Flex Duct Inspection and Correction

Purpose

This standing instruction restores a flexible duct branch to the capacity its diameter already implies, by correcting geometry rather than by buying a larger size.

Flexible duct is rated on published friction data measured with the duct fully extended, straight and supported. Almost nothing in a real attic is any of those. Excess length coiled behind a truss, a run slung loosely between supports nine feet apart, and a bend pulled tight against a joist each take a serious bite out of a branch, and together they routinely halve it. The expensive version of this call is a tech who measures a starving register, orders the next size up, installs it exactly as badly, and delivers a slightly larger disappointment.

Safety actions that gate this work

• Attic work is where the injuries on this job happen. Walk on structural members only, carry two lights, work in short cycles in heat, and tell someone in the house each time you go up and come down.
• Cutting a flex duct core exposes its glass fiber insulation, and pulling the jacket back releases fibers into the attic air you are breathing. Cut with a sharp blade rather than tearing, bag the offcuts on the spot, and use respiratory protection selected under a written program meeting 29 CFR 1910.134.
• Loose fill of unknown type stays where it is. Vermiculite in older attics may contain asbestos, so it is sampled rather than swept aside to reach a boot.
• Collars, hanger wire and truss plates cut hands and forearms. Wear cut-resistant gloves and long sleeves, and never reach blind into a bay for a fitting you cannot see.

Scope

Covers inspection and correction of flexible duct branches on residential and small light-commercial forced-air systems: length, extension, support, sag, bend radius, crushing, and the connections at the takeoff and the boot.

Does not cover rigid duct fabrication, trunk sizing or duct design, and does not cover sealing workmanship, which the duct sealing and mastic application SOP owns. The room's required airflow comes from the room too hot or too cold investigation SOP, and system-level airflow and static come from their own SOPs. Duct replacement design is a separate scope.

The five geometry defects, and what each one costs

      support                          support
      |<-------- 4 ft spacing -------->|
      ====                          ====
          \                        /
           \______________________/
            sag measured here: 1/2 in per foot
            of spacing, so 2 in at 4 ft

The connections are the sixth thing and they are not geometry. The inner core laps the collar by at least the manufacturer's stated overlap, held by a positive mechanical fastener, sealed with a closure listed for flexible duct, and only then is the jacket pulled over and secured separately. A single wrap of tape on a beadless collar is a joint waiting to come off where nobody will see it.

Roles and handoffs

Procedure

1. Establish the branch's duty before you look at it. Acceptance: the room's required CFM with its source, the run's nominal diameter, and the actual routed length paced or taped rather than estimated. Wrong looks like a defect list with no number the correction can be judged against. Stop rule: no required CFM means this starts at the room investigation SOP. Hazard: none physical; done at the truck before entering the attic.

2. Measure what the branch delivers now, at the register and at the takeoff. Acceptance: a hood reading at the register with any published correction applied, and the pressure drop from the takeoff to the register, both recorded. Wrong looks like judging a branch by hand feel at the grille. Stop rule: a delivered figure above the room gate means the branch is not the problem and this procedure stops. Hazard: ladder work at the register, feet level, buckle inside the rails.

3. Walk the run end to end and score every defect against its acceptance value. Acceptance: actual length against the shortest practical route, extension state, support interval and material width, worst sag measured against 1/2 in per foot of that spacing, every bend's centerline radius against one duct diameter, and every point of framing contact. Wrong looks like a run declared fine from one end with a flashlight. Stop rule: any measured defect is written with its value, not as a description. Hazard: attic footing and heat per the safety block.

4. Open both connections and inspect the core and the fastening. Acceptance: core lapped over the collar by at least the manufacturer's stated overlap, a positive mechanical fastener under it, a closure system listed for flexible duct, a beaded collar, and the jacket secured separately over the top. Stop rule: a collar with no bead is replaced rather than re-taped, because the fastener has nothing to hold against. Hazard: the core insulation is exposed here, so cut with a blade rather than tearing, bag offcuts immediately, and keep the respiratory protection on.

5. Correct geometry first, and only then consider size. Acceptance: run shortened to the routed length with only boot slack left, fully extended, supported at intervals no greater than the adopted code's maximum on saddles of at least the required width, sag inside 1/2 in per foot of the actual spacing, bends re-formed to one diameter centerline radius, and any crushed section replaced rather than pulled straight. Stop rule: no upsizing is quoted from a pre-correction reading. Hazard: cutting and handling release fibers, so controls stay on for the whole correction, not just the first cut.

