Hi, I'm wondering if anyone has any idea what's going on with my heating system. I have a 2 zone system (upstairs/downstairs) with a gas boiler and programmable thermostats.

A couple of months ago the circulator pump for the upstairs zone was leaking and I had it replaced. A couple of weeks after that, the heat upstairs would not turn off. It just kept getting warmer and warmer regardless of the thermostat setting, or even if the thermostat was turned off. I closed the boiler valve feeding the upstairs zone so it wouldn't keep getting hotter. I called the plumber who replaced the pump, thinking it was related to that, but he said it was not related and it was a problem with the thermostat, probably that the batteries needed to be replaced. (He also bled air from the system and said NOT to close the valve, because this would make the boiler pump air into the system.) I replaced the thermostat batteries and everything worked normally for a few weeks, then the same thing started up again ... the heat upstairs will not turn off. I tried new batteries, taping the battery compartment door shut (it seemed loose), and removing the batteries entirely. No effect, the heat just keeps cranking. Do I need to replace the thermostat? I can't figure out what's going on.