Vent Systems 12" Galvanized Steel Duct End Cap - Dryer Vent Cap - Round Metal Plug - Metal Duct Fitting - Reusable Tee Cap

Why I reached for this duct cap

Capping an unused duct run shouldn’t take a whole afternoon or a bag of specialty hardware. I picked up the Vent Systems 12-inch duct cap to shutter an abandoned branch on a basement trunk line and later used a second one to temporarily seal a dryer stub during a remodel. In both cases, I wanted something that seated cleanly, held snugly without fuss, and could be made airtight without having to wrestle with oddball fittings. This cap did the job with minimal drama.

Build and materials

The cap is galvanized steel with a shallow, 2.2-inch flange that acts like a tapered plug. It feels appropriately rigid for its size—not flimsy, not overly heavy—and the bead/groove around the side adds a bit of stiffness and helps guide insertion. Galvanizing is to a C2 corrosion-resistance class, which in practice means it’s fine for indoor HVAC and sheltered outdoor spots in mild environments. It’s not a marine-grade part and it’s not a chimney cap; it’s made for air ducts.

Edges are typical for stamped sheet metal: not razor sharp, but you’ll still want gloves during install. Mine arrived true and round with no obvious warping or out-of-round issues.

Fit and sizing: the part most people trip over

Here’s the key: this is a plug-style cap designed to insert into a female duct end of nominal 12 inches. The listed diameter is about 12.4 inches, which corresponds to the real-world fit of 12-inch ductwork and fittings. If your duct terminates in a male (crimped) end, this cap won’t slip over it; it needs to go into a female end.

How I check fit:
- Identify the duct end. If it’s crimped, that’s male. If the opening is smooth and meant to receive, that’s female.
- Measure the inside diameter of the female end (for 12-inch nominal, you should be right around that size).
- If you only have a male end, add a short female coupling, or expand the duct with a hand seamer. You can also cut back to the nearest female fitting if accessible.

On a straight, female 12-inch takeoff, the cap seated snugly without modification. On a different run that ended male, I slipped a short female coupling over the duct and the cap then inserted as expected. Trying to force the cap over a male end is where frustration starts; it’s not designed for that.

Installation in practice

The actual install took minutes:
1. Clean the duct lip. Dust or paint overspray can make insertion sloppy or gritty.
2. Test-fit the cap. The side groove helps align and start the flange. A gentle twist rocks it past any tight spots.
3. Seal. For a removable but airtight seal, I used water-based duct mastic around the seam and smoothed it with a gloved finger. Foil tape also works; just clean the surface first. For a permanent cap, a trio of 1/2-inch sheet-metal screws around the circumference is overkill but absolutely secure.
4. Verify. With the system running, I checked for whistling or measurable air loss. With mastic, it was silent and tight.

On the dryer stub, I used foil tape alone because I knew I’d remove the cap in a few weeks. No screws were needed; the friction fit held fine.

Note: there’s no rubber gasket included. If you want a compressible seal, bring your own high-temp foam gasket or rely on mastic/tape.

Performance and sealing

Once seated, the cap behaves like a factory plug. In a simple pressure test on the HVAC branch (static pressure under 0.5 in. w.c.), there was no detectable leakage with mastic. Even dry-fit without sealant, the friction fit was decent, but you’ll want either mastic or tape for long-term air sealing and efficiency.

The 2.2-inch flange depth offers enough engagement to feel secure without protruding far into the airflow when used on active systems (for example, if you’re temporarily balancing a run during commissioning). For long-term abandonment of a branch, I prefer to seal at the takeoff, and this cap makes that straightforward.

Durability and corrosion

C2-class galvanizing is appropriate for most basements, attics, and conditioned spaces. I also tested one on an exterior wall stub under an overhang for a couple of rainy weeks; no discoloration or surface oxidation showed up. If you’re near the coast or in an industrial environment with corrosive atmospheres, I’d step up to a heavier coating or stainless where feasible. For typical residential HVAC, this finish is fine.

The cap handled light handling knocks without bending out of round. If you do distort it, a hand seamer or gentle rebend gets it back into shape easily.

Where it shines—and where it doesn’t

Best uses I’ve found:
- Capping an abandoned or seasonal branch to stop energy losses.
- Temporarily sealing stubs during remodeling to keep dust and odors out of the system.
- Blocking rodent or insect access through unused interior duct ports (paired with proper exterior terminations).

What it’s not for:
- Active combustion appliances or chimneys. Don’t use this in place of a listed flue termination or on hot flue piping.
- Going over male duct ends. If your line ends in a crimp, plan on a female coupler or rework the end.

A quick safety note: never cap an active exhaust like a dryer or fuel-fired appliance. Only seal unused lines or do temporary closures with the appliance disconnected.

Tips for a clean install

• Dry-fit before sealing. If it feels too tight, check that the duct isn’t out-of-round. A few squeezes with a hand seamer can true it.
• Use mastic for invisible, long-term sealing. Foil tape is fine for temporary work or when you plan to remove the cap later.
• If you need the cap removable, run a thin bead of mastic and skip screws. If you want it permanent, three short screws plus mastic is airtight and rock solid.
• Label the cap with a marker so future you (or the next owner) knows what was capped and why.

Build quality odds and ends

• The side groove does help guide insertion; it’s not a locking feature, but it prevents the edge from catching.
• No fasteners or gasket are included. That’s normal for this category, but plan ahead.
• Roundness was excellent on the unit I used; cheaper caps can arrive ovalized. This one didn’t.

Value and alternatives

There are two basic styles for this job: inside plug caps like this one, and outside slip-on caps that go over the duct. Inside plugs are cleaner and typically seal better with mastic, but they demand a female duct end. If all you have is a male crimp and you don’t want to add a coupling, an outside cap may be simpler. For coastal installs or exposed rooftops, I’d consider a heavier-gauge or stainless option.

Pros

• Snug, reliable friction fit on female 12-inch duct
• Galvanized steel with sensible stiffness and a clean stamp
• Quick install; removable if you use tape or a small bead of mastic
• Easy to seal fully airtight
• Suitable for indoor and mild outdoor conditions

Cons

• No gasket included
• Won’t slip over male (crimped) duct ends; may require a coupling
• C2 corrosion class isn’t ideal for harsh or coastal environments

Recommendation

I recommend the Vent Systems 12-inch duct cap for anyone needing a straightforward, durable plug for unused HVAC runs or temporary closures during remodels. It seats cleanly in a proper female duct end, seals up airtight with mastic or foil tape, and stays put without resorting to an array of screws. Just be mindful of the fitment: if your duct ends male, plan on adding a short female coupling or reworking the end. For typical residential use, the build quality, ease of install, and dependable seal make this a solid, no-nonsense choice. If you’re working in a coastal environment or on hot flues, look to more specialized materials—but for standard HVAC air handling, this cap does exactly what it should with minimal hassle.