I had and still have some visible and apparent leaks in my furnace and plenum. The three tin tubes for the duct supply lines, coming out of the plenum, were each attached with only 4 screws so I sealed them with high temp cement. I also sealed the joint where the plenum and furnace connect with the same cement. However, I still have a few leaks below the plenum, there are two copper lines coming from the outside a/c unit that go into the furnace. There are 2 holes where the copper lines enter the furnace that are not sealed and air blows out of these holes, I want to seal them but I do not if any areas like that need to "breath". Please let me know if I can seal it too so I'm not loosing heating and air conditioning in the attic.
Thanks for any help,