Use compression geometry or matched pressure tests to estimate installed flex-duct loss and pressure margin.
Concepts behind this calculation
Estimate the straight-flex compression correction or compare a measured installed pressure drop with a fully extended reference at the same airflow.
Longitudinal compression increases straight-flex resistance relative to a fully stretched reference. The compression fraction compares the installed axial length with the fully stretched length of that same specimen. Separately, the reference pressure’s own length is used to normalize straight friction to the installed run before applying the correction multiplier.
Those two length references are not interchangeable. A per-length friction value does not already describe the specimen’s total loss, and fitting losses do not belong in straight-run scaling. Measured comparison instead uses equal-length complete tests at the same airflow and air basis; it captures combined installation effects without assigning them individually to sag, bends, or compression.
How to use this calculator
Document Reference / installation data source and select Check method.
Enter Airflow at comparison point, Reference pressure drop, and Available pressure budget.
For compression, enter Length corresponding to reference pressure drop, Nominal flex diameter, Fully stretched length of installed specimen, and Installed axial run length.
For measured comparison, enter Measured installed pressure drop from a compatible equal-length test and inspect the ratio and margin.
Calculation method and applicability
Kc = 100(1 − Linstalled/LFE); PDCF = 1 + 0.58Kc exp(−0.00496Dmm). First normalize the straight fully stretched pressure loss: ΔpFE,run = Δpref × Linstalled/Lref. Then Δpinstalled = PDCF × ΔpFE,run. LFE defines compression; Lref is the pressure-reference length. Measured mode compares equal-length, same-airflow complete tests without this scaling.
Kc = compression in percent; Linstalled = installed axial length (ft); LFE = fully extended length of that specimen (ft); Lref = pressure-reference test length (ft). Dmm is internal diameter converted to millimeters; PDCF is dimensionless.
Input definitions and units
Reference / installation data source
Required. Identify the reference test or measurement, duct model/diameter, airflow basis, pressure definition, installed support and date or edition. A source note does not authenticate user-entered data.
Check method
Compression uses a same-length straight-duct loss ratio after explicit reference-length normalization. Measured mode compares already measured equal-length assemblies including their actual installation effects. Choices: Compression correction for straight flex; Measured installed pressure comparison.
Airflow at comparison point (CFM)
Use the same airflow for the fully extended reference and the installed condition. This calculation does not solve airflow from compression without a compatible fan/system curve.
Reference pressure drop (in. w.g.)
Compression mode: fully stretched straight-duct pressure loss over the separate pressure-reference length. A per-100-ft value uses reference length 100 ft. Exclude fittings, entrances and exits. Measured mode: actual equal-length comparison test pressure; no automatic scaling.
Length corresponding to reference pressure drop (ft)
Axial length over which the entered fully stretched straight-duct pressure loss applies. Use 100 ft for in. w.g./100 ft data. Keep separate from the installed specimen’s fully stretched length. Only straight friction is scaled. Applies to Check method: Compression correction for straight flex.
Available pressure budget (in. w.g.)
Nonnegative allocated budget, including zero for a deficit comparison. The margin is budget minus corrected or measured loss; it does not predict delivered flow.
Nominal flex diameter (in.)
For the ASHRAE correlation, 150–400 mm (about 6–16 in.) only. Use the nominal test diameter, not an assumed compressed opening. Applies to Check method: Compression correction for straight flex.
Fully stretched length of installed specimen (ft)
Length that the installed specimen would have when fully stretched. This is used only in the compression ratio; it is not automatically the length associated with the reference pressure. Applies to Check method: Compression correction for straight flex.
Installed axial run length (ft)
Actual axial length after longitudinal compression. Used both to calculate compression and to scale the straight reference loss to this run. Do not enter flex material cut length or a fitting-equivalent length. Applies to Check method: Compression correction for straight flex.
Measured installed pressure drop (in. w.g.)
Measured method only: actual installed loss at matching airflow, air properties, axial test length and pressure taps. No inferred sag coefficient or length scaling is applied to a complete test assembly. Applies to Check method: Measured installed pressure comparison.
