A well-made flush vent does not noticeably hold back heat or AC in a normal room. Every cover, metal or wood, has less open area than the bare duct, and what matters is how much open area the slots leave and whether anything else is choking the air. Closed dampers, rugs, furniture and blocked returns cause far more airflow trouble than the vent face itself.
Why every vent cover narrows the duct a little
Air comes up the duct and leaves through whatever cover sits on top. A stamped metal register, a cast iron grille and a flush vent made from your floor all have solid parts between the openings, so none of them passes air as freely as an open hole. That's normal, and HVAC systems are designed with registers in place.
The useful question isn't whether a cover restricts air at all. It's how much open area it leaves compared with what you had before. A flush vent with long, evenly spaced slots running most of the length of the face can be close to a typical register. A cover with a few short, narrow cuts will not be.
Open area, explained
- Duct opening, 4 x 12 in48 sq inThe hole in the floor. This is the size you order by, not the outside of the old register.
- Open area (free area)Slots onlyThe combined area of the openings air actually passes through. Solid parts of the face, ribs and frame don't count.
- Percent openOpen / faceHow much of the vent face is opening. A higher number means air leaves at a lower speed for the same flow.
- Example: 20 slots, 3 x 1/4 in15 sq inA worked example of the method: slot count times length times width. It is not a spec for any particular vent.
- Air speedGoes upPush the same air through a smaller opening and it moves faster. Too fast shows up as a whistle or a draft you can feel from across the room.
Lower open area doesn't stop the air. The blower still pushes it through, only faster. That's why a restrictive cover usually shows up as noise or a pinpoint draft before it shows up as a room that won't heat.
Supply vents vs cold air returns
Your system has two kinds of floor openings, and they don't behave the same way when you put a cover on them.
Supply vent
- What it does
- Blows heated or cooled air from the furnace or air handler into the room.
- Typical size
- Small: 2, 4 or 6 inches wide and 10 to 14 inches long.
- When it's restricted
- The room runs warm or cold compared with the rest of the house, or the vent whistles.
- What helps
- Keep the damper open, keep rugs and furniture off it, and keep the slots clean.
Cold air return
- What it does
- Pulls room air back to the system so it can be filtered and reheated or cooled.
- Typical size
- Large: 6 or 8 inches wide and 20 to 36 inches long, because the system pulls all of its air back through fewer openings.
- When it's restricted
- The whole system works harder, rooms near the return feel stuffy, and you may hear the air rushing.
- What helps
- Never cover it. Use a grille with a lot of open area, such as an egg-crate pattern.
Returns are where open area matters most. A supply vent that's a little restrictive affects one room. A choked return affects the whole system. Our guide to matching cold air return covers covers sizes and the egg-crate pattern in more detail.
How to tell if a vent is really holding back air
- Open the damper all the wayMany registers have a lever or thumbwheel under the face. A half-closed damper restricts far more than the slots do. Check it before you blame the vent.
- Feel and listenWith the system running, hold your hand over the vent and compare it with a similar vent in another room. A strong whistle or a hiss usually means air is squeezing through too little opening or a partly closed damper.
- Clear what's on topRugs, beds, sofas and floor-length curtains block the air before it can mix into the room. See whether you can put furniture over floor vents for the details.
- Look down the ductLift the vent out and use a flashlight. Construction debris, a dropped toy or a crushed flex duct below the floor will cut airflow no matter what cover sits on top.
- Check the returns and the filterA dirty filter or a blocked cold air return starves every supply vent in the house at once. If several rooms are weak, start here.
- Do the open-area math if you're comparing coversCount the slots, multiply by their length and width, and compare that number with the cover you're replacing. Comparing like for like tells you more than any label.
What keeps heat and AC flowing
- Open area matters more than looks
- Dampers fully open
- Nothing on top of the vent
- Clean slots and duct
- Returns never covered
- Clean HVAC filter
Where matching flush vents fit in
Vents.com builds flush vents from a piece of your own hardwood, LVP or LVT, so the vent sits level with the floor and matches it. The slots run across the face of the vent, and the vent sits in a frame set into the duct opening, level with the finished floor. Order by the duct opening size, keep the damper and slots clear, and the vent does the same job as the register it replaces without the raised metal edge.
If you're replacing an older register and want to compare, measure its slots the way the example above shows. Questions about a specific size are welcome through our contact page.
Frequently asked questions
Do flush floor vents restrict airflow more than metal registers?
Not inherently. Both have less open area than the duct. A flush vent with long, evenly spaced slots can pass about as much air as a standard stamped register. Compare the open area of the two covers, not the material.
Will a matching wood or vinyl vent make my room colder in winter?
Not if the slots leave enough open area and the damper is open. If a room was already cold before the new floor went in, the cause is usually the duct run, the damper or a blocked return, not the cover.
Why does my floor vent whistle?
Air is moving fast through a small opening. A partly closed damper, debris in the duct or a cover with very little open area are the common reasons. Opening the damper fully often fixes it.
Should I close vents in rooms I don't use?
Closing a few supply vents rarely saves much energy in a forced-air system and can raise pressure in the ducts. If you do it, close only a few and never close or cover a cold air return.
Do cold air returns need more open area than supply vents?
Yes. Returns are larger because the system pulls its air back through fewer openings. That is why returns use high-open-area patterns such as egg-crate grilles.
What sizes do matching cold air returns come in?
Vents.com builds matching returns in 6x20, 6x24, 6x30, 6x36, 8x20, 8x24, 8x30 and 8x36 inches.
Related guides
- Do vent covers affect HVAC efficiency?How blocked, closed or undersized covers make your system work harder.
- Matching cold air return coversEgg-crate returns made from your LVP, LVT or hardwood, in all 8 sizes.
- Can you put furniture over floor vents?What blocking a vent does to airflow, and what to do instead.
- Floor vent sizes and how to measureA size chart, how to measure the duct opening, and the sizes we build most.
- Flush vents on floating LVP floorsHow a flush vent fits around the duct without pinning a floating floor.
- How to clean vent coversHow to clean floor, wall and ceiling vents, including wood and vinyl flush vents.