Custom Duct Wye Fittings, Made to Order in Any Size

Custom Duct Wye Fittings: The Complete Guide

A duct wye, also called a Y-branch, a Y-split, a pants fitting or a 45 degree lateral, is the fitting that divides one trunk into two legs that leave at 45 degrees each side of the trunk centerline. All the air entering the trunk leaves through the two legs; there is no straight-through run and no flat wall for the air to strike. That geometry is why the wye is the lowest-loss, quietest way to split a supply trunk or merge two returns, and why every duct design manual recommends it over a tee wherever the room exists to fit it. It is also a fitting that is rarely in stock in the size a job needs, because the trunk size, the leg size and the depth all have to match. Every wye on this page is cut and formed to the trunk and branch sizes you enter in the designer above, priced as you type, and made to order in our Pittsburgh shop.

This guide covers what a wye is and how it differs from a tee and a cross, where it is used, how to size the two legs, why the pressure drop and noise are so much lower than a tee, which gauge and material to specify, how the three openings connect, how much room it needs, and how to measure for it. If you already have your numbers, the designer prices them instantly and the popular-sizes table links to a page for each common size.

What a duct wye actually is

The fitting is a rectangular trunk opening on one end and two rectangular branch openings on the other, joined by a body in which the two legs diverge at 45 degrees from the trunk centerline, so the included angle between the legs is 90 degrees. The top and bottom are flat plates and the sides are the trunk walls swept out to each leg, with a splitter where the two legs meet. The legs are the same depth as the trunk in the sizes the designer builds, so the whole fitting has a constant height and the change of area happens in width only; a 20" x 12" trunk splitting into two 12" x 12" legs stays 12" deep throughout. That flat-top construction is what makes a wye simple to hang, easy to insulate, and cheap to build.

The four numbers that define it are the trunk width and depth and the branch width and depth. All three openings are finished for slip-and-drive connection in the published sizes; TDC, plain slip, raw and flanged are available in the designer for every opening.

Where duct wyes are used

• Splitting a supply trunk in two. The classic use: a trunk leaves the air handler and divides to serve two wings, two floors or two zones. The wye keeps the pressure balance between the two halves and avoids the flat-face loss of a tee.
• Merging two returns. Two return trunks joining at 45 degrees into one lose far less pressure and make far less noise than two streams colliding in a tee.
• Exhaust headers. Two hoods or two booths joining into a common exhaust main; in stainless for grease or process exhaust.
• Zone splits with dampers. A wye at the split with a damper in each leg gives each zone the same fitting and the same pressure loss, so the balance holds when one damper modulates.
• Extended plenum systems. Where a Manual D design changes from a single trunk to two branch trunks.
• Long runs that fork. Anywhere a trunk ends by dividing rather than continuing.

Wye versus tee versus cross

The decision is about whether the trunk continues. A tee has a straight-through run and one collar off the side; the trunk keeps going. A cross has a straight-through run and two opposed collars; the trunk keeps going with a branch each side. A wye has no straight-through run; the trunk ends by dividing into two legs. If your trunk stops and splits, this is the fitting, and it is a much better fitting for that job than a tee with a capped end, which forces all the air to turn 90 degrees into one collar and pile up against the cap. Where a 50/50 split at a tee with the run capped loses on the order of one velocity head of static pressure and makes an audible rush, the same split through a wye loses a fraction of that and is nearly silent. The trade is room: a wye needs the two legs to spread, and the width across the two legs at the end of the fitting is roughly the trunk width plus the length of the legs, so a 20" trunk with 12" legs occupies about 36" across at its widest. If both downstream ducts must run parallel to the trunk, follow each leg with a short 45 degree elbow; the wye plus two 45s still loses less than a tee.

Sizing the two legs: equal area at the trunk depth

The rule is the same one that sizes every branch fitting: give each leg the share of the trunk free area that matches its share of the air, at the same friction rate as the trunk, and keep both legs the same depth as the trunk so the fitting stays flat-topped. For a wye that means:

Leg area = trunk area × (leg CFM ÷ trunk CFM), and leg width = leg area ÷ trunk depth, rounded up to the next even inch.

For the common 50/50 split the two legs together have the same free area as the trunk, so each leg is half the trunk width at the full trunk depth. Some reference values:

Pure equal area holds the velocity constant through the split, which is what an equal-friction design wants. In practice the published sizes and the designer round each leg up, so the legs together carry a little more area than the trunk and the air slows slightly as it divides; that is harmless and gives the two downstream runs a little more margin. The calculator at the top of the page does the arithmetic for any split, shows the size of each leg, and, if you enter the CFM, the velocity in the trunk and in each leg so you can see whether a leg is running fast. Keep residential branch trunks under about 700 to 900 fpm; commercial low-pressure systems run higher.

Because the designer builds both legs the same size, the calculator loads the larger of the two legs when the split is uneven, which gives the smaller leg extra area rather than starving it. If the two legs must genuinely differ, a 60/40 wye with a 12" and an 8" leg for example, we build it as a custom piece; call with the trunk size and both leg sizes.

