What is this part of Air Duct?

[ccpyue]

I found this Air Duct ending in my basement, seems is an opening. Could someone tell me what is this for? will the heat or air-con escape from here in to my basement? Shall I seal it totally? Thank you.

 

[user 4883]

Can't see to be sure but, in either case it's not being used.
If it's a supply duct, then seal it up.
If it's a return duct & that is dampered, it may have been for a fresh air supply. If it is the return, it is missing it's hose to the fresh air source (i.e.- outdoors).

 

[asbestos]

I would seal it if it has conditioned (heat or AC) air in it. by seal I mean the joints and seams. you can use approved tape (not duct tape, but tape for ducts) or mastic goo. I have had to do this for some home sales because of inspection reports. It is not that big of a deal. I used the good and a disposable brush. Insulation might not be a bad idea. Here it is all crawl spaces not basements so any heat lost is straight to the outside. Even on a return line you don't necessarily want it sucking in air from the basement. there are a number of good articles about that in various energy saving type websites