I am planing to change the 18 years old mid-efficient gas furnace to high efficient one.
Now the old furnace and water heater share the 5'' chimney. After changing to high efficient one, only water heater will use the 5'' chimney. I have following opinions about resizing the chimney liner.
1. It should be rezied to 4'' one, and quote is 200-300$
2. 4'' won't fit in 5'' one.
3. 5'' can be kept.
My house is a two-story. Water heater input is 44,000 btu and old mid-efficient input is 75,000 btu.
Thanks for your suggestions.