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Downdraft From Ceiling Registers

K T | Posted in General Questions on

We have R8 insulated flex-ducts in a vented attic feeding ceiling registers on the 2nd floor. They were properly installed and pressure tested. I have noticed when outside and attic temps are significantly lower than indoor temps there is a slight down draft from the register. The registers are air sealed to the ceiling and the ducts run vertically through about 16″ of loose cellulose above the registers.

I am theorizing that when the heat is not running the volume of air in the ducts cools off in the cold attic and the vertical duct run above the register creates some stack effect. Maybe there’s another cause?

Is this a common problem and is there an easy solution? Maybe a backdraft damper above the register (or would this harm the AHU?) 

thanks!

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Replies

  1. mgensler | | #1

    We had this same issue. No amount of insulation or air sealing would fix it. We did a kitchen remodel this year and removed all of the ductwork from the unconditioned attic. It wasn't a cheap or easy solution but it worked.

    1. K T | | #2

      Well, that's not exactly the answer I was hoping for. Surely this is a common issue- in a perfect world heat registers would be on the floor and A/C in the ceiling and everything inside the envelope but not in our case.

      Even though it didn't work were you able to add insulation to insulated duct?

      1. mgensler | | #5

        We had about 14" of cellulose on top of the flex. I think also as the air in the ducts cool, it falls out of the register. I never noticed it in the summer.

        1. K T | | #6

          Wow that's a lotta insulation! Yes you're describing the situation I'm envisioning- that the duct air get's cold and exchanges with the warmer air at the top of the ceiling resulting in downdraft. Not an issue in summer in fact probably gives the warm room air somewhere to go.

          Wondering if a backdraft damper might help. Even though it's not a backdraft per se and both the supply and draft are the moving in the same direction, the latter is very very light pressure so may be held back by the damper. The fabric kind might work.

  2. ScottOHara | | #3

    How much cellulose is on top of the ducts? There is no problem with deep burying the ducts in cellulose. I usually try to maintain the attic R value over the ducts including the return which will most likely require some kind of damn be built.

    1. K T | | #4

      Unfortunately the ducts are not buried. On the plus side, no condensation risk :-)
      They are just R8 flex ducts. The boots and vertical run above them have 16" cellulose

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