Custom Duct Reducers, Made to Order in Any Size

Custom Rectangular Duct Reducers: The Complete Guide

A duct reducer, also called a trunk reducer, a step-down, a concentric transition or a taper, is the fitting that steps a rectangular duct down to a smaller rectangular duct over a short tapered length, with the smaller end centered on the larger one. It is the fitting that makes an extended-plenum trunk work: every time a branch takes air off the trunk, the trunk has less air to carry and should get smaller, so that the velocity and the static pressure stay where the last register needs them. A well-designed trunk has a reducer every few branches, which is why a single house can need five or six of them in sizes no supply house stocks. We build every reducer to the exact numbers you type in, and this page explains how to pick those numbers.

Everything below is what we tell customers on the phone: what the fitting is, how it differs from a transition, where it is used, how much to reduce after each branch, how long the taper should be, which gauge and material to specify, how the ends connect, and how to measure the two openings you have. If you already know your numbers, the designer above prices them instantly and the popular-sizes table links to a page for each common size.

What a duct reducer actually is

Geometrically the fitting is a rectangle at one end and a smaller rectangle at the other, joined by four flat trapezoidal walls that slope inward. When both dimensions get smaller all four walls slope; when only the width changes, the two side walls slope and the top and bottom are straight. Our designer builds the outlet centered on the inlet, so the taper on each dimension is shared equally between the two opposite walls; that is a concentric reducer, and it is what nearly every trunk step-down needs. The walls are laid out flat from the five numbers you enter, cut, formed, and joined with locked seams, so the two openings are square and true to each other and the taper is continuous from end to end.

The five numbers that define the fitting are the inlet width and depth, the outlet width and depth, and the overall length from the inlet face to the outlet face. Both ends are finished with the connection you specify, slip-and-drive being the default. Install the reducer with the large end upstream. The same part turned around is an expansion, which is legitimate on a return trunk that gathers air as it goes, but an expanding taper should be gentler than a reducing one; if that is your case, the transition page and its calculator handle the expanding rule.

Reducer versus transition

We use reducer for the concentric case: the outlet is smaller than the inlet in one or both dimensions, centered, with a symmetrical taper. We use transition for the general rectangular size change, which includes one dimension growing while the other shrinks (a 24" x 8" trunk reshaped to 16" x 12" for a chase) and includes expansions. Both are the same construction from the same five numbers, and the same shop builds both. The practical difference on this site is the defaults: the reducer sizes are published at 12" and 18" lengths, because a reducer usually sits in a trunk where there is room for a gentle taper, while the transition sizes are published at 8" and 12" for equipment connections where space is short. A square to round is a different fitting again, changing the cross-section from four sides to a circle.

Where reducers are used

• Trunk step-downs. The extended plenum trunk drops from 24" x 8" to 20" x 8" to 16" x 8" to 12" x 8" as branches take air off it. Each step is a reducer in the same depth, tapering on the width only. This is the reducer on most orders.
• Reducing in both dimensions. A 20" x 16" main dropping to 16" x 12" where the duct leaves the mechanical room, or a 12" x 12" riser becoming 10" x 10" on the upper floor.
• Equipment and plenum outlets. A 24" x 24" supply plenum reducing to the 24" x 16" trunk that leaves it. Where the change is large, use the longer 18" taper.
• Branch takeoffs. A short rectangular branch reduced from the collar size to the register box size before the last elbow.
• Commercial low-pressure duct. Any place a rectangular main steps down on an engineered drawing, the drawing calls out the reducer by inlet, outlet and length, and this fitting is exactly that.

How much to reduce after each branch

The trunk should get smaller so that the velocity stays roughly constant along its length. Velocity is airflow divided by free area, so the size the trunk wants after a set of branches is the remaining CFM divided by the velocity you are designing to, converted to square inches. Some reference values at about 900 feet per minute, which is a comfortable residential trunk velocity:

In practice the trunk is stepped in two-inch or four-inch increments of width, and a reducer goes in every time the remaining airflow has dropped enough to justify the next step. Reducing too little leaves the far end of the trunk slow and starves the last registers of static pressure; reducing too much makes the trunk fast and loud. Enter the downstream CFM in the calculator at the top of this page and it shows the velocity at the outlet, so you can see which side of that line you are on.

