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:

Register from upstairs hallway: AC register

Electrical runs penetrate cavity without seal: Electrical Run

Stud gaps are not sealed in upper portion of cavity: Upper cavity

Lower part of cavity appears to have been sealed: Lower Cavity

The runs to the main trunk that I am on top of appear to be sealed as well: Run1 Run2

All those clean marks on the ducting is where I stuck the vacuum cleaner.

  • It's common to see stud and joist cavities used as return air ducts. I can't say from your description whether there's a problem in this case. It should be a single clear route.
    – isherwood
    Commented Jul 3, 2017 at 22:34
  • Pictures always help.
    – Tyson
    Commented Jul 3, 2017 at 23:02
  • 1
    @isherwood - its a clear route from the register to the return air duct, but the cavity is not sealed from the rest of the interior walls. I see some caulk in a couple places, but there are large gaps and some holes for electrical wire that allow unfiltered air in from the interior walls to be returned right to the furnace and AC. Also it is still pretty dusty in the return vent cuz I couldn't get all the area down in the horizontal run.
    – Ian
    Commented Jul 3, 2017 at 23:04
  • I dislike and do not trust most of the so-called HVAC companies. To them any short cut is okay. Your installation should work unless the cavity is accessible to the outside, a bathroom, the kitchen or to the garage. certain areas are not supposed to have return vents. I would call a REAL installation company and have the installation checked. Check with your neighbors to see if they can recommend a good company.
    – d.george
    Commented Jul 5, 2017 at 9:48
  • I'm not sure what you mean by "unfiltered air". Filtration happens at the furnace, and some pull from other parts of the house isn't necessarily a bad thing.
    – isherwood
    Commented Jul 5, 2017 at 13:25

2 Answers 2


I don't think it's illegal if done right. The problem is that it seems to have been done wrong. If you have construction debris in the intake that's lazy and sloppy at best. Eventually some of that junk is going to end up in your AC unit with predictably bad results. Let's hope your filter is a good one.

I have seen (shudder) old houses where the intake went along cavities but the builder or owner had neglected to ensure that every last hold along the joist run had been sealed. In the next cavity over was fiberglass insulation. You don't want to be breathing that stuff.

  • This cavity is behind the register filter though. Are there other filters right in front of the furnace and AC unit that never get changed? And yes I don't see what is keeping this intake from being exposed to insulation through the electrical runs and stuff.
    – Ian
    Commented Jul 5, 2017 at 16:53

If it is done correctly and sealed it is perfectly acceptable in my area in California however in Colorado it is unacceptable and must have actual duct work to be up to code. Different areas take a different view so best to contact the building inspector if you want the real answer. In either case it was obviously done improperly and was left to other trades to hopefully finish their work properly.

  • Yeah, but you can obviously see that in the upper portion of the cavity no sealing has been done. There gaps between studs and holes for wiring. There's probably a good 8 feet of interior wall between the cavity and insulation, but the air filter is quite a bit of resistance and I wouldn't be surprised if a significant amount of air is getting pulled through gaps between the studs and drywall and the wiring holes.
    – Ian
    Commented Oct 22, 2017 at 14:50

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