I'm trying to understand how the supply and the return ducts connect on my home central air. I can see that the fiberboard is taped to the joist to create a plenum, but then how does that connect to the main return plenum hanging from the joists?
I assume that there is a hole cut on the top side of the return plenum right where the return duct would be for each room return. Is it just a matter of taping the return duct to the return plenum?
Next question is that I have an 8' section of return coming off the blower unit (which is on one side of the house center support beam) and then a 40' return plenum on the other side of the support beam thats against the supply plenum. It goes return plenum, beam, supply plenum and the return plenum. As far as I can see there is only one bay in between the joists that runs from the small return plenum over to the longer return plenum. Is a 16x10 area big enough to connect to a 21x9 main return plenum?