HVACQuick Custom Square to Round Duct Transitions

MCB Series built-to-order square to round ceiling box transitions. Allows connections from rectangular grilles to round ducting.

• Available in a wide range of custom dimensions: round connections for 4, 5, 6, 8, 10, 12, or 14-inch diameter duct; rectangular dimensions from 6 to 24 inches in half-inch increments, depending on the circular diameter
• 0.030 galvanized steel or 0.040 aluminum construction depending on the size
• Some final assembly required
• Mounting brackets for easy installation are available as an optional accessory

Typical production time is approximately 1 week.  Please call us if you need one quicker or if you have a custom application.

Adjust Transition Parameters

Grille Sizing
Pricing and Model#
Note: These custom square to round transitions are made-to-order. They are NON-RETURNABLE. If you have questions, please call us at 877-711-4822 before ordering.


MCB square to round transition shown WITH OPTIONAL mounting brackets/bars:

square to round transition with bars

square to round transition with bars

MCB square to round transition shown with no optional mounting brackets/bars:

square to round transition with no bars

square to round transition with no bars

square to round transition with no bars


Do you have questions about the products on this page?

Click on the ? image on the left or this link to ask us your questions!
#1 Q:   Im looking for a tapered transition from 8 x 24 rectangle to 12 round duct. Is this possible?
A:   Sorry, but unfortunately we do not do tapered transitions like this.

       Asked on 2023-06-15 by JD
#2 Q:   can the mounting brackets be shipped loose so that they can be customized on site?
A:   They come unattached so the level may be done on site...

       Asked on 2023-02-13 by F
#3 Q:   Can these be made watertight? I see a bunch of screws in it so I am guessing not.
A:   They can be made airtight for typical hvac applications using tape or sealing caulk/putty but watertight no.

       Asked on 2021-07-01 by Graham
#4 Q:   Your description says for use in ceiling. Any reason this couldnt be used in a floor application?
A:   Floor application is fine as well.

       Asked on 2020-09-10 by Rob
#5 Q:   I need Square to Round Transition Box w23 L26 for 12 inch hose. Can you make it and what is the price? Thanks.
A:   We only go up to 24 inches on these transitions. Sorry.

       Asked on 2020-08-20 by Mykyta
#6 Q:   Do the mounting brackets come pre-attached? If so: would it be possible to have them attached so they are flush with the bottom of the unit?
A:   They do not come attached. We cannot change the location of the pilot holes for mounting them but you can drill your own pilot holes where it suits you.

       Asked on 2020-05-03 by William
#7 Q:   Hi, I am fabricating an hot air exhaust duct for an air compressor. The outlet is rectangular 4.6 x 19.6. I can go over on the narrow dimension but no room on the longer. Ideally, I would like a 6 x 19.5 rectangle x 12 round. Is there any way I can get to the increased round size? Thank you.
A:   The round connection is not flexible and cannot be ovalized. The max diameter is the smallest dimension which is 6 inch in your case. Sorry.

       Asked on 2020-04-16 by Greg
#8 Q:   Can there be more that one duct opening for use as a return air plenum on a mini split air handler?
A:   No. These are setup for a single exhaust point. Sorry.

       Asked on 2019-02-09 by Jeremy
#9 Q:   Do the X and Y dimensions include the thickness of the material? For instance, if I were to order a 20 by 20 transition, would I be able to fit a true 20 by 20 filter inside of it?
A:   Yes, these are outer dimensions so anything inside would have to be undercut. These are typically used with a filter grille that has the filter slots. The transitions themselves do not have filter holding racks/slots.

       Asked on 2018-09-13 by CS
#10 Q:   Trying to visualize this in a ceiling with an Airscape grille.Would the flanges sit above the sheet rock, and then the grille would screw through the sheet rock into the flanges (mounting brackets)? For example: a 14 X 14 square transition, with a 14 X 14 LIST sized grille... would the screw holes on the extra 1.75 of the grille line up with these flanges (mounting brackets)?
A:   The metal mounting brackets on the transition are screwed to the top of the wood/joists that frame out the opening. See the image in the Pictures tab showing screws going into a 2x4. The grill is typically screwed from below, through the drywall into the same wood framing pieces. Hope this helps.

       Asked on 2017-04-04 by Colorado

why buy from us?

We're engineers passionate about home ventilation and serious about customer service – so you can count on us for carefully-selected equipment and expert advice to ensure your project is a success from specification to installation.

Your call is answered by qualified people who will gladly discuss your project in detail, advise you professionally and never sell you inappropriate products.