Picture is my HVAC plenum chamber. My contractor is going to install new ceiling in few days. I wanted to see if there is any leak in that area, so I used flash light inside service panel door after turning off light in room. The bright area you see is gap in plenum chamber framing. How can I seal it ? What can I use?
Upon close inspection of the first picture, it appears that the backing piece behind where light is escaping was cut too small, which is unfortunate.
I'd recommend a sheet metal screw or two where the two pieces of metal do overlap, just to be sure gravity doesn't work against you over time, then a layer or two of duct tape to cover the gap and metal on both sides of it. Use the proper silver foil & plastic stuff available in the HVAC aisle at a hardware store, not "duck tape" available in 8,000 colors and patterns at your local convenience store/big box everything store. You can use it on the small corner, where light is peeking through, as well.
I'd actually recommend using tape on all joints to keep them from allowing air to escape. I believe that this is a fairly standard practice these days, and it's rather unfortunate that your HVAC contractor didn't do this for you.
There are duct sealing mastics available at your home store. I would use that to seal both leaks and after it dries, cover it over with duct tape, the real stuff found in the HVAC section, not the cheap stuff. The top leak actually looks like they originally used a mastic but didn't do a good job.