My home is a tri-level ranch with cold air returns in various spots around the house. The furnace is in an unfinished basement section under the main living space. I just noticed the top of the return venting is completely opened in one joist opening. I can only think someone thought there was an issue with how much air was being returned to the furnace. I'm wondering if this negatively impacts how the system runs and is something I need to address. Any and all thoughts are much appreciated.