I have a 3 zone boiler heat system, one t-stat for each floor. It's worked fine for years except twice in the past few months. The first time I was out of town and my wife called and said the upstairs was very hot. I can't remember how I got it straightened out over the phone but I did. I figured the old mercury t-stat on that floor was sticking on so I replaced it with an electronic version.

Then today we had the same problem, t-stat set on 65, temp is 75. I switched that t-stat from heat to off and then took a look at the boiler and valves. The manual override on that zone valve is in the 'auto' position, making me think it's not stuck open but that's the only cause I can think of.

Can the valve be stuck open without the manual lever moving from the auto position? Doesn't the lever move when the valve opens, or does the lever stay in the 'auto' position whether the valve is open or closed? The only idea I have is to rebuild or replace the zone valve. 

The control for the system looks like it's just a junction box, no relays. Do the t-stats just control the power to the valves directly, not provide a control signal? There must be a relay to activate the pump then, right? Or am I completely confused?

Thanks