That's not really a problem you can fix with a single thermostat. You've got multiple issues here. You really need multiple zones. I've got a three story house, though the third "story" is a finished attic. You really want a separate zone for each story. In my case, the same heating/cooling system serves the 2nd and 3rd floors - there's an electronic gate which determines whether the heated/cooled air goes to the 2nd or 3rd floor. The 2nd and 3rd floors each have a thermostat. The thermostats don't know that they're being serviced by the same system, they just call for heat or cooling. The first floor is another zone, but it's served by its own system. That's a total of 3 thermostats and 3 zones. The other problem is that your AC system is too small for your house. Left to its own, my 3rd floor gets really hot - it's a finished attic, as I said. 90+ easily. However, if the cooling system is running, it has no trouble keeping the heat under control, and never runs 24 hours a day. Not everything works perfectly here, though. While I don't have AC issues, during the winter heating the 2nd floor ends up sending about half the heat to the 3rd floor. Probably because the vents are in the ceiling. I put a door closer on the door to the attic, but it didn't help much. I deal with this by keeping the 2nd floor somewhat cold. The 1st floor is generally 68, the 2nd floor 64, and the 3rd floor ends up being 70 just keeping the 2nd floor at 64. It's not really a problem I see as fixable short of converting back to radiators on the 2nd floor.