Custom Duct Tee Fittings, Made to Order in Any Trunk and Branch Size

Custom Rectangular Duct Tees: The Complete Guide

A duct tee, also called a rectangular tee, a trunk tee, a branch tee, a tee takeoff or a bullhead tee depending on how it is used, is the fitting that lets a rectangular trunk give up part of its air to a rectangular branch at a right angle while the trunk keeps going. It is the fitting a supply house stocks in three or four sizes and a duct job needs in a dozen, because the trunk comes in two-inch steps on two dimensions, the branch comes in its own two dimensions, and the combination on any given job is one of hundreds. That is why customers tell us we are one of the very few shops that will build a tee to the exact trunk and branch they type in, and why this page exists.

Everything below is what we tell customers on the phone: what the fitting is, how a tee differs from a takeoff collar and a wye, how to size the branch, when to add turning vanes, which gauge and material to specify, how the three openings connect, and how to measure the trunk 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 tee actually is

Geometrically the fitting is a length of rectangular trunk with a rectangular collar cut into one side and seamed in, so that air entering one end of the trunk can leave either through the far end or through the branch. The trunk run and the branch share one depth, so the top and bottom panels are continuous and the branch is flush; the branch has its own width and its own length from the trunk wall to its mouth. The two run ends can be different widths, which is how a tee also serves as the point where a trunk reduces, and the clear run from each trunk mouth to the branch opening, the throat, is set so the branch collar does not land on a joint. Our designer builds the flat pattern from those numbers, the shop cuts and forms it, and the branch is seamed in rather than screwed on, so the fitting is airtight at the one place a field-made tee never is.

The numbers that define the fitting are the trunk width at each end, the shared depth, the branch width, the branch length, and the throat on each side. Every opening is finished for slip-and-drive connection to rectangular duct, or TDC or flanged if the system uses them. Air can flow either direction: a tee installed with air entering the branch and leaving both run ends is a bullhead tee, and it is the same part.

Where duct tees are used

• Trunk to rectangular branch. The classic case: a 20" x 12" or 24" x 12" main trunk feeding a 12" x 8" or 14" x 10" branch trunk down a wing of the house, the branch then carrying its own round takeoffs. Where a stamped collar only fits a round pipe, the tee is what a rectangular branch needs.
• Bullhead at the plenum. The fitting at the top of a supply plenum or at the end of a riser where one trunk has to split left and right down the basement. Set the branch to the full trunk width and each run end to what its direction carries.
• Trunk reduction with a branch. Where the trunk steps down right where a branch leaves, one tee with a smaller downstream end replaces a tee and a reducer and one joint.
• Return trunks. Return air is collected the same way, with the tee joining a return branch into the return trunk; sizing here is looser and larger, since return velocity should stay low.
• Exhaust and process duct. Joining two hoods or two machines into one exhaust run, usually in heavier gauge or stainless, and usually as a wye where the two flows merge at a shallow angle; see below for when a tee is the wrong part.
• Commercial rectangular trunks. Rectangular low-pressure duct out of a rooftop unit or an air handler, splitting to zones, with TDC or flanged connections.

Sizing the branch: match the velocity

The rule that resolves nearly every tee question is to size the branch for the air it carries, not for the trunk it comes off, and to keep the velocity in the branch about equal to the velocity in the trunk. If the branch is too small the air speeds up, the fitting adds static pressure, and the branch whistles; if it is much larger the air slows and the branch steals more than its share, starving the registers at the far end of the trunk. The equal-velocity branch area is

A_branch = A_trunk × ( CFM_branch ÷ CFM_trunk )

and since the branch shares the trunk depth, the branch width is that area divided by the depth, rounded up to the next inch. Some quick reference values:

You will notice a branch carrying half the air is half the trunk area, not half the trunk width and half the depth. That is normal and it is why the depth is shared: the branch width alone carries the size change. When the branch is a whole secondary trunk with its own takeoffs, size it from a room-by-room load and use the branch CFM that comes out of it; when it is one register, the branch is simply the collar that makes that register's duct fit the trunk, and a 20" x 12" trunk feeding an 8" x 8" branch is a perfectly normal tee even though the ratio is small.

The calculator at the top of this page does the arithmetic, rounds the branch width up to a whole inch, and shows the velocity in each opening so you can see whether the branch is too small. As a rule of thumb keep residential supply trunks under about 900 feet per minute and branches under about 700; commercial low-pressure systems run higher.

