"Bill" wrote in message
...
I noticed quite a large opening (8" x 8") in the cold air return going into
my furnace in my basement.

It's been missing a while, and it's in a ackward spot to fix.

What is the prblems associated with this hole, and what improvements will
I gain by spending the time to fix it (seal the opening)?

Is your basement heated by forced air? If not, I'd close that rascal.