ZOEYES 4 PCS 4 Inch Duct Connector Flange Inlet Fitting Metal Straight Pipe Flange Ventilation Pipe Connector for Heating Cooling Ventilation HVAC System（Silvery Gray）

A simple, sturdy way to add 4-inch takeoffs

I put these 4-inch duct flanges to work on a couple of small ventilation projects: adding a new branch to a basement return and outfitting a plywood plenum for a DIY inline fan setup. In both cases, the flanges did exactly what I want a takeoff to do—create a rigid, clean, and reasonably airtight transition from a flat surface to 4-inch duct—without fuss.

Build quality and finish

These flanges are stamped from iron and finished in a silvery gray baked enamel. In the hand, they feel rigid and consistent—no tinny flex or odd weld seams to work around. The finish is evenly applied and held up well to clamping and screw-driving during installation. It’s paint, not galvanization, so I was mindful around sharp corners and edges; a deep scrape will expose bare metal. That said, the enamel seems tough enough for typical HVAC use and adds a bit of corrosion resistance and a cleaner look than raw steel.

The dimensions are right where they need to be for 4-inch duct work. The mounting ring measures about 5.2 inches outside diameter, the throat opening is about 3.7 inches inside diameter, and the collar stands 2 inches tall. In practice, that 2-inch height gives enough bite for a worm-gear clamp around flexible duct without crowding. The pre-bored mounting holes save time—no need to pilot through steel—and they accepted my self-drilling sheet metal screws without complaint.

Edges are reasonably smooth for a stamped part. I ran a thumb lightly around the collar and mounting ring and didn’t find burrs that would snag flex duct. As with any metal fitting, a quick check and a swipe with a file if needed is a good habit, especially if you’re installing dozens.

Fit and compatibility

On the duct side, I used standard 4-inch flexible aluminum dryer duct, 4-inch insulated flex, and 4-inch semi-rigid. All three slipped over the collar, and a hose clamp cinched them down firmly. The 3.7-inch ID spec can be confusing at first glance—remember you’re sliding duct over the outside of the collar, not into it—so a 4-inch duct fits as expected. With semi-rigid, I found it helpful to gently flare the first inch of the duct for an easy start.

On the mounting side, the ring sits flat and seals up well to sheet metal, plywood, or MDF. The outer ring’s 5.2-inch footprint is modest enough that I could fit flanges fairly close together on a small plenum face, but do make sure you’ve got at least that much clear space on your mounting surface. If you’re retrofitting to existing round duct, you’ll want a different style (a saddle or draw collar) rather than this flat-mount flange.

Installation experience

The steps were straightforward:

• Trace the internal opening on the surface using the collar as a guide.
• Cut the hole: hole saw for wood, aviation snips for sheet metal.
• Dry-fit the flange, then run a bead of duct mastic or apply butyl/foam gasket tape under the ring for an airtight seal.
• Drive screws through the pre-bored holes into the substrate.
• Slide on duct, clamp, and tape as needed with UL 181 aluminum foil tape.

A couple of notes from the installs:

• The pre-bored holes are clean and de-burred; screws seated flat and didn’t walk.
• The collar height is sufficient for two clamps if you’re adding a sleeve or filter sandwich, but typically one clamp plus foil tape is enough.
• If you’re mounting to thin sheet metal (existing plenum or trunk), a backing ring or short run of rivets can help distribute load and stiffen the area, but on 26–28 gauge steel I didn’t find it strictly necessary.

I checked for leaks with a smoke pencil after sealing. With mastic under the flange ring and foil tape on the duct-to-collar connection, leakage was negligible. If you skip the mastic, expect minor edge leaks—about what you’d see with any un-gasketed takeoff.

Use cases that suit it

This style of flange is ideal when you’re transitioning from a flat panel to 4-inch duct:

• Dryer exhausts, bathroom fans, and range hood knockouts on custom-built panels.
• Hydroponics tents or boxes where you want a rigid pass-through for inline fans.
• Small branch additions off a custom plenum or filter box.
• Shop air and fume extraction from benchtop enclosures.

It’s less appropriate for:

• Saddling onto round trunks (use a saddle takeoff or a draw band collar).
• High-temperature or combustion venting where a listed/galvanized fitting is required by code.
• Exterior penetrations exposed to weather, unless you’re building a protected housing and sealing/painting edges thoroughly.

Performance and durability

There are no moving parts here, so performance is about airflow resistance and sealing. The collar throat is unobstructed and transitions cleanly from panel to duct. With a proper seal, I didn’t observe measurable static pressure penalties beyond the inherent impact of adding a branch and a fitting. The enamel finish reduces the chance of surface rust in damp basements or bathrooms, but I’d still avoid scratched or raw edges in humid environments. If you do mar the finish, a dab of primer and paint or foil tape at the scratch is a simple insurance policy.

Noise transmission is more about your system than the flange itself. A rigid connection can pass vibration into a box or plenum, so a thin foam gasket under the ring or butyl tape is a smart add if your fan is hard-mounted.

What I like

• Solid, consistent stamping with a tough enamel finish that resists scuffs.
• True-to-size collar that works with common 4-inch flex, semi-rigid, and clamps.
• Pre-bored mounting holes that speed installation and reduce slipped screws.
• Practical 2-inch collar height that gives a secure clamping surface without protruding excessively.
• Four-pack quantity makes sense for small projects and keeps per-flange cost down.

Where it could be better

• The painted finish can chip if you drag a clamp over the edge; a slightly radiused lip would help. Work around this by loosening clamps fully before placement and tightening in place rather than sliding.
• No gasket included. I prefer adding butyl or a foam ring for an airtight seal; it’s an extra step and material to source.
• Iron with enamel is fine indoors, but for damp, coastal, or exterior-adjacent use, a galvanized or stainless option would be more carefree long-term.
• The 2-inch collar is adequate, but if you rely on double-clamp terminations or want a long barbed collar, you’ll want a dedicated long-collar takeoff.

Tips for best results

• Use duct mastic under the mounting ring for a reliable seal, not just caulk. It remains flexible and bonds well to painted metal and wood.
• Deburr your cutout and the flange edge if needed to protect flex duct.
• If you’re connecting insulated flex, strip back insulation and vapor barrier cleanly, clamp to the bare collar, then pull the insulation back and tape the vapor barrier to the ring for a neat, insulated termination.
• Pre-plan screw locations so they land in solid material; on thin sheet metal, consider rivets for a low-profile, rattle-free attachment.

The bottom line

These 4-inch duct flanges are exactly what I look for in basic HVAC hardware: true sizing, clean finish, and no surprises during installation. They make it easy to add a sturdy, airtight 4-inch takeoff to a flat surface, and the four-pack is practical for typical home ventilation projects.

Recommendation: I recommend these flanges for anyone building or modifying small ventilation runs—bath fans, dryer hookups, range hood transitions, and DIY plenum work—where a flat-mount 4-inch takeoff is appropriate. They’re sturdy, accurately sized, easy to install, and finished well enough to resist the bumps and abrasions of a typical install. If you need a listed fitting for combustion venting, exterior exposure, or a saddle for round trunks, choose a more specialized part; otherwise, these are a dependable, good-value staple to keep in the shop.