Custom Duct Transitions, Made to Order in Any Size

Custom Rectangular Duct Transitions: The Complete Guide

A duct transition, also called a rectangular reducer, a taper, a size change, a duct adapter or a transition piece, is the fitting that joins a rectangular duct of one size to a rectangular duct of another size over a short tapered length. Every trunk line needs several of them, because the trunk gets smaller as branches peel air off, and every equipment connection needs at least one, because the furnace or air-handler opening is never the same size as the duct that leaves it. Stock transitions exist for a few square sizes; the rest of the combinations, which is most of them, have always been shop-made. We build every transition 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 reducer and a square to round, where it is used, how long the taper should be, what the size change does to velocity, 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 transition actually is

Geometrically the fitting is a rectangle at one end and a different rectangle at the other, joined by four flat trapezoidal walls. When the outlet is smaller in both dimensions all four walls slope inward; when only the width changes, two walls slope and two are straight; when one dimension grows while the other shrinks, two walls slope in and two slope out. 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 transition, and it is what almost every trunk and plenum connection 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. Air can flow either direction through the fitting; a transition installed with the small end upstream is an expansion, and it is the same part. Enter whichever end you consider the inlet, and the designer builds it.

Transition versus reducer versus square to round

The names overlap, so here is how we use them. A transition is the general rectangular size change: any inlet to any outlet, including the cases where one dimension grows while the other shrinks, and including expansions. A reducer is the specific case where both dimensions get smaller and the taper is symmetrical; it is the same construction, and the reducer page defaults to longer lengths because a reducer usually sits in a trunk where there is room for a gentle taper. A square to round changes the cross-section from four sides to a circle and is a different fitting with a different pattern. If your drawing says reducer but one dimension stays the same or gets bigger, it is a transition, and this is the page for it.

Where transitions are used

• Trunk step-downs. The extended plenum trunk drops from 24" x 8" to 20" x 8" to 16" x 8" as branches take air off it. Each step is a transition in the same depth, tapering on the width only.
• Equipment connections. The furnace or air-handler supply opening is one size, the plenum or trunk is another. A short transition makes the change right at the equipment collar. Return-side connections at the filter rack are the same problem in reverse.
• Reshaping for a chase. A 20" x 10" trunk has to pass through a stud wall that only allows 14" x 14". A transition reshapes the duct on the way in and another on the way out, keeping the area close to constant so the velocity does not jump.
• Register and grille boxes. A rectangular branch feeding a diffuser neck of a different rectangular size, or a return grille box converting to the return duct.
• Commercial low-pressure duct. Any place a rectangular main changes size on an engineered drawing, the drawing calls out a transition by inlet, outlet and length, and this fitting is exactly that.

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 contracting transition is that no wall should slope more than about 15 degrees from the duct axis (a 30 degree included angle). For an expanding transition, where the air is slowing down, 10 degrees per wall is better, because slowing air separates from the wall far more readily than accelerating air does. Convert that to a length and the minimum for a 15 degree taper is

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

and for a 10 degree taper the multiplier is 5.7 instead of 3.7. Some reference values, contracting at 15 degrees:

Eight inches is our default length because it satisfies the 15 degree rule for the two-inch and four-inch steps that make up most trunk work, and the 12" length is published for the larger changes. Where space is tight, a shorter, steeper transition still works; it just costs a little more static pressure. Where you have room, longer is always quieter. The calculator at the top of this page flags a length shorter than the minimum so you can decide with the number in front of you, and it switches to the 10 degree rule automatically when your outlet is larger than your inlet.

What the size change does to velocity

Velocity is airflow divided by free area, so a transition only changes the velocity if it changes the area. A 20" x 10" to 16" x 12" transition goes from 200 to 192 square inches and barely changes the velocity; the same 20" x 10" inlet to a 12" x 10" outlet drops the area by 40 percent and raises the velocity by two thirds. Neither is wrong; the trunk is supposed to get smaller as it carries less air, and the whole point of stepping it down is to keep the velocity roughly constant so the static pressure stays available at the last register. What you want to avoid is a step-down that outruns the branches taken off, which leaves the downstream trunk running fast and loud. Enter the CFM in the calculator and it shows the velocity at both ends; as a rule of thumb keep residential trunks under about 1,000 feet per minute and branches under about 900, and let commercial low-pressure systems run higher.

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 transition 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 transition 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 transition joins a trunk section or a plenum collar 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.

How to measure the openings you have

1. Inlet: measure the inside width and depth of the duct, plenum collar or equipment opening on the upstream side. Its nominal size (the number written on the duct) is the inside dimension.
2. Outlet: the same on the downstream side. If the outlet is bigger than the inlet, enter it anyway; the calculator switches to the 10 degree expansion rule and the designer builds the same part.
3. Length: measure the space available between the two connections and subtract the overlap at each end. If the length is not critical, use the calculator's minimum or 8", whichever is greater.
4. Orientation: a concentric transition does not care which way it is turned. If one wall must stay flat, that is an eccentric transition; call us with which side is flat.

Why field-made transitions 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 transition 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 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 transition 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 transition to the cart with the rest of the job, or order any of the 404 published sizes from its own page. For eccentric transitions, 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 transitions: types, sizing and when to use them: the taper rules in more depth, with the concentric-versus-eccentric discussion.
• Made-to-order duct transitions and reducers: what to specify and where the fitting shows up on real jobs.
• Duct reducers: eccentric versus concentric and square to round transitions: the two neighbouring fittings and when each is the right one.
• Every published duct transition size with its price.