I have an oil-fired forced air furnace in the basement of a 2 floor, 2100 square foot house. I have about 16 floor vents to deliver heat, and 2 first floor returns, one 14x14, the other 8x14. Both feed into standard 8x16 return ducts. The furnace has a 20x20 filter.

My question is, do the two cold air returns deliver enough air to the furnace? This week, I covered up a 6x12 vent that had been cut into one of the basement return ducts. At some point the basement was heated. But I'm not sure if the return vent was to aid heat circulation in the basement, or to deliver more air to the furnace. (I figured I would be better off pulling cool air out of the house, not the cellar.)

Does the furnace need more air than the two 1st floor returns deliver? The other question: with a stairwell bringing cooled 2nd floor air to the 14x14 1st floor return, will I gain anything from a 2nd floor return, if I were to add one?

Thanks for staying with me this far! I appreciate the answers.