I have a twenty year old Thermo Pride Furnace powered by oil.Recently when I have the thermostat,a Honeywell T87 turned up to sixty eight degrees the furnace and blower will shut off at sixty eight degrees not seventy like it use to.When the house temp drops two degrees below my setting the furnace comes back on like it should.What is causing the furnace to shut off sooner than it should.I change the fliter every month.
Technically, the thermostat should shut off at the set point as you stated 68 deg. It is normal for the temperature to rise above the set point from residual air flow. The thermostat should reenergize the unit at 2 degrees below the set point. There is a heat anticipator in the thermostat that controls what is known as overshoot. This has to be adjusted by the amp draw on the heating circuit for proper operation. I would put a good thermometer at the thermostat and check the actual temperature against what the thermostat is reading. Believe me, those thermostats do get out of adjustment. Hope this helps.
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