The oil furnace has been venting into this brick chimney for 50 years. It appears to be double-whythe brick, with a bit of mortar fallen out. Now there is an added gas water heater vent. I called for an inspection by a heating technician, along with a checkup of the old furnace. What scares me is the water heater was installed by a plumbing and heating company, and they obviously thought this was acceptable?