The house we bought almost 2 years ago has 2 intake air registers for the furnace and AC (not one for each, but both feed both the units). There is a big 20x20 intake downstairs and a smaller 12x20 intake upstairs. The 20x20 pokes out the bottom of the main trunk and looks pretty standard, but the smaller 12x20 upstairs is really weird.
The upstairs register doesn't directly go to the main trunk. Instead it feeds air into an interior wall cavity which is connected to the main trunk via 2-3' of horizontal ducting. If this cavity was sealed so that it made a sort-of sideways J to draw air into the trunk from upstairs I wouldn't have any problem. But it is not so. First off the square cavity was littered with construction debris and dust (which I just now vacuumed out). Second, it is not sealed, so it can draw unfiltered air from all the interior walls.
How normal is this setup? I'm guessing someone just made a mistake and something got left undone, but I'm having trouble figuring out how the previous owners changed this filter year after year without being bothered to at least clean out the cavity. I'm thinking I need to get my whole system cleaned and sealed since drawing all this unfiltered air could very well be making us sick.
I found my camera so here are some pics:
All those clean marks on the ducting is where I stuck the vacuum cleaner.