Tee, takeoff collar or wye

A round takeoff collar, the stamped saddle or spin-in that screws to the side of a trunk, is the right part for a 5", 6" or 7" round branch feeding one register. It costs less than a tee and the small branch does not disturb the trunk much. A tee is the right part when the branch is rectangular, when it carries a large share of the trunk airflow, when the trunk changes size at the branch, or when the branch needs a real straight run with its own throat instead of a saddle. A wye splits the air at a shallow angle instead of a right angle, so the branch flow does not have to turn 90 degrees; it costs about half the static of a tee for the same split and is the right part on exhaust, dust collection and any high-velocity system, or wherever the branch runs in the same general direction as the trunk. The tee is the right part wherever the branch has to leave at a right angle, which in a basement or a ceiling with joists in the way is nearly always.

Turning vanes and the throat

The air that leaves through the branch has to turn 90 degrees, and it does not do that willingly; it piles up on the downstream wall of the branch and leaves a dead zone on the upstream wall, so the branch effectively runs at about two thirds of its area for the first foot. Two things fix that. The throat, the clear run of trunk between each mouth and the branch opening, gives the air room to settle before and after the split; six inches is the default and more is better on a large branch. Turning vanes in the branch opening, a set of curved blades that steer the air around the corner, cut the pressure loss of the turn roughly in half and take the whistle out. They are worth specifying when the branch takes more than about a third of the trunk airflow, in every bullhead tee, and on any tee within a few feet of the blower. On a small branch off a large trunk they do little. The designer has a turning-vanes option; the price updates when you turn it on.

Gauge

Sheet-metal gauge for low-pressure duct is set by the largest dimension of the fitting, which on a tee is the trunk. 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 trunk: 24 gauge up to about 14" on the long side, 22 gauge to about 26", and 20 gauge above that. Because the branch opening removes a panel from one side of the trunk, a tee is less stiff than a straight duct of the same size, so where a straight section could get away with the next lighter gauge, the tee should not. 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 nine out of ten tees are made of. It is the least expensive, it takes duct sealant and tape well, and it is what the rest of the system is made of. 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. It is softer, so we hold the same gauge number, which in aluminum is a slightly thicker sheet. 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 openings connect

All three openings 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 tee joins the trunk on both sides and the branch duct on the third the same way any rectangular joint does. Each opening's connection is chosen independently in the designer, so a tee can be TDC on the trunk and slip-and-drive on the branch where a commercial trunk feeds a residential-style branch. The branch collar is seamed into the trunk in the shop; the only field joints are the three ends. Seal each joint with mastic or UL 181 foil tape, and seal the branch corner seams on the outside as well; a tee has more seam length than any other fitting of its size.

How to measure the trunk you have

1. Trunk width and depth: the nominal size of the rectangular duct the tee will join, which is its inside dimension. Give the tee those dimensions; the slip-and-drive edge adds the connection allowance. If the trunk reduces at the branch, give both run widths.
2. Branch width: the nominal width of the branch duct, or the width the calculator gives for the branch airflow. The branch depth is the trunk depth.
3. Branch length: how far the branch collar has to reach from the trunk wall to the first branch joint. Twelve inches is the default and clears most joists; shorter is fine where the branch duct joins immediately.
4. Throats: the clear trunk on each side of the branch. Six inches each is the default; make the upstream throat longer if the tee follows an elbow closely.
5. Orientation: tell us which side of the trunk the branch leaves, looking in the direction of airflow. A branch out the top or bottom of the trunk is the same fitting rotated.

Why field-made tees go wrong

Every installer has cut a rectangular hole in the side of a trunk, bent tabs in and screwed a piece of duct to them. It gets air into the branch. It also leaks at every tab, the hole is never quite square so the branch comes off at an angle and the next joint fights it, the tabs project into the trunk and catch the air, and there is no throat, so the branch usually lands on a trunk joint and the drive cleat has to be cut. A shop-built tee has the branch seamed in airtight, square to the trunk by construction, with the throat placed so the collar clears every joint. 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, the field version is not acceptable at all.

Ordering, price and turnaround

Set the trunk, branch and throats in the designer, choose material, gauge and the connection on each opening, and the price shown is the price you pay; there is no quote step for standard configurations. Each tee 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 tee to the cart with the rest of the job, or order any of the 474 published sizes from its own page. For a branch out the top or bottom, a tee with two branches, 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

• Wye versus tee duct fittings: when to use each: the pressure-drop comparison and where each belongs.
• Supply versus return ductwork: how trunk and branch sizing differ on the two sides of the system.
• Every published duct tee size with its price.