I have a two story house with two heating zones. The downstairs is controlled with a digital thermostat and the upstairs is an old mercury turn style thermostat. In the summer I turn the downstairs off and the upstairs one I turn all the way down. This summer it was 90 degrees outside and I came home and could hear the furnace running and the upstairs was almost 100 degrees. I turned the thermostat all the way down and the furnace wouldn't kick off so I had to go down to the basement and turn the power off. Is this a problem with the thermostat since the heat was only on upstairs?

I would start by replacing the mercury stat with a new stat. Over time the spring in the mercury controlled stats wears out and can cause this problem. They don't even make mercury stats anymore. Start there and if the problem persist, at least you have a good stat to start with. Please let me know how things work out.

I would start by replacing the mercury stat with a new stat. Over time the spring in the mercury controlled stats wears out and can cause this problem. They dont even make mercury stats anymore. Start there and if the problem persist, at least you have a good stat to start with. Please let me know how things work out.

Since they don't make these anymore, would changing the thermostat to a new digital one cure the problem?

Sorry, I should have been more clear. By "replacing the mercury stat with a new stat" I indeed meant upgrade the stat to a digital. Either way, its time for the old stat to go, whether it cures the problem or not is yet to be determined. One way to know for sure is to simply unhook the wires from the old stat and see if the problem is solved. If it is, then a new stat should cure all. Make sure when removing wires or installing new stat, that the power is off to the furnace and damper control box so you don't short any wires.

Oh, one more very important piece of info I will need. Is this a forced air furnace with dampers, or is it hot water heat (boiler) with zone valves?