Summary: I'm doing some remodeling in our home and was surprised to find a return air opening in a strange spot (in a sealed joist bay between my main floor and basement), I'd appreciate any suggestions.
Situation: The house is a raised ranch in Ohio, built in 1959. It had no central air when built. The basement was "semi-finished": Painted concrete block walls (no insulation), but with an original fireplace and a ceiling attached to the joists. Supply and return ducts are in the basement. The basement has a few supply registers (about 100 sq inches for the 1000 sq ft basement) with no obvious return air opening in the basement. The main floor has floor supply registers in all rooms and the return air comes through three large grills in the central hall to a single central duct. The door to the basement has a gap of 1 1/2" at the bottom, and I just assumed the air from the basement flowed through this gap to reach the return air grills on the main floor. However, when I was doing some work in the basement I removed the ceiling and found about 32" of the return air duct had been opened up on top so that it was drawing air in from two joist bays between the main floor and basement. There are no grills or other signs that these bays were being deliberately used for return air.
a) The opening was cut before the ceiling was installed in the basement and was meant to serve as a return air point in the basement (if so, it is about 3 times larger than the supply registers in the basement)
b) It was a mistake.
Right now the basement has a slight negative pressure when the fan is running. When I close off the hole the basement is neutral (i.e. I can't detect any airflow under the door gap leading upstairs using smoke/tissue.) I'm not sure why the air isn't rushing under the door to reach the main floor return grills, unless maybe there are just a lot of leaks in the return duct.
I've got a natural-draft water heater in the basement. I'm thinking I'd be much better off by sealing up this basement return air source. Any thoughts or suggestions would be appreciated. Thank you.