I am building a custom shelving unit that will sit over an inconveniently located floor vent/diffuser. My thought is to raise the bottom shelf up on stubby legs so that it does not block the vent. The question is, how tall do these legs need to be to gain sufficient clearance?

The register itself is a standard 4" x 10" opening. It seems to me that the space I am creating under the shelf is functionally equivalent to if I were extending the vent with a 90 degree turn. By this logic, I believe I should only need 4" of vertical clearance under my shelf. As long as I do not create a bottleneck that is any smaller than the cross-sectional area of the vent itself, it should be fine, right?

Is this reasoning correct? I've read various recommendations, some saying that I need as much as 18" clearance. This seems like ridiculous overkill to me.

  • 2
    You could always get a rounded 90, put it on the vent, and blow the air out from under the shelf across the floor instead of vertically.
    – stevieb
    Dec 13 '18 at 19:43

You don't need nearly 4". You can basically take the width and height of the gap on three, or even four sides of the floor vent into consideration, depending on how much of the undershelf area is open to the room. The air that comes through the grate can distribute to the sides as well as the front, and eventually make its way out from under the shelf.

Air movement looks something like this (assuming that that shelving is tight to the wall):

|               *               *               |
|               * ############# *               |
|               * ############# *               |
|               *****************               | 

| : shelf perimeter
* : outflow area perimeter
# : vent area

The math looks like this:

original vent area (disregarding the grille) [av]: 40 square inches
distance between the vent and the wall (hypothetical) [dv]: 2"
outflow area perimeter length [op]: 2 * (4 + dv) + 10 = 22

required height: av / op ≈ 1.8 (1-13/16) inches

With that in mind, an inch may be enough if the vent is further from the wall than I've assumed, or if the rear of the shelf is open to the room (up behind the shelving unit). It would be different if it was a return, where vacuum pressure is low, where resistance due to airflow path bends is a more significant factor.

If the shelving unit has side panels that reach to the floor, you can simply calculate the entire frontal area of the void below the bottom shelf. If it's over 40 square inches, you're probably golden. The exception would be if the vent is very close to the wall, wherein you'd go back to the math above.

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