Standard Operating Procedure.

A power vent has to be installed because the existing exhaust duct work goes behind drywall, which is apparently a big no-no.

I am trying to determine if he destroyed the regulator on purpose or if that is part of the job.

Not a part of the job.
Under what circumstances does the vent go behind drywall? Most chimneys are behind drywall at some point.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13