High efficiency pvc venting through old

Is it acceptable to run your pvc vent on a new high efficiency unit through the old vent stack? Seems like a potential for debris/rain to fall in with no type of cap. Thoughts?

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That would be acceptable in most areas except the original duct around the PVC vent would have to be sealed as any other roof penetration would

BTW that old class B double wall vent would make a nice pitch box like used on commercial roofs for refrigerant lines and electrical conduit roof penetration