Quick question on adding a second water heater in my house for some radiant floor heating in the basement. I currently have a 100,000 BTU furnace and a 40,000 BTU hot water heater both venting out the same chimney. If I add another hot water heater, how many BTU's can I get away with and still allow it to vent with the other two out that chimney? Thanks a bunch!

Quick question on adding a second water heater in my house for some radiant floor heating in the basement. I currently have a 100,000 BTU furnace and a 40,000 BTU hot water heater both venting out the same chimney. If I add another hot water heater, how many BTU's can I get away with and still allow it to vent with the other two out that chimney? Thanks a bunch!
Before I could begin to answer that question. I'd need to know the diameter of the pipes, the length of the pipes and weather or not anything is power vented.