How to interpret the result
The budget margin is available pressure minus corrected or measured loss at the selected flow. A large ratio highlights excess resistance but does not diagnose its cause or prove an airflow deficit.
Worked examples
Compression correction for straight flex
Inputs: ASHRAE 2021 Example 6 geometry: installed length 1.8 m (5.905512 ft), compression 15%, nominal diameter 250 mm, airflow 470 L/s. Fully stretched specimen length = 1.8 / 0.85 = 2.117647 m (6.947661 ft). A fully stretched reference loss of 11.4 Pa applies over a separate 1.8 m reference run.
Calculation: Installed length / pressure-reference length = 1, so the reference run loss remains 11.4 Pa. PDCF = 3.517642696; corrected loss = 40.101126735 Pa = 0.160991217 in. w.g. Excess relative to a fully stretched 1.8 m run is 28.701126735 Pa.
Result: 0.161 in. w.g. corrected drop and 0.039 in. w.g. margin for a 0.20 in. w.g. budget. The source 15% table row prints 40.0 Pa; this continuous calculation is about 0.253% higher. It is not a test of a particular installed product.
Measured installed pressure comparison
Inputs: Constructed equal-length measured tests at 470 CFM, same air and pressure taps: reference 0.0458 in. w.g., installed 0.160 in. w.g., budget 0.20 in. w.g. This is not an ASHRAE field-test record.
Calculation: Ratio = 0.160/0.0458 = 3.49345; increase = 249.35%; excess = 0.1142 in. w.g.
Result: 0.160 in. w.g. installed; 3.493× reference; 249.345% increase; 0.040 in. w.g. margin. Compression, sag, bends and support are not separated without additional comparable tests.
Assumptions and limits
Compression PDCF compares resistance on the same installed-length basis. The reference pressure must be straight friction over its stated reference length; the calculator scales that length once. The installed specimen’s fully stretched length defines compression, not the pressure-reference length.
If 11.4 Pa is measured over a 1.8 m fully stretched specimen then the same specimen is shortened to 1.53 m, its straight-length-normalized reference is9.69 Pa, so this model predicts34.085957725 Pa. That constructed scenario is different from ASHRAE Example 6, which holds installed length at 1.8 m.
The compression correlation is applied only to 150–400 mm nominal diameter and 0–45% longitudinal compression. It is a pressure-drop multiplier for a fully extended straight-flex reference at the same airflow, not a universal product curve.
Equal-length measured comparisons are not length-scaled. The measured method does not identify whether excess resistance came from compression, sag, bend radius, support, fittings, connections or measurement uncertainty. It requires comparable reference and installed measurements.
No airflow prediction, fan-curve intersection, sag geometry, bend coefficient, product selection, acoustic result or code approval is generated.
ASHRAE Table 2 is a separate DFDB-based result set. For example its 30% row lists PDCF 5.9 while Eq22 at 250 mm gives6.035285; that does not agree by rounding to one decimal. The discrepancy remains unresolved and the calculator reports the named equation, without forcing table agreement.
The source correlation does not establish suitability for every flex product, insulation construction, support arrangement or installation. Confirm the applicable manufacturer and project standard.
FAQs
Where do I put a pressure drop quoted per 100 feet?
Enter that pressure in Reference pressure drop and 100 feet in Length corresponding to reference pressure drop. Keep Fully stretched length of installed specimen as its separate physical measurement.
Does the correction predict the branch’s reduced airflow?
No. It compares pressure losses at Airflow at comparison point. Delivered airflow requires a compatible fan/system analysis or measurement.
Quietflex DuctulatorPeer calculator exposes diameter, CFM and adjusted CFM; states FD-72R1/ASHRAE 120 and fully extended assumption. No output was copied.
Texas A&M / TAMU repository study recordIndependent study record describes 12, 14 and 16 inch flexible duct tests at 0, 4, 15, 30 and 45% compression under ASHRAE 120 methodology. Full numeric tables were access-limited in this run; not used as a product preset.
Abushakra, Walker and Sherman — ACEEE 2002 research paperTables and test results express straight resistance on a length-normalized basis. Corroborates the distinction between friction per length and total specimen loss. Its older size-specific correlation is not substituted for the named2021 equation.
