Tim,
"Wiz" is right, the stack size should never be reduced!
We have found with the Alkota 5305 we were able to run a flex exhaust pipe from the muffler to a custom built stack adapter which was bigger than the burner exhaust stack, and next we added a slip on stack adapter which made a 90 degree bend and went out about 12" and made another 90 degree bend to exit the van.
(With the exception of the flexible exhaust pipe, everything else was custom made at a local sheet metal shop.)
No soot problems ever! The increased size on the exhaust stack and the engine exhaust being injected in to the burner exhaust helped keep a clean soot free burn. (Kind of like an afterburner effect!)
I will post a picture if I can find one.
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Call or e-mail me if I can be of assistance!
Mark McIntyre: AKA: Pressure Washer Zone Man.
PressureWasherZone.com EasyCleanSystems.com
Mark@easycleansystems.com
916 638-0828