Adding a hallway door to increase air conditioning efficiency

[MN Renovator]

Originally Posted by gtojohn

Is it possible to add a return air duct to the downstairs? Or if there are 2 seperate supply ducts make one a return? The idea being to help remove the extra air which is too cold and allow it back into the system.

Careful. If you add a return and there is no corresponding supply air, you'll create negative pressure in the basement which will pull air through every leak imaginable which can add humidity, radon, drafts, etc. to the space. If there are any natural draft appliances(most natural gas water heaters are a common example), you can backdraft that exhaust into the living space.

If there is no supply venting, don't add a return.