venting 4" stainless thru existing 6" B vent

[superfitter]

I'm going to be installing a noritz tankless water heater in a small laundry room. side wall venting is not possible. There is an existing 6" B vent (which will be abandoned when tank water heater is removed). Can I run 4" stainless through the B vent piping to the roof? I can remove the B vent, but where the vent goes up through the attic I would have to install a sheetrock chase. looking for a simpler installation

Noritz Model # N-084M

 

[havasu]

I would call Noritz and ask them if it is OK. I know many (including my Navien) were very specific on venting and what is allowed or not allowed, and I would hate to give you the incorrect answer.