Multiple vents in chimney

[Mike Lamb]

I thought I had seen a reference where a Cat 4 vent cannot share the same chimney flue with another gas appliance – in this case a newer lined vent for water heater. Any help?

[Bill Kibbel]

The masonry chimney flue has become a chase for added vents, so they're not technically sharing a flue.  

The big problem is that's probably an uninsulated single-wall metal flue liner, so there's clearance issues to the PVC vent pipes.  Also, the metal liner is not secured with a proper collar and the cap isn't attached with a proper fitting.