When I turn the heat on on my central heat and air unit, the heat came on then continued to run getting really hot. It would not shut off unless I turned from auto to on then back. Sometimes I had to turn from auto to on and leave it for heat to work. I thought it was the thermostat, so I changed with a new digital one which worked fine for a few days. Tonight I had it set at 70°F but the temperature in the house was 80°F; it's doing the same thing again, not shutting off.
What could it be?