How much does compressed flex duct affect airflow?

Compressed flex duct dramatically increases pressure drop and reduces airflow. A flex duct compressed to 85% of its extended length has about 1.5 times the pressure drop of a fully extended duct. At 70% extension the pressure drop is 3-4 times higher. At 50% extension — a visibly sagging duct — pressure drop is 10 times higher and the duct effectively blocks airflow to that room. This is why standard duct design practice requires flex duct to be fully extended and supported at maximum 4-foot intervals. Most flex duct installation problems in Canadian homes come from excess duct length left coiled or sagging in the attic.

What is the maximum length for flex duct in residential HVAC?

Standard duct design practice limits flex duct branch runs to 6 feet maximum for residential systems. SMACNA allows up to 14 feet in some configurations. Beyond 6 feet, the combination of flex duct roughness, fittings at each end, and installation compression creates excessive pressure drop that cannot be recovered. For longer runs, use sheet metal duct with flex duct only at the final 2-3 foot connection to the register boot. Using 8 feet of flex duct where a designer calculated 6 feet increases pressure drop by 35% on that branch, reducing room airflow by 15-20%.

🌸 Flexible Duct

Flexible Duct Calculator

Calculate friction loss in flex duct accounting for compression. Shows exactly how much pressure drop a sagging or kinked flex duct creates versus a fully extended run — and what it costs in airflow to that room. Includes maximum run length guidance per standard duct design practice.

🌸 Flex Duct Parameters

Airflow *

CFM

Flex Duct Diameter *

Extended Length *

Max 6 ft per standard duct design practice for branch runs

Extension % i

🔴 Compressed 50%🟡 85%🟢 100%

Flex Duct R-Value

Number of 90° Bends Each 90° bend adds ~30-45 ft equivalent length

🌸 Flexible Duct Results

Condition	Friction Rate	Total Pressure Drop	Effective TEL

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Why flex duct installation quality matters more than diameter

Flex duct is the most abused material in residential HVAC. It is ordered in 25-foot sections, and installers often cut it longer than needed then push the excess into the joist cavity or attic, creating a compressed, sagging run. A 6-inch flex duct fully extended handles 175 CFM at 0.10 IWG/100ft. The same duct compressed to 70% extension handles about 80 CFM at the same static pressure — less than half. The room starves for air and no amount of damper adjustment, duct cleaning, or system upgrades will fix it until the duct is re-stretched and properly supported.

Standard requirements for flex duct

Standard duct design practice specifies that flexible duct must be fully extended (100% of its rated length) and supported at no more than 4-foot intervals to prevent sagging. Maximum branch run length is 6 feet. Any run requiring more than 6 feet of flex duct should use sheet metal for the majority of the run, with flex duct only at the final connection. When calculating TEL for a flex duct run, use a roughness factor of 0.003 ft (versus 0.0003 ft for sheet metal) and add the equivalent length of the two end fittings. Use the equivalent length calculator for complete TEL analysis.

Frequently Asked Questions

How do I know if my flex duct is properly extended?+

A properly extended flex duct should have no visible ripples or sags in the outer jacket when supported at 4-foot intervals. The corrugated inner liner should be fully stretched so the corrugations are at their minimum depth. If you can push the duct along its length and it compresses easily, it is too long for its space and needs to be shortened. Measure the actual installed length and compare to the fully extended length marked on the duct label. Any duct installed at less than 90% of its rated extended length is performing below specification and should be re-stretched or replaced with a shorter section. See the duct sizing calculator to verify the correct diameter for the run.

Can I use flex duct for my main trunk duct?+

No. Flex duct should never be used for trunk ducts or runs longer than 6 feet. Trunk ducts carry the full system airflow (1,000-2,000 CFM or more) at high velocity. The roughness and compression limitations of flex duct at these flows create unacceptable pressure drops and noise levels. Trunk ducts must be sheet metal or rigid fiberglass duct board. Flex duct is only appropriate for the final branch connection — typically 4-6 feet from a rigid sheet metal branch to the register boot. Using flex duct for longer runs is one of the most common installation defects found during HVAC commissioning in Canadian homes. Use the air velocity calculator to check what velocity your trunk carries.