zone valve that opens up all by it self and turns the room it is serving into a sauna. I changed the head and thermostat. Had my electrician check all the wiring for shorts and it still opens up every once in a wile. They never turn the heat on unless they have a guest. The thermostat is one of the new Honeywell digital round type that shows the temp in the window. Could the theromostat be the culprit? I tryed it with and without the resistor and still the same thing. Oh yea ond one more thing. If you turn the heat on and wait long enough the windoe in the thermostat will go blank. There must be some sort of duty cycle or thermal switch in the zone valve that cuts the power when it gets too hot. What gives???

heatboy_10

Digital thermostats.........

.....can't be wire directly to the zone valve. You need a relay in between.

jr76

resistor

is the new thermostat t8775a? If so there's a resistor that should be installed between terminals 1 & 2 on the Taco powerhead.

Big Ed_4

Thermostat Test

Disconnect the thermostat and connect the the two wires to open valve. Then disconnect the wires and see if it closes , if it does close now,then the thermostat is the problem. Before replacing the thermostat swap the polarity (swap terminals) and see it will works properly now . If its the thermostat try a different model...