First I will say that I am sorry to be slow to post in the thread after these great responses. I thought I was going to get emails that notified me of people posting in the thread, but that didn't happen.
First a few more details about my system. I have an Aprilaire 2200 that sits right next to the unit, which I guess from Bob's post is actually called the air handler (sorry I am new at this). Basically it's MERV 10 (@ 1200 CFM), and uses a 20x25x6 filter. It seems from what I read that this system does pretty well at cleaning the air, and I was actually planning on purchasing this: http://estore.aprilaire.com/catalog/...grade-kits/302
... to upgrade the thing to be even better.
Although Bob clearly has a great solution in his post, I really don't have the money or expertise to do something like that. Some day I may just have to get this thing taken care of by a real HVAC company, but right now I must do it myself for budgetary reasons. That means I am trying to do something that will perform well, but hopefully not reconfigure the whole system or where the intake comes from. Mostly I am trying to determine if my idea for a fix is going to work OK and not cause problems that I don't know about. I know it won't represent the ideal solution.
As biggles points out, I don't want to be drawing air that's 20 degrees cooler and paying higher energy bills. That's the main thrust of why I want to fix this, along with making the air in the house cleaner so I don't have as many allergic issues. Biggles mentioned that I might even be mistaken and this place I am working on is not the place that draws the return air back into the unit. It seems like it has to be, but to make sure I am not missing something I am going to go up into the attic in a few minutes and examine the system to make sure I am not mistaken about how the air flows.
Assuming this place in the video IS the return air, I would love to know if my idea for a solution can work. Basically this is my idea:
1. Seal up the hole in the floor from under the house so no air can enter from below the floor
2. Cut a larger hole in the wall in my hallway and put in a new return air filter grill that holds a one inch thick filter. Maybe I make it 20x30 or so? I know I already have the 20x25x6 filter up in the attic next to the unit, but I thought as long as I was putting in a new return grill, why not throw a small filter on that too?
It would seem that if I followed these steps the return air will still flow through this open wall into my system, but I will have forced it to come from only inside the house via the new grill in the hallway. Can this work or am I missing something major and am somehow going to damage my system? Thanks again for the help!