I am working on a Re-Verber-Ray XTS Radiant Tube heater.

When I originally got the call, the location had 4 of 8 of theses heaters not working. 3 of them I was able to clean the intakes, blow out the section of the box with the gas valve, checked wiring, etc and they all fired right up.

The fourth one, EVERYTHING was burnt up in it. The wiring, silicon control tubing, top of the gas valve, and the ignition control module.

I rebuilt the unit, replacing everything but the blower and burner tube.

The unit ran several hours.

And it did it again.

The Baffle seems intact, Blower running fine and is clean, flue clear. What would cause this blowback?