Furnace "leaking" hot air

Hello. I have a gas furnace that blows hot air well enough. The problem is it blows hot air through one of two grommeted "holes", for lack of a better term. One of these holes is connected to the A/C condensate pipe through which condensed water drains away, and the other isn't connected to anything. It's through this second "hole" that lots of hot air blows out, making the area around the furnace toasty warm while the upstairs is considerably less warm. I'm used to the basement (where the furnace is) being the coolest part of the house. Now it's the warmest. What could be the problem?

I've checked the filters and they're ok.

Any advice appreciated. Thank you.