Pilot won't stay lit. Removed glass inspection cover still no help. Removed burner, finally got enough air flow to keep burner lit. When I try to reassemble pilot goes back out. Burner ignites and stays lit for approximately 10 seconds. The model is a Richmond 50 gallon gas Integra system. Model 12G50=40F1 509465. What can I do to get more air flow to keep it lit? Is there a sensor that needs replaced or something need cleaned? I have had this unit less than 7 years. It has a 12 year warranty.