I am finishing a 1100 sq ft basement (2 yr old house). It has 2 current vents running directly off existing duct work. The furnace room will be partitioned off (19'x5') and contains the gas furnace, gas water heater, and sump pump. The furnace room will be closed off by a solid door, and the basement door at the top of the stairs will always be open. Does anyone know how many return air vents I will need to install? If I'm not providing enough information, please let me know if more information is needed. Thanks in advance.
As many as it needs to return as much air as those 2 supplies are putting into the room.
Plus, since the basement door will always be open, about another 10% additional return. Since cold air will fall to the basement.
Where is the furnace and water heater getting their combustion air from?
If they were getting it from the basement. You will have to provide air vents from the finished area to teh utility room, or another means for them to get their combustion air from the outside.