FKUMLUN 6 Inch Duct Connector Flange, Metal Straight Pipe Flange Square to Round Interface Air Duct for Heating Cooling Ventilation HVAC System,whit Adjustable Hose Clamp

Why I Reached for This 6-inch Flange

A recent retrofit in my workshop required tying a 6-inch round flex duct into a square cutout on a plywood return box. I didn’t need a full transition boot or a bulky register box—just a clean, square-to-round interface I could screw down, seal, and forget. That’s exactly why I picked up this 6-inch flange. It’s a simple piece of galvanized hardware with a square mounting plate, a round stub for duct, and an included hose clamp. No gimmicks, no moving parts—just the right shape, finish, and fastener layout to make a common HVAC connection straightforward.

Build Quality and Design

The flange is made from galvanized steel with a smooth finish that fends off corrosion in typical interior settings like bathrooms, kitchens, and garages. The plate feels stiff enough for the task; it won’t fold up in your hands, but like most sheet-metal connectors, it will deform if you crank down on it with pliers. The round stub has a consistent diameter and accepts both 6-inch flex duct and standard 6-inch rigid pipe. The lip is formed cleanly—no awkward ripples that make duct fitting a fight—and the edges were reasonably deburred. I still wore gloves during handling because galvanized edges can surprise you.

Pre-drilled holes on the square plate are a nice touch. They’re laid out in a way that makes sense for mounting to wood, a wall surface, or thin sheet-metal plenums. I would have liked to see a neoprene gasket or foam strip included, but at this price point, bare steel with holes is what I expect. The included adjustable hose clamp fits typical 6-inch flex duct. It’s the standard band-and-screw design; it did its job without slipping, though if you routinely work with thicker, insulated duct, keep a longer clamp on hand.

Installation Experience

From unboxing to sealed connection, this was about a 20–30 minute task, most of that spent on prep and sealing. Here’s how it went:

• I squared up the opening and dry-fit the plate to check hole alignment and coverage. The flange’s footprint gave me just enough room for screws while still keeping the transition tidy.
• I ran a bead of construction sealant around the square opening (butyl or polyurethane works well) and set the plate in place. The pre-drilled holes accepted #8 sheet-metal screws without fuss. Into plywood or studs, wood screws are fine; on thin metal, I prefer self-tapping hex-heads.
• For the round connection, I slid the flex duct over the stub, tightened the clamp with a nut driver, then sealed the joint with mastic and foil tape. If you’re running insulated flex, strip back the insulation and jacket before clamping, then re-tape the jacket to the flange plate for a neat finish.

Everything lined up and tightened down cleanly. No slotting or drilling new holes was required, though if you’re matching an existing pattern on a fan housing or a register box, verify spacing first. This isn’t a universal bolt pattern; it’s a general-purpose plate.

Fit and Compatibility

The round portion is sized correctly for 6-inch duct. Flex duct slid on without a wrestling match, and 6-inch snap-lock pipe also nested well. The clamp cinched to an airtight seal with mastic.

On the square side, the plate covers a modest opening with some wiggle room. Because the exact plate dimensions and hole spacing aren’t specified, measure your specific application before you commit. If you’re trying to hit factory holes on a particular fan housing or a metal plenum with a predefined pattern, you might need to punch one or two new holes in the flange—even so, the metal takes a drill bit easily.

This flange is best suited for straight-through transitions. If you need an offset or a direction change, consider a purpose-made transition boot. For through-wall terminations, the flange behaves like a mounting escutcheon; it keeps the installation clean and provides a solid mechanical connection for the duct.

Air Sealing and Performance

By itself, the flange is a structural connector. Airtightness depends on your sealing. With a bead of sealant under the plate and mastic plus foil tape at the round stub, leakage was negligible in my smoke test. I didn’t see any whistling or vibration at typical bathroom-fan or inline-fan static pressures.

Because the piece is a simple stub rather than a long, tapered transition, it doesn’t “improve” airflow—but it doesn’t get in the way either. Pressure drop across the connection is minimal and mainly dictated by the duct and attachments upstream and downstream.

Durability and Corrosion Resistance

Galvanized steel is the right choice for most indoor ventilation scenarios. In damp environments like bathrooms, it holds up well. In coastal or semi-exterior applications, I’d either add a protective coat (cold galvanizing spray or paint) or choose stainless components for peace of mind. The clamp is stainless-style banding with a typical screw housing; keep it out of direct weather and it should last.

Safety and Code Considerations

This flange is for ventilation air—not for flue gases, grease ducts, or high-temperature exhaust. Use appropriate listed components for those systems. If you need a fire-rated assembly or a UL-listed transition for a specific appliance, verify certifications before installation. I also like to use fire-blocking foam or mineral wool around wall penetrations where required by code and then treat the flange as the finish plate.

What I Liked

• Straightforward square-to-round interface that just works
• Galvanized finish and cleanly formed round stub
• Pre-drilled holes speed up installation
• Included clamp is convenient and fits standard 6-inch duct
• Takes sealant and tape well for airtight results

What Could Be Better

• No gasket or foam strip included for the plate-to-surface seal
• Plate dimensions and hole spacing aren’t specified; exact fit is on you
• Edges are still sheet-metal—gloves recommended
• Clamp is adequate, but insulated flex may need a longer band

Practical Tips

• Run a continuous bead of butyl or polyurethane sealant under the plate, then tool the squeeze-out for a neat fillet.
• Use mastic on the round joint, then finalize with UL 181 foil tape for a durable seal.
• Strip back insulation before clamping; re-wrap and tape the jacket to the plate for a clean finish.
• If you’re mounting to drywall, add blocking behind the opening so the screws bite solid wood.
• Support the duct within a foot or two of the flange to prevent sagging and stress on the plate.

Who It’s For

This flange suits DIYers and pros who need a quick, dependable way to bridge a 6-inch round duct to a square opening without a bulky transition. It’s perfect for bath fans, inline fans, small ERV/HRV hookups, and simple wall or plenum terminations. If your job needs a custom offset, a long taper, or certified fire-rated hardware, look to a different class of fittings.

Recommendation

I recommend this 6-inch flange. It’s a no-nonsense connector that installs quickly, seals up tightly with standard materials, and offers the right balance of rigidity and workability. The lack of a built-in gasket and unspecified hole pattern are minor drawbacks, easily mitigated with sealant and a drill if needed. For everyday HVAC ventilation tasks where you need a clean square-to-round interface, it hits the mark.