6. Re-measure the same two things on the same register. Acceptance: delivered CFM against the room's requirement using the 80 percent gate, and takeoff-to-register pressure drop, both compared against the step 2 figures. Wrong looks like a report quoting the corrected CFM without the number it started from. Stop rule: still short after correction is when an upsizing or re-route scope is written, and it cites the post-correction figure. Hazard: ladder work again at the register.

7. Restore, verify and record. Acceptance: jacket and vapor barrier continuous and sealed over every joint opened, insulation replaced where disturbed, attic access closed, filters in, panels on, blower-door interlock proven by opening the door on a running blower, and the branch's before and after values on the ticket. Stop rule: an open jacket over a cold supply in an attic will condense, so no branch is left with a vapor barrier gap. Hazard: this step puts the system back with the family present, so restore at the disconnect with the panel closed.

The record this produces

• Room requirement, its source, branch diameter and routed length as found
• Delivered CFM and takeoff-to-register pressure drop before correction
• Defect inventory with a measured value for each: length, extension, support interval and width, worst sag, bend radii, framing contact
• Connection findings at both ends, including collar bead and closure type
• What was corrected, with photographs of each support and connection before insulation went back
• Delivered CFM and pressure drop after correction, against the requirement
• Any remaining scope, quoted from the post-correction figure

The advisor quotes upsizing only from the second reading. The next tech in that attic can see whether the supports were ever brought to spec, and a later condensation complaint is answered by the jacket photographs in a minute.

Worked pass: bedroom branch delivering about half its requirement

Step 1: the room's requirement from the room-by-room load is 165 CFM. The branch is 7 in nominal flex. Taped along its actual route it measures 34 ft, where the shortest practical route between the takeoff and the boot is 19 ft, so 15 ft of it is surplus, coiled behind a truss.

Step 2: register reads 96 CFM. That is 96 divided by 165, about 58 percent of requirement, against a gate of 80 percent, which needs 0.80 times 165, or 132 CFM. Takeoff-to-register pressure drop reads 0.19 in w.c.

Step 3: supports are 1/2 in wire at 9 ft intervals, and worst sag measures 7 in. The interval itself already exceeds the 4 ft maximum in the adopted code, so acceptance is computed at a compliant 4 ft spacing, which is 0.5 times 4, or 2 in; 7 in fails that and would fail even the 4.5 in a 9 ft spacing would allow. Two bends are pulled to roughly half a duct diameter of centerline radius, and the wire has indented the duct at three points.

Step 4 FAILS. At the takeoff the core laps the collar by about 1 in against the manufacturer's stated overlap, held by a single wrap of tape with no mechanical fastener, on a collar with no bead. Stop rule taken: the collar is replaced with a beaded one, the core is lapped to the stated overlap over a positive mechanical fastener, sealed with a closure listed for flexible duct, and the jacket is secured separately.

Step 5: run cut to 21 ft, which is the 19 ft route plus 2 ft of slack at the boot, a reduction of 13 ft from 34, or about 38 percent shorter. Fully extended, re-hung at 4 ft intervals on 2 in wide saddles, the two bends re-formed to at least 7 in of centerline radius, and the crushed section at the truss chord cut out and replaced rather than pulled straight.

Step 6: register re-reads 158 CFM, which is 158 divided by 165, about 96 percent of requirement and above the 132 CFM gate. Takeoff-to-register drop falls from 0.19 to 0.06 in w.c. System TESP moves from 0.61 to 0.56, a small change, which is what one branch out of six should do and is recorded as such rather than claimed as a system fix.

Step 7: jacket continuous and sealed at both ends, photographs taken before insulation went back, interlock proven. No upsizing is quoted, because the geometry correction alone cleared the gate.

Checking the pass against the sections above: every defect in the table was measured and reported in the unit the table names, including the sag computed at a compliant spacing rather than the as-found one; the failing step is step 4 and its stop rule replaces the collar instead of re-taping it; the delivered figure is reported before and after on the same register, as step 6 requires; and no size increase was quoted, which is the rule step 5 states.

References

• The mechanical code your jurisdiction has adopted, in its adopted edition, for flexible duct support intervals, support width and the maximum permissible sag of 1/2 in per foot of spacing between supports
• Air Diffusion Council Flexible Duct Performance and Installation Standards, in the edition the manufacturer's instructions reference, for extension, bend radius and connection detail
• The duct manufacturer's installation instructions for core overlap, listed closure systems and published friction data
• 29 CFR 1910.134 for the written respirator program governing protection used when the core insulation is cut or the jacket is opened
• See related: the room too hot or too cold investigation SOP, the duct sealing and mastic application standard SOP, and the attic and crawlspace duct insulation repair SOP