Return Air question


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Old 11-06-13, 08:47 PM
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Return Air question

I'm currently remodeling my family room and after removing the drywall from the ceiling I found a cold air return that is using a joist as the passage instead of an actual duct.

The family room is right below a bedroom. So the cold air return in the bedroom runs to the return in the family room. Is this correct? I'm trying to learn and understand more about this type of thing. Does an actual duct need to be made and ran between the joist?

Here are pics of what I'm referring to.
First pic is of the cold air return duct in the bedroom above the family room.
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Second picture is the same just showing its using the opening in the joist, no duct.
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Picture shows the cold air return running down the wall and feeding out the very bottom on the right side of the two hvac ducts.
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Old 11-06-13, 11:45 PM
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Using stud and joist spaces for return air ducts is fairly common in residential systems. There ARE rules that need be followed however. You CAN have electrical wiring run across the space but you cannot have it run parallel in the space. All parts of the space should be air sealed to prevent drawing in any outside air or other contaminants. The cross-sectional area of the"duct" must be at least as large as if a more conventional duct were used. Unfortunately, this last bit of information is often ignored resulting in undersized return air ducts.
 
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Old 11-07-13, 05:29 AM
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When you say 'air seal all parts of the space' could you elaborate on how this is done? Thank you for your response.

Also, the cross-sectional part would be the space running down the wall? What steps would you take to ensure this joist cavity is correct for the cold air return?

Some additional questions I had concerning supply runs.

You can see the HVAC supply runs in the pic below. What advantages would I gain from replacing these with insulated flex duct? Are there restrictions on the length of the run? One line supplies heat to the family room from the ceiling, another line runs to the garage (which is covered up currently), and the third line supplies the bedroom above the family room.

Is this flex duct appropriate?
Master Flow 6 in. x 25 ft. Insulated Flexible Duct R6 Silver Jacket-F6IFD6X300 at The Home Depot



 
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Old 11-07-13, 12:40 PM
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Books have been written on duct design yet it seems as if residential ductwork is more often a matter of just fitting something in the space to conduct the air from one area to the other. Generally speaking flexible duct will have a poorer performance than will rigid duct. What little you might gain by the insulation you will lose in the less-smooth interior of the flexible duct. Sealing the seams and joints of the ducts is generally far more important than insulation. Do not use the cloth "duct" tape but instead use foil tape or mastic.

I suggest that you check your public library for books on duct design and also do a thorough Internet search of the subject.
 
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Old 11-09-13, 03:01 PM
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Thanks for your response. I was wondering if I could get some additional info regarding using a joist cavity as a return air duct.

I understand this cavity needs to be sealed. Would it be ok to add furring strips inside the cavity that are attached about a 1/2" up from the bottom of the joist. After that attach drywall to the bottom of those furring strips. Before adding the drywall the seams where the subfloor above meets the joists would be sealed with caulking. What type of caulking is best for this?


Thank you.
 
  #6  
Old 11-09-13, 08:52 PM
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I do not advocate using a stud/joist space for return air ducting. Most have little to no sealing so anything that you do is going to be an improvement. Mostly you want to seal any obvious holes where pipes or wires enter the space or similar things. The other problem is that the stud/joist space is often just plain too small for the required airflow and the rough surfaces (compared to sheet metal) further impede the flow of the air. The total cross-sectional area of all the return air ducts should be a minimum of 1-1/4 times that of the discharge outlet of the furnace and 1-1/2 times is even better.
 
 

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