Yes i have a exhaust preheat. I built a shroud around the catalytic portion with 2 of the 3”blowers and a check valve upstream.
I only use the blowers for startup
My burner will overheat if i pull more than 5” of h2o for more than a few minutes
This limits me to 45 -50 MPH on a level road
I used 3” thinwall tubing everywhere except 5” at the exit rom the burner