Gas Furnace Vent Size
I have a 90+ Rheem gas furnace. The contractor came off the unit with a 2" pvc vent pipe, but one foot later upped it to 4" for a 12 foot horizontal run to the outside of my house. I am confused as to why they would go from the furnace's 2" discharge to a 4" pipe. Why couldn't they just run with 2" all the way, since that is what comes off of the unit.
If it is related to the pitch of the pipe, could I keep with 2" if I went directly into the nearby fireplace flue and kept it mostly vertical?