contemplating installing a natural gas tankless hot water heater to replace the existing hot water tank heater. The old hot water heater has a B-vent system and the new tankless high-efficiency heater has a PVC venting system. Can I connect to the existing vent system, and if so who makes an appropriate adaptor?
You cannot connect high efficiency tankless water heater to your existing chimney / venting system, it requires its own / dedicated venting system as per its manufacturer’s installation guidelines. This system produces large amounts of condensate and requires PVC venting.
I don’t know WH’s manufacturer but most of those tankless systems require 3/4″ gas supply line, if you only have 1/2″ at this location you may need to run a larger diameter from the gas meter.
Let me know if you have any other questions.