Vent Glossary: HVAC Register & Grille Terms
Sizing charts and product pages throw around a lot of terms — register, grille, diffuser, net free area, CFM. This page defines each one in plain language, with the specific numbers that matter when you're choosing or sizing a vent for your home.
Core Distinctions
Vent
The general term for any opening that moves air into or out of a room. Register, grille, and diffuser are all types of vents; "vent" just means the category.
Grille
A cover with fixed openings (slats, mesh, or a decorative pattern) that lets air pass through without controlling the amount. Grilles have no damper — they're either fully open or covered, nothing in between. Most return air covers are grilles.
Register
A grille with a built-in damper (a lever or knob) that lets you open, close, or throttle the airflow. Registers are used almost exclusively on supply vents, where you may want to redirect or reduce airflow to a specific room.
Diffuser
An outlet designed to spread air in a pattern rather than let it pass straight through, so a room gets even airflow instead of one direct blast. Green Vent's linear slot diffusers use this design for supply air on floors, walls, and ceilings.
Louver
A single angled blade or fin. A grille or register is made up of multiple louvers arranged in a frame — the louver angle is what determines how much the airflow gets redirected versus how much it stays hidden from view.
Damper
The adjustable part inside a register — usually a lever you move by hand — that opens, closes, or partially blocks airflow. Damper is a part, not a product: it's the mechanism, register is the vent it's built into.
Supply vs. Return
Supply Air
Conditioned air being pushed out of your HVAC system into a room. Supply vents are almost always registers, since you often want to redirect or reduce airflow into a specific space.
Return Air
Air being pulled back into your HVAC system from a room, to be reconditioned and recirculated. Return vents are grilles, not registers — a damper on a return would restrict the system's air intake, which the system needs to stay fully open at all times.
Supply Register / Supply Grille
A vent installed on the supply side of the duct system. "Supply register" is the more common term since supply vents almost always include a damper.
Return Air Grille
A grille sized and placed to pull air back to the HVAC system. Green Vent's aluminum return air grilles come in sizes from 6×4 to 14×6, with an optional washable mesh pre-filter.
Return Air Filter Grille
A return air grille built with a removable, washable filter behind the face — it catches dust before it reaches your system's main filter, without replacing it.
Sizing Terms (the numbers that matter)
Duct Opening
The actual hole in your wall, floor, or ceiling — not the faceplate of your old cover. Every Green Vent size is named after the duct opening it fits (a 10x4 cover fits a 10" × 4" opening). See the full sizing guide for the four-step measuring process.
Face Plate
The visible frame of a vent cover, which sits on top of the duct opening and overlaps it by roughly ¾" on each side so the rough edge of the hole disappears. The face plate is always larger than the duct opening — this is the most common source of ordering the wrong size.
Net Free Area (NFA)
The actual open area air can pass through, after subtracting the frame, louvers, and any filter mesh. A grille's net free area is typically 70–85% of its visible face size, not the full face size — the tighter the louver spacing or filter mesh, the lower the percentage.
Face Velocity
How fast air moves across the full face of a vent, measured in feet per minute (fpm). Return air grilles are generally sized for around 500 fpm; pushing air faster than that (600–800 fpm) moves more volume through a smaller grille but increases noise.
CFM (Cubic Feet per Minute)
The unit HVAC systems use to measure airflow volume. To size a grille for a target CFM: grille area (in²) = CFM ÷ (face velocity × net free area %) × 144. Your HVAC technician or system documentation can tell you the CFM for a given room.
Boot / Register Box
The metal duct transition box behind the wall, floor, or ceiling opening that the grille or register frame attaches to. The boot determines the duct opening size — it's fixed in place, so it's what you measure, not the cover you're replacing.
Placement
Wall Vent Cover / Wall Register
Installed on a vertical wall surface, at any height. Common for both supply and return, in living rooms and hallways especially.
Ceiling Vent Cover / Ceiling Register
Installed in a ceiling. Ceiling placement is common for return air, since warm air naturally rises toward it.
Floor Vent Cover / Floor Register
Installed in the floor, always walkable. Floor placement works well in colder climates, where drawing air from floor level improves heating efficiency — but floor vents also collect more dust, so a washable filter is worth considering.
Materials
Aluminum (Powder-Coated)
Green Vent's primary material. Aluminum resists rust and corrosion in humid rooms like bathrooms and basements, and holds its shape better than thin steel under everyday foot traffic or vacuum bumps.
Steel Vent Cover
A heavier, lower-cost alternative to aluminum. Steel covers are more prone to rust in moisture-prone rooms and dent more easily than aluminum, but cost less upfront.
Brass Floor Register
A decorative floor register finish, typically chosen to match existing brass hardware or older-home interiors.
Green Vent Product Types
Linear Slot Diffuser
A modern, minimalist supply diffuser design using one or more narrow slots instead of a traditional louvered face. Works on floors, walls, and ceilings; Green Vent's linear slot diffuser line runs 1 to 4 slots depending on size.
Linear Slot Diffuser — S-Type
A slimmer, trimless version of the linear slot diffuser, designed for a flush, low-profile look.
Eggcrate Grille (Cube Core)
A grille made of a fine square-cell grid, giving high net free area and an even, low-restriction airflow pattern. Common for both supply and return.
2-Way Floor Register
A floor register with a damper that directs airflow toward two sides instead of straight up, useful for redirecting air away from furniture or walkways.
Washable Mesh Pre-Filter
A removable mesh filter fitted behind a return air grille. It's rinseable with warm water and catches dust before it reaches your system's main filter — it supplements, not replaces, that filter.
Decorative Vent Cover
Any vent cover chosen primarily for its finish or pattern (color, material, or design) in addition to its airflow function — the term describes a use case, not a distinct product category.
Terms Green Vent Doesn't Sell (so you don't order the wrong thing)
Transfer Grille
A grille installed in a door or interior wall to let air pass between rooms without ductwork, not over a duct opening. Green Vent's registers and grilles are sized for duct openings, not door transfers.
Gable Vent
A vent installed in an attic gable or exterior wall for passive ventilation, protecting a home from trapped moisture and pests. This is a building-ventilation product, separate from the supply/return duct vents covered elsewhere on this page.
Magnetic Vent Cover
A cover that clips on with magnets to fully block airflow, typically used to seal an unused vent. Return vents should never be fully blocked — doing so restricts the system's air intake and can strain the HVAC blower.
Exhaust Fan Cover
The grille over a bathroom or kitchen exhaust fan, connected to its own fan motor rather than the central HVAC duct system. Sizing and installation differ from a standard register or grille.
Measuring for a replacement? Start with your duct opening, not the old cover's faceplate — the full walkthrough is in the sizing guide.
