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I have recently come across an issue with my thermostat and when and how my heat comes on.

My thermostat seems to be an old round Honeywell. I set my temp at 65 typically. As of last night my furnace has been coming on what seems like too early as it's already above the temp at 67-68 degrees and turning off at around 72-74. What would be causing something like this to happen?

Appreciate any help as it's cold up here in New England.

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    So this is a new development? Mercury thermometers of that type aren't particularly accurate, especially if they're old. They rely on a metal coil that expands and contracts with temperature. You might swap in a simple digital.
    – isherwood
    Nov 16, 2018 at 19:02
  • Yes very old and fairly dusty. It just started happening yesterday I was going to go pick up a simple Honeywell digital this weekend. should this fix the issue?
    – user93182
    Nov 16, 2018 at 19:19
  • Well, if that's the actual problem. There may also be an adjustment in your current thermostat. familyhandyman.com/heating-cooling/furnace-repair/…
    – isherwood
    Nov 16, 2018 at 19:27
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    Are you using a separate thermometer to check house temperature? Or the ancient temp indicator on the Honeywell?
    – Kris
    Nov 16, 2018 at 23:01

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Mercury thermostats are VERY accurate and last forever. Either it is now not level, or the heat anticipator is mis adjusted or not making good contact. If set at 65 degrees, the furnace should kick on at 64 and shut off at 66. If the swing is wider than that, the heat anticipator needs to be moved towards shorter. My Honeywell mercury thermostat is 65 years old and still works perfect.

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  • They do fail but most last a lifetime, I wish they still made them, inexpensive and if set up correctly quite accurate.
    – Ed Beal
    Nov 29, 2018 at 20:28

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