Gas Equipment venting to an exterior metal chimney

[norske]

I am looking for advice for venting a boiler and wh into an outside chimney 40 Ft high
the boiler is fan assisted 233 MBh and wh is Nat draft 75.1 MBH , total gas load 308 MBH
the outside design temp is 12 deg f

the chimney is not specified yet . a 7" interrnal type B vents works inside, but owner doesn't want to lose space.
i was going to spec a stainless steel chimney for outside
the only tables i can find in NFPA 54 are for exterior masonry chimneys and the and i cannot satisfy the minimum gas input in table for a 40 ft high chimney

[BaldLoonie]

You need some kind of liner or you will condense the gases from both units. B vent down the chimney will be great.

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[norske]

You need some kind of liner or you will condense the gases from both units. B vent down the chimney will be great.

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ok
what about using a class A dbl wall ext chimney w/o a liner

[gravity]

that piping job is gonna be expensive!