That has been covered already in my posts. The following quote is from one of them (check full article here: furnace and water heater venting)
#5. If one appliance has been disconnected from the chimney previously serving 2 or more appliances, make sure that a professional re-evaluates your chimney flue. It may need to be down-sized to prevent flue gas condensation and to provide proper draft (typical condition when upgrading to a high efficiency furnace)
And follow to this article: chimney flue sizing for calculations.
Let me know if you need further assistance, just click “ask related question” or “comment” below my answer