Flame goes out, or will not light. I replaced the thermocouple and tested the burner assembly by hooking up the piping outside the heat exchanger. Pilot worked fine. I re-installed the burner assembly in the heat exchanger and secured the face plate. It would not light. I loosened the face plate screws and pulled the assembly away to create a gap in the faceplate for air, and everything worked fine. What is the problem & how do I correct it. The room is very well kept