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I reinstalled an old mercury thermostat. Heat and air both work, but they won't cut off until I move the thermostat switch to Off. In other words, when the switch is on Heat, the furnace and blower run regardless of which end of the tube the blob of mercury is in. Likewise, when the switch is on Cool, the A/C and blower run regardless of which end of the tube the blob of mercury is in.

  • sound like you mis-wired the switch – jsotola Dec 29 '18 at 23:38
  • The most likely scenario is that you connected the thermostat incorrectly. It is also possible that the thermostat is defective. You should disconnect the thermostat and then check its operation with an ohmmeter. The mercury should be making and breaking connections as it moves from end to the other. If it isn't then it is defective. If it is, then carefully recheck how you are wiring it to your system. – Barry Dec 29 '18 at 23:38
  • Thanks for the suggestions. Problem solved. I tested the mercury part with an ohmmeter and determined it is working properly. I reinstalled the stat using fewer wires. It behaved even more strangely. I examined the incoming wires more closely. Someone had removed too much insulation from one. I taped it with electric tape. Everything is working as expected now. – Richard Hevener Jan 1 at 19:44

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