TVKJHG 3 1/4-Inch x 10-Inch to 4-Inch Diameter Galvanized Rectangular to Round Duct Transition Microwave Vent Kit Round Transition Vent Adapter for Connecting HVAC Range Hood Ducting Fittings

A practical fix for tight duct runs

I ran into this adapter during a small kitchen refresh where cabinet layout left me with limited space to route a vent line. The rectangular outlet on the hood was 3-1/4 x 10 inches, but the only available chase to the outside would accept a 4-inch round run. Rather than rebuild cabinetry, I tried this simple rectangular-to-round adapter. It’s a basic piece of galvanized sheet metal, but on jobs like this, the basics matter: dimensional accuracy, clean seams, and an interior that doesn’t turn airflow into a noisy mess.

Build and design

The adapter is exactly what it claims to be—3-1/4 x 10 inches on one side, 4 inches round on the other—pressed from galvanized steel. The finish is a straightforward silver galvanization that shrugs off fingerprints and minor scuffs. Seams are straight and tight on the unit I received; there were no sloppy gaps that would require excessive sealant. The interior is noticeably smoother than bargain-bin transitions I’ve used in the past, and that matters. Any ridges, steps, or crimp overlaps inside a duct become turbulence points and add static pressure. This piece keeps that to a minimum.

A few practical observations from the bench:
- Edges are reasonably deburred for shop-made sheet metal, but they’re still metal edges—gloves are a good idea.
- The 4-inch round collar accepts standard 4-inch rigid, semi-rigid, and many foil-lined flex ducts without drama.
- The rectangular opening is true to size and slips over a standard 3-1/4 x 10 takeoff or hood outlet as expected.

There’s no mounting hardware or gasket included. That’s typical at this price point, but plan to supply your own foil tape, mastic, screws, and/or clamps depending on your application.

Installation experience

I used the adapter in two scenarios: a bath fan upgrade that needed a rectangular transition at a plenum box, and the kitchen hood job mentioned above. In both cases, installation was straightforward.

My steps and tips:
1. Dry-fit both ends. Check that the rectangular face seats flat against the mating flange. If your mating surface is warped or painted, a thin bead of butyl tape or HVAC gasket helps.
2. Secure the rectangular side. I used four #8 x 1/2-inch self-drilling sheet-metal screws at the corners and mid-span, driving through the outer flange into the hood outlet/takeoff. Keep screw tips short so they don’t protrude into the airflow path.
3. Seal the joint. For a kitchen hood, I prefer mastic (water-based duct sealer) brushed around the seam and screw heads; for a bath fan, UL 181 foil tape is usually fine. The goal is an airtight seal.
4. Connect the round side. Slide the 4-inch duct over the round collar, use a stainless worm-drive clamp, and seal with foil tape. For dryer applications, skip screws on the round connection—protrusions can catch lint; rely on clamp and tape.
5. Support the run. Add hangers within a couple feet of the adapter so the piece isn’t carrying the weight of the duct.

The fit on both ends was snug without forcing. On the hood job, I appreciated that the adapter’s body isn’t overly long, which helped it tuck under a shallow cabinet bottom panel. Short is good for clearance, but remember that shorter transitions are also more abrupt, which impacts airflow (more on that below).

Performance and airflow

No adapter can cheat physics. A 3-1/4 x 10 rectangular opening has about 32.5 square inches of area; a 4-inch round duct has about 12.6 square inches. That’s a significant downsizing, and you’ll feel it on higher airflow systems.

Here’s how it played out for me:
- On a 90–110 CFM bath fan with a short run, the adapter didn’t cause any noticeable drop in performance. Airflow at the exterior hood measured within a few CFM of the fan’s spec, and noise didn’t increase.
- On a modest under-cabinet range hood rated around 200 CFM, the adapter worked, but the fan ran at a higher pitch than with a 6-inch round transition. The pressure increase from the downsizing is audible and will reduce effective CFM.
- I would not use this on high-output (300+ CFM) range hoods; you’ll choke the system and likely violate the manufacturer’s ducting requirements.

The smooth interior and clean taper help. The piece doesn’t add unnecessary turbulence beyond the inherent reduction. Sealed properly, I had no leaks on a smoke-pencil test. But if you’re trying to maintain high flow, opt for a 6-inch (or larger) round transition or stay with the full 3-1/4 x 10 rectangular run.

Where it fits (and where it doesn’t)

Good fits:
- Bath fans running 4-inch duct over short to medium distances.
- Dryer vent connections when you need to interface with a rectangular plenum or wall takeoff. Use smooth-walled duct and clamps; avoid interior screws on the round side.
- Tight soffits or cabinet cavities where the rectangular profile saves vertical space before transitioning to a 4-inch chase.

Use caution:
- Range hoods above ~200 CFM. Many hood manufacturers require 6-inch or 3-1/4 x 10 ducting minimum. Check the manual. Downsizing to 4 inches adds noise and reduces capture efficiency.
- Long duct runs with multiple elbows. The pressure penalty stacks fast; consider a larger round duct and a longer, gentler transition.

Durability and maintenance

Galvanized steel is the right material for this job. It resists rust in typical indoor or covered outdoor conditions. After cutting and handling, I had a few minor scratches at the edges; if you’re installing in a damp environment or near salt air, a quick touch with cold-galv spray on raw edges is cheap insurance.

For dryer use, keep connections smooth and check for lint buildup during your routine vent maintenance. The adapter’s interior is smooth enough that it won’t be the primary lint trap point, provided you avoid protruding fasteners.

What I’d change

• Include a thin foam or butyl gasket strip. It would make sealing the rectangular flange faster, especially on slightly uneven mating surfaces.
• Offer a long-taper version. A transition with more length reduces turbulence and pressure loss. Space is often tight, but having the option helps in custom installs.
• Clearly label airflow direction and end type. The collar is intuitive, but printed guidance helps DIY users get it right the first time.

Practical buying and install notes

• You’ll need: UL 181 foil tape, mastic (optional but recommended for hoods), four to six short sheet-metal screws, a worm-drive clamp for the 4-inch connection, gloves, and a drill/driver.
• Keep screws out of the airstream where codes require smooth interiors (dryers and many hoods). Use clamps and tape instead.
• If you can, transition to round as soon as you have vertical space, then upsize the round duct to match your appliance’s spec. Don’t force a 4-inch system to do a 6-inch system’s job.
• Always check your appliance manual and local code. Venting requirements are often specific and worth following to the letter.

The bottom line

This rectangular-to-round adapter is a nicely made, no-drama fitting that installs cleanly and seals up tight. The galvanized build feels sturdy, the interior is smoother than most budget transitions, and the dimensions are true. It’s an excellent choice for bath fans, dryer connections, and space-constrained runs that are already designed around 4-inch duct.

I recommend it with a clear caveat: use it where 4-inch duct is appropriate. If your range hood or exhaust system calls for 6-inch or higher, step up to the proper size and a longer transition. Within its intended envelope, this adapter does exactly what it should—save space, connect dissimilar duct shapes, and keep airflow moving without adding headaches.