How to Install a Takeoff and Flex Branch to a New Register

Why this matters

A new branch run is the most common duct job a residential shop does: a finished bonus room, an addition, a cold bedroom that never had enough supply. It is also the job where the room's complaint gets locked in for twenty years. Flex duct is forgiving to install and unforgiving in service: a run left partly compressed, kinked at the boot, or pinched by a narrow strap carries far less air than its size promises, and nobody sees it once the insulation is back. The other half of the loss is leakage: an unsealed collar or a boot gapped to the drywall blows conditioned air into the attic and pulls attic air into the room. This article installs the branch so the flow hood at the register agrees with the design on the first reading. The inspection and correction of existing flex is its own card (See related: Flex Duct Inspection and Correction).

The job this article walks through

A bonus room over a garage needs a new supply register. The Manual D design for the room calls for 110 CFM through a 7 inch flex branch, read off the shop's flex sizing chart for the run length and the system's available friction rate. The attic route from the sheet metal supply trunk to the boot location is 14 ft along the path the duct will take, with one bend down to the ceiling. Everything below uses those numbers.

 SUPPLY TRUNK, sheet metal (collar on its side)
 ====[ START COLLAR ]=======================
          |  mastic over the collar seam
         [D] balancing damper, handle marked
          |
          +--- 7 in flex, core pulled taut ---+
               |               |              |
             strap           strap            | bend,
             1.5 in          1.5 in           | radius
          <-4.7 ft-> <-4.7 ft-> <-4.7 ft->    | 7 in+
                                           [ BOOT ]
 == ceiling drywall ==============[seal]--REG--[seal]==

The collar and the boot each hold one end; the two straps split the 14 ft between them.

Start state and hazards

Attic hazards, each with its control. Walk only on joists or on boards laid across them; a foot between joists goes through the ceiling and often the person follows it. Attics in warm weather reach heat-stress temperatures, so this work is scheduled for the morning, with water and a set limit on time up there. Fiberglass insulation irritates skin, eyes and airways when disturbed: long sleeves, gloves, eye protection, and a filtering facepiece respirator worn under the shop's respiratory program or the voluntary-use provisions of 29 CFR 1910.134 (applied to construction by 29 CFR 1926.103). Sheet metal edges cut: gloves on whenever snips are in hand.

Before cutting the ceiling, look from above for cable, pipe and framing at the boot location, and confirm what the ceiling is made of. A textured ceiling of unknown age gets tested for asbestos before anyone cuts it; if it cannot be tested today, the boot does not go in today.

Shut the system off at the thermostat before cutting into the trunk, so the blower is not throwing metal shavings down the supply or pressurizing the opening while you work. The restore is the airflow check at the end.

What you need: a start collar sized to the branch (7 inch here) with a balancing damper, or a separate in-line damper; 7 inch flex in the insulation value your energy code requires; listed tie bands and a tensioning tool; duct mastic; support strap at least 1.5 in wide; a ceiling boot with a 7 inch collar; and the register.

The steps

1. Pick the collar location and cut the trunk. Put the takeoff on the top or side of the trunk where the air is moving steadily, not tight against a transition, elbow or end cap where flow is turbulent and the takeoff gets less than its share. Mark the circle, drill a pilot, cut with snips, and keep the cut inside the line; an oversize hole cannot be sealed tight. Why location matters: a takeoff in a dead spot behaves like a smaller duct, and no amount of damper adjustment downstream adds air that never entered the branch.

2. Fasten and seal the collar. Seat the collar, fasten it with sheet metal screws on the spacing its maker calls for, and coat the whole joint between collar and trunk with mastic, working it into the seam. Why mastic and not tape alone: cloth-backed tape dries out and lets go on hot trunks within a few seasons, and the leak it leaves blows into the attic every hour the blower runs.

3. Set the damper and mark it. Install the damper with its handle where you can reach it from a board, set it fully open for now, and mark the handle so its position is readable later. The damper exists for the balancing pass (See related: How to Proportionally Balance Branch Duct Runs); an unmarked damper inside insulation is one nobody adjusts again.

4. Pull the flex fully extended and cut it to length. Stretch the run out along its route until the inner core is taut, then cut. A compressed core has its spiral wire bunched into ridges, and those ridges are where the resistance comes from. Cutting flex to the straight-line distance plus a little and then shoving it into place is how most new runs end up compressed.

