Heil furnace: model N9MP2100J20A3, serial L0241 30932
Programmable thermostat set to 65 degrees overnight, 68 degrees during the day. Woke one morning to indoor temp at 80 degrees. Set thermostat to 'off', burners did not turn off. Had to shut off power switch to furnace to turn off burners.
Called our regular HVAC service company. Furnace worked okay for them, but technician saw some dark areas around a resistor on the main circuit board and suggested replacing it. Did not see any sign of problem with the thermostat. Replaced the circuit board and it worked okay for a few weeks, then started doing the same thing. Replaced thermostat, but did not fix the problem.
Regardless of position of switches (on/heat/cool, fan on or auto) the burners will ignite when the main power is turned on.