I have 3 zones in my system, last spring I had a zone in the family room that was getting hot by itself. I was running wide open, disconnected the thermostat and didn't make any difference. The taco circulator wasn't running that I could tell but only this zone was getting so hot you couldn't touch the pipes in or out.

I figured it was somehow gravity bleeding and shut the valve off at the circulator. My question is this: Is this a bad circulator or is this a bad flow control valve on the other side near the expansion tank? Not sure what that flow control valve does..

Sounds like a bad control valve. To isolate it, switch another thermostat that you know works and see if it operates the valve. If it does, your thermostat is broken. If nothing changes it's your valve.
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I am thinking the valve is stuck wide open right now, as soon as I put the petcock for that zone back on it's going to heat that zone again.

When the service guy was here last he "flushed" the system. Yea I think he flushed all the crap into that valve..

There is a little lever on the zone valve that he may have opened to f;ush the system and forgot to close. That lever will leave it wide open unless pushed back closed. the lever simply opens the valve mechanically rather than with the magnet in the head that opens it when the thermostat calls for heat.

Scott each zone should have a flow check valve

Does each zone have it's own circulator or does it have Zone valves with one big circulator?

each zone has it's own circulator the flow valve is on the other side it's marked taco 19 with an arrow so this is what's bad then.

I would have to say with out looking at it yes

do you have a boiler mate or a tankless?

Tankless

It sounds like the check valve/ flow control to me

flow control valve. on the top there is a knob or lever.turn it down clockwise.if it heats up switch it out.

Try stroking the petcock.
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nebe you can't post pictures anymore :rotf2:

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Try stroking the petcock.
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Nebe can never resist the urge to stroke his petcocks!