We have a Heil unit that has begun to blow cold air occasionally when the heat is on. The unit, when it does heat, goes on and off 3 times and will eventually get to temperature, instead of running until it gets to temperature. It'll come on in the morning and only blow cold air. When I go down to the unit it has a green light that flashes 3 times. If I switch off the unit and turn it back on again, it works for most of the day. Pressure switch has been replaced, duct work has had extensive restructuring to improve the intake. Several technicians have been out & run all sorts of tests but cannot figure what is going on. Their only solution was to fix the duct work, which we did, but we are still having issues.

Help...

Check for vent obstruction. I would guess your high limit is tripping and resetting