Radiant

What is the difference between the Vision and Vision VSE? 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?