Unit not heating. When I arrived unit would run just fine for a couple minutes then flame would go out and it would restart. Checked for proper draining. Checked pressure switch tubes for clogged. Tapped on pressure switch. After tapping on switch unit ran fine. Turned unit on and off 4 times and unit ran fine every time. Checked exhaust pipes for freezing. Cleaned flame sensor. Put in parts order form for new pressure switch.