By code. chimney is not allowed to be more then 7 times the smallest appliances breech opening. So a 3" breeched/draft hooded/dilution hooded water heater. May not be connected to a flue pipe or chimney that is more then 49 sq in in cross section.
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that is not going to be an easy install at all.... needs to be plumbed up to code..the old tank has to lifted over the boiler to get it out....same with new one going in.150.00 bucks....not a chance...