5. Connect the inner core at the collar. Slide the core over the collar past its bead, fasten it with a listed tie band tensioned with the tool, and seal the joint with mastic or listed flexible-duct tape. Then pull the insulation and outer jacket over the joint and secure the jacket with a second tie, so the vapor barrier is continuous. Listed closures are those meeting UL 181B, which covers tie bands, mastic and tapes for flexible duct. Skip the outer tie and the jacket gaps, the cold core sweats in summer attic air, and the condensate ends up as a stain on the ceiling below.

6. Support it without pinching. Hang the run on strap at least 1.5 in wide so the strap does not cut into the insulation or squeeze the core. The Air Diffusion Council's installation standard, which most flex makers reference, calls for support at intervals of no more than 5 ft and no more than 1/2 in of sag per foot between supports; check the flex maker's instructions and the mechanical code as your jurisdiction adopted it. Make every bend gentle, with a centerline radius of at least one duct diameter, and never bend at the collar or boot itself.

7. Set and connect the boot. Fasten the boot to framing so it holds its own weight, connect the flex to the boot collar exactly as in step 5, core then jacket. Cut the ceiling opening to the boot's size from below, inside your layout, and seal the gap between the boot and the drywall with mastic or caulk. Why: that gap pushes supply air into the attic while the blower runs and lets attic air into the room when it stops, and it is a leak no inspection of the collar end will find.

8. Insulate and restore. Re-lay attic insulation around the boot and the run without burying the damper handle. Turn the system back on at the thermostat and confirm the blower starts and the existing registers are still delivering air before you take any readings.

Worked through: the bonus room run

Support count. The 14 ft run is held at the collar end and at the boot. With supports no more than 5 ft apart, two intermediate straps split it into three spans of 14 / 3, about 4.7 ft each, inside the 5 ft limit. Allowed sag at 4.7 ft spacing is 1/2 in per foot x 4.7 ft, about 2.3 in. The crew set the two straps and checked sag with a tape at each span's middle: about 1 in, well inside.

Bend. The drop to the boot needed one bend; at 7 inch duct, the centerline radius has to be at least 7 in, so the crew laid the bend over a curve at least 14 in across rather than folding it at the boot collar.

First flow reading. With the damper fully open and the system on cooling-speed airflow, the flow hood at the new register read 72 CFM against 110 designed, 72 / 110 = 0.65, about a third short. Fully open damper, so this was not a balancing problem.

What the attic showed. Back up with a light, the crew found the last 3 ft before the boot pushed together like an accordion: the run had been cut to the straight-line distance plus a foot, and pushing the boot end into place compressed it. One strap had also been hung from a nail on a rafter with ordinary plumber's tape, narrow enough to pinch the core.

Fix. A new length was cut with the run pulled taut along its full route, the boot connection remade core then jacket, and the narrow strap replaced with 1.5 in strap. Second reading: 104 CFM, 104 / 110 = 0.95, 5 percent under design. Inside a plus or minus 10 percent of design acceptance band, which is a common shop default for a residential branch; the balancing pass that follows can trim the rooms that are over to bring this one up further.

On the ticket: design 110 CFM, as-built readings 72 then 104, the cause of the first reading, damper position (fully open, marked), support spacing, and a photo of the collar and boot joints sealed before the insulation went back.

How to know the run is right

• The flow hood reading at the register is within the shop's acceptance band of design, with the damper's position recorded.
• Every joint (trunk to collar, core to collar, core to boot, boot to drywall) is sealed, and the jacket is tied at both ends.
• Supports are no more than 5 ft apart, at least 1.5 in wide, with sag inside 1/2 in per foot, and no bend is tighter than one duct diameter.
• Insulation is re-laid and the damper handle is still visible.

When a new run comes in short and the attic looks right, the problem is upstream: a trunk already starved, a blower at the wrong speed, or a system short of total airflow. A new branch cannot create air the system does not move. Measure total external static before blaming the branch (See related: How to Measure Total External Static Pressure on a Furnace).

References

• Air Diffusion Council, Flexible Duct Performance and Installation Standards, current edition, as referenced by the flex maker
• UL 181B, Closure Systems for Use With Flexible Air Ducts and Air Connectors
• 29 CFR 1910.134 Respiratory protection
• Flex duct and boot manufacturer instructions; the mechanical and energy codes as adopted by your jurisdiction
• See related: Flex Duct Inspection and Correction; How to Proportionally Balance Branch Duct Runs; How to Measure Total External Static Pressure on a Furnace