Proper Cold Air Return Sizing

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Old 04-23-06, 06:50 PM
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Proper Cold Air Return Sizing

Hello all.

Renovating a one and a half story cape cod attic for master suite. Recently added a bathroom, and and open master suite. I am approximating the master suite has about 4,000 cubic feet of space in in. (Did my best estimates to include roof pitch, knee walls, etc. dormers) I have added three supply runs to the upstairs. One is in the bathroom, and the other two near the outside walls of the upstairs but still in the heated space. I have one cold air return run in the middle of the room where my stairs go down. Trying to estimate how large i should make the vent for the cold air return. Should it be a standard size grill or should I make it larger to pull more. Is there any standard formula to determine correct grill sizing? If I have not supplied enough info, please ask away.

Appreciate any feedback.
 
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Old 04-24-06, 01:06 PM
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You need AT LEAST as much return as your supply.

Is you have 3 pipes, 6" round--then you need equivalent return air volume.

If you've got 100sq.in of new supply--then you need at least 100sq.in. of return. 125--150sq.in. would be better. I always like the return to be as free-flowing and unrestricted as possible.
 
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Old 04-25-06, 12:53 PM
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Additional Question

Thanks for the reply. This may seem like a dumb question. Will there be any noticeable difference if I place a larger grate in the wall. My situation is that I have one return right in the middle of the room that goes along my stairwell. It is a 2 X 4 wall cavity that terminates in the basement and extends into my return for the furnace. Will having a larger opening in the wall there create any additional airflow or fill the space simply not allow for any more to be pushed through? Not really looking forward to rip open another wall cavity at this stage in the project.
 
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Old 04-27-06, 08:31 AM
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I don't think a larger grate will signicantly increase airflow into the return duct. Air flowrate in the return duct will be affected by friction losses of the duct material itself, it's diameter, it's length and how many bends/turns there are. Granted a grate size that is too small WILL affect the airflow, but making it significantly larger won't help the airflow.
 
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Old 04-27-06, 01:50 PM
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Thanks

Thanks much for the reply. I was expecting that to be the general answer. The good news is that it is the full wall space in between two 2X4's and only wallboard is on the outside of the cavity with nothing else. It is a bit wider than a standard rectangular duct so I might be a bit closer on my return needs.
 
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