Can forced air supply duct be run through panned returns?

I am trying to clean up a low hanging heat duct running through my shop and increase the air flow to a distant cold bedroom. One duct in particular taps off the supply trunk line and runs several feet below the floor joists before connecting to the wall stack up to the second floor bedroom. Apparently, it was routed this way because the corresponding joist cavity was panned out for a return to a different room.

Does code allow the supply line to pass through the any portion of the panned return channel? I know it’s not the best/easiest approach, but this would certainly solve my problem with the supply line. This is a 5†round duct and should have minimal impact on the air flow capacity of the 9x14.5 panned return channel.

Any thoughts would be appreciated!

Jim H.

DigitalJim said:

I am trying to clean up a low hanging heat duct running through my shop and increase the air flow to a distant cold bedroom. One duct in particular taps off the supply trunk line and runs several feet below the floor joists before connecting to the wall stack up to the second floor bedroom. Apparently, it was routed this way because the corresponding joist cavity was panned out for a return to a different room.

Does code allow the supply line to pass through the any portion of the panned return channel? I know it’s not the best/easiest approach, but this would certainly solve my problem with the supply line. This is a 5” round duct and should have minimal impact on the air flow capacity of the 9x14.5 panned return channel.

Any thoughts would be appreciated!

Jim H.

Click to expand...

Well that's an interesting question. TECHNICALLY this would be the same as running any branch line through a return air plenum above, say, the ceiling tiles of an office space. The code does require a minimum insulation value for all supply ducts anyway, so you would still need to insulate your round supply - which would eat up more space in your return duct. Depending on how much air that return duct is supposed to be handling, you may in fact need the full 14.5 x 9 dimension. I don't have enough information to answer that portion of the issue. Why not just relocate it to an adjacent joist space? Because you would still need to dip below to cross over?

I don't believe there would be any code issues in NY with a nested supply/return approach (it would be a code issue in CA, which requires Manual-D duct design.)

Sealing all seams & joints on the existing ductwork with duct mastic or (where appropriate) FSK tape would be an important first thing to try. The seams of the panned-joist returns should be sealed with duct-mastic too. In typical ducted HVAC systems 15-25% of the air ends up going somewhere other than intended due to leakage, and 40% is not unheard of.

Panned joist returns don't meet code even as return-ducts anymore in some areas for performance/mold/contamination reasons not sure about NY. If the vertical run on the supply duct is inside exterior wall stud bay, or if the vertical duct IS the stud bay, that's a huge heat loss and mold hazard worth doing something more serious about than monkeying with the ducts in the basement.

Sharing that #1 slot would be finding and fixing any heat leaks in that remote bedroom. Air-sealing is the first part, followed by spot-insulation where it's doable, then window losses: If the windows are clear-glass double-panes in reasonable condition, and low-E exterior storm window would cut the heat loss of that window by more than 30%.

If the heat loss characteristics are dramatically different from the rest of the house where the thermostat lives it may be better to cap off the ducted run and heat/cool the room with a ductless mini-split heat pump. Heating with mini-splits is far cheaper than heating with oil or propane, comparable in operating cost to heating with gas (your actual gas & electricity rates might make it tip one way or the other.) But the temperature stability, noise & comfort will be better with the mini-split. (There are models that are rated to run at outdoor temps of -15F now, so don't worry that they'll crap out during cold snaps.) If the heat load of the room at Rochester's +5F 99% outside design temperature are at least 5000 BTU/hr it would be a reasonable way to go. If it's a high-R/low loss room even a 3/4 tonner would be too much.

Gravity said:

Well that's an interesting question. TECHNICALLY this would be the same as running any branch line through a return air plenum above, say, the ceiling tiles of an office space. The code does require a minimum insulation value for all supply ducts anyway, so you would still need to insulate your round supply - which would eat up more space in your return duct. Depending on how much air that return duct is supposed to be handling, you may in fact need the full 14.5 x 9 dimension. I don't have enough information to answer that portion of the issue. Why not just relocate it to an adjacent joist space? Because you would still need to dip below to cross over?

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Well, any cross over is going to take the same number of elbows, so I assume will dimish the flow to the far bedroom. They will also still be in my way. I've been staring the the basement ceiling for weeks trying to come up with a simple solution. Also, the return channel has to route through an interior wall space to pull from the target room. That dimension will be 3 1/2 x 14 1/2 (even smaller if you look at how the sole plate is hacked away). So I'm figuring that would be the limiting factor in the return air flow - much more so than a joist bay with a supply line in it.

So, would there be that much heat loss from the supply if running through just a few feet of return?

The higher the temperature difference between the supply and return air streams, the greater the heat loss. If it's only a few feet you could probably live with it but it's a total hack job and I would never do it myself. I would be most concerned about messing up the sizing of your return duct which could throw your system off balance (IF it's even balanced). Ultimately you can do anything you want to, how well it works will be discovered after it's installed. Sorry I cant help more. Dana offers some good advise about just installing a dedicated mini split system. Those things work pretty well and you wouldn't be messing with the rest of the house if you install one.

A minisplit wouldn't be nearly as cheap as hacking on the tin, but is guaranteed to work. A DIY installation commissioned & tested by a pro would be in the ~$2000 range for a name-brand higher efficiency 3/4 ton like a Fujitsu AOU-9RLS2H or Mitsubishi MUZ-FE09NA.

If the comfort issues are truly a duct problem or can be fixed by reducing the heat loss out of the room it's probably going to be both cheaper & better to do it that way. (Unless your furnace is an oil or propane burner.)