Why is my thermostat turning the heat on too soon?
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.