Why a wye loses so much less pressure than a tee

Air does not like to turn. In a tee the branch air turns 90 degrees into the collar, separates from the inside wall of the collar, and forms a contracted jet that has to re-expand; the loss coefficient for a 90 degree branch takeoff is on the order of one velocity head. In a wye each leg turns only 45 degrees, the walls that guide the turn are the swept sides of the fitting rather than a flat face, and there is a splitter where the two legs meet so the air divides cleanly instead of stagnating against a wall. Published loss coefficients for a 45 degree wye at a 50/50 split are a quarter to a third of the equivalent tee. On a residential system that is the difference between a static pressure the blower is comfortable with and one that pushes it to the top of its curve; on a return it is the difference between a quiet hallway and a rush at the grille. It also means less turbulence downstream, so a damper or a coil close to the fitting sees smoother air.

Gauge

Sheet metal gauge for a wye is set by the trunk width, because the widest flat plate in the fitting is the trunk end. The published size pages on this site default to 24 gauge for trunks up to 16" wide and 22 gauge for trunks from 18" to 30" wide, with both legs in the same gauge as the trunk. That is one gauge heavier than a straight section of the same width, because the flat top and bottom plates of a wye are longer than a straight section's and a wye carries three joints. Step up one gauge for medium-pressure systems, exhaust that runs warm, an exposed fitting, or a wye carrying dampers in both legs. The designer lets you change the gauge and the price updates on the spot.

Galvanized, aluminum or stainless

Galvanized steel is the standard for heating, cooling and general exhaust duct and is what most wyes are made of; it is the least expensive, takes mastic and foil tape well, and matches the rest of the system. Aluminum weighs about a third as much for the same gauge number and does not rust, so it is specified for rooftop duct, pool and spa air handlers, coastal installations, and fittings that have to be lifted into place by hand. Stainless steel is the material for commercial kitchen grease exhaust, laboratory 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.

How the three openings connect

All three openings of a published wye are finished with a slip-and-drive edge: the two long sides of each opening receive an S-cleat and the two short sides receive a drive cleat, exactly as on rectangular duct, so the wye joins the trunk on one side and the two branch ducts on the other the same way any rectangular joint does. Hang the wye first, cleat and seal the trunk joint, then bring each branch duct to its leg. Seal every joint and the leg seams with mastic or UL 181 foil tape. If the job calls for TDC flanges, a raw edge for a flexible connector, or a flanged opening at equipment, choose it per opening in the designer.

How much room a wye needs

The legs diverge at 45 degrees, so the fitting spreads. The overall width at the branch end is approximately the trunk width plus 1.4 times the branch length, and the fitting is as long as the branch length plus the transition from the trunk. In the published sizes the legs are 6" to 36" long; a longer leg gives a gentler transition and more room for a damper, a shorter leg fits a tighter ceiling space. Check the width across both legs against the joist bay or the space above the ceiling before you order, and if the two downstream ducts must run parallel to each other, plan a 45 degree elbow on each leg immediately after the wye.

How to measure the wye you need

1. Trunk: measure the inside width and depth of the trunk duct feeding the wye. Those are the nominal size and the numbers you enter; the slip-and-drive allowance is added in the shop.
2. Legs: use the calculator, or your Manual D or equal-friction design, for the leg width. Keep the leg depth equal to the trunk depth unless the branch duct is shallower; if it is, enter the branch duct's actual depth.
3. Leg length: long enough to get a cleat and, if used, a damper on each leg; 12" is a common choice, and more if the ceiling space allows.
4. Connections: match what is on the duct you are joining at each opening. Slip-and-drive to slip-and-drive, TDC to TDC.

Wye versus a field-made split

The field alternative to a wye is a tee with the run capped, or a plenum box with two collars. Both work, both are loud, and both cost static pressure the fan would rather spend on the rest of the system. A shop-made wye has a real splitter, swept sides, and three finished openings; it drops into the trunk in the time it takes to cleat three joints and it is the fitting the design manual assumed when it sized the trunk. On a return, the difference is something the homeowner hears every time the blower starts.

The other field alternative, a pair of 45 degree elbows welded or screwed together at the ends of a short straight, makes a fitting that looks like a wye from the outside but has no splitter and no swept interior; the air hits the seam between the two elbows and half of it stalls. If a wye is what the design calls for, a shop wye with a real splitter is the only fitting that delivers the loss coefficient the design assumed. It also arrives with three finished, matching openings, so the trunk and both legs join it the same way and nothing has to be adapted on the ladder.

Ordering and price

Set the trunk and branch 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 wye 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 stock fittings. Add the wye to the cart with the rest of the job, or order any of the 375 published sizes from its own page. For two legs of different sizes, a different branch angle, dampers in the legs, or a takeoff from a drawing, call 412.725.2744 or email sales@pmxductwork.com with the dimensions.

Read more

• Wye versus tee fittings: the pressure loss and noise comparison in more depth.
• Wye fitting installation: hanging, sealing and dampering a wye.
• Manual D duct design and duct pressure drop calculation: where the fitting losses come from.
• How to reduce duct noise: why the split fitting matters.
• Every published duct wye size with its price.