I just bought a new to me house, built in 1970. I replaced the old mercury thermostats with new programable digital ones. They worked great for the first 12 hours. Kept and read temps like it was set. Called for heat when needed. Then yesterday the middle floor just kept heating up. Thermostat is set at 60 but is reading 79 and heat is still coming from the radiators. I checked the zone and it does open and close manually with no issues.
Update: So the thermostat will open send power to the zone valve to open, but the valve does not close. The zone valve is a White Rodgers 1311-102.
Update 2: So I just replaced the zone valve. It will open the valve, but not close it. All the wires are the same as upstairs, but the middle level will not close it. New ideas!!!