How long the taper should be

Any change in duct size should happen gradually, or the air separates from the walls, the fitting adds static pressure, and the register at the end of the run gets noisier. The guideline for a reducer is that no wall should slope more than about 15 degrees from the duct axis (a 30 degree included angle). Convert that to a length and the minimum is

L_min = (largest change in either dimension) ÷ 2 × 3.7

So a 20" x 12" to 16" x 12" reducer, changing 4" in width, needs about 7.5"; a 24" x 16" to 18" x 12", changing 6" in width, needs about 11.25"; a 24" x 24" to 16" x 16", changing 8" in both, needs about 15". Twelve inches is our default length because it satisfies the rule for nearly every trunk step-down, and the 18" length is published for the larger plenum and main reductions. Where space is tight, a shorter, steeper reducer still works; it just costs a little more static pressure. Where you have room, longer is always quieter. The calculator flags a length shorter than the 15 degree minimum so you can decide with the number in front of you.

Gauge

Sheet-metal gauge for low-pressure duct is set by the largest dimension of the fitting. The size pages on this site default to the gauge that a two-inch-water-column residential or light-commercial system would use for the inlet: 26 gauge for inlets up to about 14" on the long side, 24 gauge to about 26", 22 gauge to about 40", and 20 gauge above that. Because the walls of a reducer are trapezoids joined at four corners over a short length, the fitting is stiffer than a straight duct of the same gauge and size, so these defaults are conservative. Step up one gauge for kitchen exhaust, dust collection, outdoor or rooftop work, anywhere the fitting will be walked on or leaned on, or any medium-pressure system. Every size page lets you change gauge in the designer and the price updates on the spot.

Galvanized, aluminum or stainless

Galvanized steel is the standard for heating and cooling duct and is what nearly every reducer we build is made of. It is the least expensive, it takes duct sealant and tape well, and it matches the rest of the system. Aluminum weighs about a third as much, does not rust, and is specified for pool and spa air handlers, coastal installations, some laboratory exhaust, and anywhere a fitting has to be lifted into place by hand. Stainless steel is the material for commercial kitchen grease exhaust, chemical and process exhaust, washdown environments, and any fitting that will be exposed and must stay bright. Each material is priced live; the popular-sizes table above shows all three for two common sizes so you can see the difference.

How the ends connect

Both ends are finished with the connection you choose, and the two ends can be different. Slip-and-drive is the residential and light-commercial standard: the two long edges receive an S-cleat and the two short edges receive a drive cleat, so the reducer joins a trunk section the same way any rectangular joint does. Raw leaves the edge plain for a field connection or for a collar to slip over. FoldOut and DoubleFoldOut turn the edge into a flange for larger commercial duct, and a DuctMate-style flange is the four-bolt transverse connection. Install with the drive cleats on the short sides, seal the joint with mastic or UL 181 foil tape, and remember that the two ends are different sizes: the cleats and drives for each end are sized to that end, so the reducer is the place on the trunk where the next size of drive cleat starts.

How to measure the openings you have

1. Inlet: measure the inside width and depth of the upstream duct. Its nominal size (the number written on the duct) is the inside dimension.
2. Outlet: the same on the downstream duct. Keep the depth the same if the trunk has to stay flat against the joists and reduce the width only.
3. Length: measure the space available between the two connections and subtract the overlap at each end. If the length is not critical, use 12" or the calculator's 15 degree minimum, whichever is greater.
4. Orientation: a concentric reducer does not care which way it is turned. If one wall must stay flat, that is an eccentric reducer; call us with which side is flat.

Why field-made reducers go wrong

Every installer has taken a piece of the larger duct, slit the corners, folded the walls in until the smaller duct fits, and screwed and taped the result. It gets the air from one size to the other. It also leaks at every slit, the folded walls are never at the same angle so the outlet is not centered or square, the overlapping metal makes a step inside the duct, and the taper is far steeper than 15 degrees because a folded wall cannot be long. A shop-made reducer has walls laid out to the true trapezoid, corners that are locked rather than overlapped, and an outlet that is centered and square by construction. On a supply trunk the difference is measurable in static pressure and audible at the last register. On a kitchen or process exhaust, where the duct must be sealed grease-tight or airtight, the field version is not acceptable at all.

Ordering, price and turnaround

Set the five dimensions in the designer, choose material, gauge and connections, and the price shown is the price you pay; there is no quote step for standard configurations. Each reducer is made to order in our Pittsburgh shop from full sheets, so an unusual size costs the same as a common one of the same amount of metal, and quantity does not change the unit price the way it does for stamped fittings. Add the reducer to the cart with the rest of the job, or order any of the 392 published sizes from its own page. For eccentric reducers, TDC flanges, heavier gauges than the designer offers, or a takeoff of a drawing, call 412.725.2744 or email sales@pmxductwork.com with the dimensions and we will price it the same day we read it.

Read more

• Duct reducers: eccentric versus concentric and when to use each: the taper and placement rules in more depth.
• Made-to-order duct transitions and reducers: what to specify and where the fitting shows up on real jobs.
• Duct transitions guide: the general size-change fitting and the cases a reducer does not cover.
• Every published duct reducer size with its price.