Radiant

Why isn't my radiant tube firing or running? Do the Vision Herringbone units have the same burner spares as the standard U Tubes? How do you install a tube coupler on my radiant tube heater? I have a radiant heater that is failing to ignite, what should I do ? What could cause the flue fan on my radiant tube to stop running? What size gas connection is on a Radiant Tube Burner assembly?