I’m researching exhausting a water heater up an old chimney that is unlined. When I read installation manuals for water heaters they recommend using a 3-inch diameter B-Vent. I notice in a lot of older homes people just plug the vent into an unlined chimney that’s no longer being used. That’s a much larger diameter exhaust than the recommended 3 inches. I also think there would be a CO consideration for health and damaged as well deterioration from the inside you can’t see half way up as the exhaust cools because of the wrong sizing before it can get out.
Can you recommend some resources and possible pictures for this?