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Discussion Starter #1
Most of the Dead Vent articles I've read seem to be exclusively focused on cooling a room. I live in an area where it drops to -40 in the winter and can occasionally get up to 30C in the summer so we need both heating and cooling. It would seem a dead vent for supply and return may not be sufficient to heat a room in the really cold months. The room in question has 2 6" supply lines and one cold air return. I was wondering if it would be sufficient to use 6" flexi duct on the supply lines and insulate around them and then use a dead vent for the return. Will I be compromising soundproofing to a great degree?

Thanks for the help
 

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Elite Shackster , HTS Moderator Emeritus
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Just FYI, I have an unsealed theater with 2 live vents and no return (the return is sucked up the stairs to the main floor and then back down to the air handler -- very poor design for the basement HVAC). I can barely hear the air handler running through the vent in the theater, especially with a movie playing. Sadly the AC compressor is right outside and I CAN hear that in the summer.

I mention this so you don't worry too much. Priority should be the heating and cooling.

That being said, the flexible duct will attenuate quite a bit of the noise on the supply, and I can't see why having a dead vent return would be bad. The trick to HVAC isolation is not having a clean path in or out for the sound. Both the dead vent and flexible ducting help accomplish this. I don't believe it should matter which one is supply and which is return.

Hopefully bpape can chime in with more advice. He's definitely our resident expert on soundproofing.
 

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The trick to HVAC isolation is not having a clean path in or out for the sound.
Interesting thought... Sound is movement of air, so at first thought bends in the duct should not make that much difference. But, bends increase resistance to air flow so I suppose that is the mechanism in play.

I would think that fiberglass lining inside the duct would be important to air flow noise - does flex duct have that, or is the insulation on the outside?
 

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Elite Shackster , HTS Moderator Emeritus
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The flexible duct usually has a lining of fiberglass on the outside.

That being said, the biggest attenuator of not having a "clean" path is impedance changes in the flow path. A meandering pipe with no insulation could still conduct the sound just fine. But a curved pipe going into a larger ante-chamber, and then another curved pipe exiting causes a "shock" to the sound wave and limits the transmission. That's really what I meant by no direct path.
 

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Discussion Starter #5
So what would be the best way to accomplish the changes within the confines of 16 X 12" joist cavities keeping in nind that I also need room for insulation. I don't think a 12" flex duct would fit. I was under the impression that the curves in the flexiduct caused the soundwaves to bounce off the walls of the duct which are absorbing fiberglass and thus absorbed sound. Have I been misled?
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Elite Shackster , HTS Moderator Emeritus
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The combination of curves and fiberglass help absorb the sound. But a long straight fiberglass lined pipe will attenuate sound as well. Glasspack mufflers in cars work on this principal. The sound wave is often pictured as a sine wave or as a straight line compression wave, but while the wave does move forward, the air wants to expand in all directions (including sideways) and if there's fiberglass there, it will take out some of the energy.

But yeah, curves, with the lined ducts will be your best bet. Sadly with that joist space limitation, I don't know what is available, but I thought I saw flexible ducts smaller than that at Home Depot.
 

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Discussion Starter #7
I think there is 6" flexi duct which I assume will have an outside diameter of 8 or 9 inches. That should fit in the joist spaces and still allow a little curvature and leave room to place a 4-inch layer of insulation on one side of the duct.
 

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What will make the system more effective is if the area around the insulated flex duct is also insulated. Flex in and of itself will only absorb the highest frequencies. Better success if you have additional insulation in the soffit air cavity.
 

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gobrigavitch,
Do you have open stud access to the supply and return ducts? I had similar duct noise isolation concerns for my project (still in progress). There are a few thoughts from folks at the project thread. The link below should jump you to a post with a pdf attachment of HVAC boxes I used to 'wrap' the duct with insulation and provide the bends suggested to attenuate the noise. It was a bit of work but easier in the new construction stage.

A few other pictures follow that post after the ducts were installed.

http://www.hometheatershack.com/forums/home-theater-design-construction/13639-new-ht-project-construction-questions-2.html#post128059
 

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Bpape had some very appropriate thoughts in that thread. The insulation in the flex is only effective if there's a LOT. The best dust system has the flex, surrounded by more insulation, and all of that surrounded by significant damped mass.

Often times ducts are run in walls and in ceiling joists where the insufficient mass allows the ducts to bleed badly. The mid and lower frequencies need to be contained in a separate massive chase.

Bpape also pointed out that the twists and turns in the flex need only be subtle to be effective. Dramatic bends will result in increased resistance. The ribbed interior surface of the flex already increases resistance.
 

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You could always run rectangular ducting (ie: the kind used for cold air return) and use Linacoustic insulation or other duct liners.

The problem is that if you use duct too wide is that the velocity of air escaping will drop. In a large room that could mean poor HVAC circulation in the room. The cold air return should be at the opposite side of the room to help circulate the air. If you built silencers into the cold air return you would need to ensure that it was still taking in the right amount of air. The room could get too hot in the winter, too cold in the summer, or the opposite. There is a lot of engineering behind a well designed HVAC system.
 

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That could be done, but that 1" of insulation won't absorb much, I'm afraid. You need a few inches
 
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