The heat cuts out after 15-20 minutes and just blows cold air until I shut it off.I then press a very tiny reset button[roll out switch?] on the top side of a sensor located on a heat shield in front of the burning chamber.This will correct the problem temporarily.I justb installed 2 new ones but still have the same problem.I also have tried operating the furnaces with out the front covers and this did not make a difference.Any ideas?