Cold air return grille area
herringchoker
| Posted in General Questions on
Hi:
I’d like to make a wood grille for a 24 x 6-in cold air return duct. Welcome advice as to maximum area it should obstruct. No doubt as little as possible, but is there a rule-of-thumb?
Thanks!
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Replies
What really matters here is the amount of airflow in CFM that the grille needs to pass, not the amount of open aperture it has for a given total size. Do you have an idea of how much airflow you need to move through this vent?
Bill
A good first approximation is that the free area of the grill has to match the area of the duct that serves it. If you look at return boots, they're bigger than the duct going in. For example, a 12" round duct will use a 12x20 rectangular boot.
A circle 12" in diameter has an area of 113 square inches. A 12x20 rectangle is 240 square inches. The ratio of 113 to 240 is 47%, so the expectation is no more than 53% of the area of the boot is covered by the grill.
If you want to use a grill with higher coverage than that you can use a larger boot and larger grill to get the same opening.
I've found that even with equivalent net free area for air flow, the larger dividers of wooden grilles create more noise than stamped metal grilles. The larger you can make the return, the less you will hear it.
You can cut down on noise by using a router to round over the corners of the slats in the grille. Ideally, you want an airfoil shape, or at least a narrower profile on the intake side of the grille. Slats with square corners will make the most noise. A somewhat ideal arrangement would be to use wooden slats such as those used for window blinds, which tend to be pretty thin, with rounded edges.
Bill