first time poster here. I'm currently replacing the wall registers throughout my house. Each opening is a standard size except for one, so I'm looking for feedback on my plan to effectively decease the size of this uncommon register boot from 6" x 17.5" to 6" x 14" (which is the closest size available in the registers I'd like to purchase). The register is fed from a 9" diameter duct connected to a heat pump which provides both heating and cooling throughout the house. This is a ~275 sq ft room with two registers; the other register in the room is 6"x14" and fed through the same ductwork as the one I'm looking to alter. I'd like to avoid removing the duct boot if possible.

I understand that downsizing a register can have negative implications on the system as a whole and cause excess noise, though discussion on this thread shows that it's been done before successfully when the size of the new duct is still large enough to handle the airflow. To that end, I've covered 3.5 inches of the register with cardboard for the past few days and haven't noticed any differences anywhere in the house.

So first question: is there anything else I should be considering before making this change?

And second question: how would you go about it?

As you can see in the photo below, the ductwork comes in on the left side of the opening. So my current plan is to cut down some scrap 2x4 to fill the rightmost 3.5" inches of the duct boot, line the left side with aluminum to effectively create a smaller boot, secure it in place somehow, and then put joint compound on the exterior facing side, paint it, and then install the 6x14 register.


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  • Would a pair of 6" x 8" registers fit side-by-side? If they do then all you'd need is a vertical support in the middle.
    – brhans
    Mar 24, 2022 at 0:33
  • 1
    since the supply line is much smaller than the existing opening i wouldn't worry too much about a slight reduction in grille size. I'm not sure why the cove would be so large, maybe a good hiding place?
    – dandavis
    Mar 24, 2022 at 19:28

3 Answers 3


Treat it as a drywall repair.

There's probably no need to fill or wall-off the extra space, but if you want to do that spray foam is the easy answer. next easiest is sheet metal held in with self-drilling screws (if you get the metal cut and bent at a sheet metal shop, or use a thin grade of aluminium that can be bent by hand) you'll probably need a angle attachment on your drill-driver.

However the main part is drywall repair. so step one it to get a patch piece and anchor it in place, (it might be easiest to to cut back the existing drywall a bit to do that)

Attaching the drywall patch to metal boot seems bass-ackwards it needs to connect to the drywall.


You are going to spend money on adjusting the vent opening and then refinishing it so it looks finished… Or you could just buy custom wood or metal grilles.

Try this place, but you can just Google it:


  • You could just put the next larger standard size grill!
    – Gil
    Apr 23, 2022 at 5:15

Rather than put cardboard or wood in your vent, I would cover that hole with a custom vent cover using sheet metal, sheet metal screws and air vent caulk.

Perhaps a sheet metal company locally could make a transition size for you or you could cover it with a metal cover with folded sides on 3 sides.

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