I bought my house (ranch with basement) last year and the previous owner had mostly removed the plaster/drywall ceiling in the basement to access electrical and plumbing when renovating. I noticed the other day that this "disconnected" the cold air returns from the duct running along the one side of the basement back to the furnace area.
The cold air returns open up to the joist bays, which used to be sealed by the drywall ceiling. In each bay where a cold air return vent was, there was an opening in the cold air return duct in the basement. This duct runs across the house and then opened to another joist bay that ran over to the top of the furnace.
My question is: when the ceiling was removed and the joist bays were no longer sealed by the drywall and the cold air returns essentially just open into the basement, how does this effect the system? I was assume its probably less efficient since more cold basement air is mixing with the warmer air from upstairs. Is it hurting anything for now? I'd rather not have to spend the money right now to run ductwork to reconnect everything. Also the furnace seems to run fine and my gas bills arent excessive (especially considering my leaky single pane windows)