Flexible Duct Selection Guide| SamcoFlex

How to Select the Right Flexible Duct for an HVAC Project?

Selecting flexible duct for an HVAC project requires more than matching the duct diameter to a diffuser neck. The selected product must suit the system’s airflow, velocity, pressure, temperature, insulation, fire-safety, indoor-air-quality and installation requirements.

For HVAC consultants, a flexible duct specification should identify the required construction, performance ratings, applicable standards and documentation. Product availability and published performance values vary by model, so the current technical datasheet, certification and installation instructions should always be reviewed for the exact product proposed.

This guide explains how to evaluate flexible duct for residential, commercial, industrial, healthcare and infrastructure projects, including flexible duct sizing, insulation, pressure ratings, fire classifications, condensation control, installation and product documentation.

What Is Flexible Duct in HVAC?

Flexible duct is a factory-made air duct or air connector used in HVAC and ventilation systems. It may connect rigid ductwork, plenums, air-handling equipment and terminal devices such as diffusers and grilles.

Compared with rigid sheet-metal ductwork, flexible duct can accommodate limited changes in direction and alignment. Its permitted length, bend radius, support spacing, pressure capability and connection method depend on the product and the manufacturer’s installation instructions.

A flexible duct assembly may include:

An inner air-carrying core
Helical wire reinforcement
Thermal insulation, where applicable
An outer jacket or vapour-retarder layer, where applicable
Joining, fastening and sealing materials

Construction varies substantially between products. The term “flexible duct” should therefore not be used as a complete technical specification.

Why Flexible Duct Selection Matters in HVAC Design?

Incorrect flexible duct selection or installation can increase system resistance and reduce airflow at the terminal. It may also contribute to:

Higher pressure loss
Reduced delivered airflow
Increased fan energy demand
Heat gain or heat loss
Condensation where thermal and vapour-control requirements are not met
Air leakage at joints
Noise
Sagging, compression or deformation
Premature material damage
Non-compliance with fire or building requirements
Indoor-air-quality concerns

Flexible duct should be evaluated as part of the complete air-distribution system, including the duct layout, terminal device, fan capacity, pressure-loss calculation and installation method.

1. Define the HVAC Application Before Selecting Flexible Duct

The first question should be: Where will the flexible duct be used?

The appropriate product may differ between a residential apartment, commercial office, hospital, laboratory, hotel, industrial facility or high-temperature application.Typical considerations include:

Residential HVAC Flexible Duct: Residential systems commonly require flexible duct with suitable thermal performance, airflow characteristics, fire performance and installation practicality. The required properties should be established from the applicable code, project specification and system design.

Flexible Duct for Commercial Buildings: Offices, shopping centres, hotels and other commercial buildings may require specific fire classifications, insulation values, pressure capabilities, leakage performance and durability. These requirements should be stated in the project specification and verified against the proposed product documentation.

Flexible Duct for Healthcare Facilities: Healthcare projects require particular attention to fire performance, cleanliness, maintenance, indoor air quality and material emissions. The project’s infection-control, ventilation and material requirements should be reviewed before selecting a flexible duct. SamcoFlex positions SAMCO-‘O’ PURE Insulated Flexible Duct for applications including healthcare facilities, laboratories, clean environments and food-processing facilities. Any claim concerning fibre shedding, emissions or suitability should be confirmed against the current product datasheet and supporting test documentation.

Flexible Duct for Industrial Applications: Industrial projects may involve elevated temperatures, chemical exposure, mechanical hazards, unusual pressures or demanding service conditions.

The consultant should evaluate:

Minimum and maximum operating temperature
Pressure and velocity
Chemical compatibility
Mechanical protection
Fire performance
Moisture exposure
Required service life
Cleaning and maintenance requirements

A product should not be selected for an industrial or high-temperature application solely because its nominal diameter is suitable.

2. Decide Whether Insulated or Uninsulated Flexible Duct Is Required

The need for insulation depends on the air temperature, surrounding conditions, energy requirements and condensation risk.

Insulated Flexible Duct for HVAC: Insulated flexible duct may be appropriate where thermal control is required, including:Supply-air distribution, Conditioned-air distribution through unconditioned spaces, Ceiling voids with temperature or humidity differences, Applications requiring control of heat gain or heat loss, Applications where condensation control is part of the design

Insulation reduces heat transfer between the air inside the duct and the surrounding environment. It does not, by itself, guarantee condensation control. Insulation thickness, thermal conductivity, vapour-retarder continuity, joint sealing and installation conditions must all be suitable.

The outer jacket may provide vapour-retarder functionality where the product is specifically designed and tested for that purpose. This should be confirmed from the product documentation.

SamcoFlex lists insulated flexible duct products including SAMCO-GR, SAMCO-21, SAMCO-31 and SAMCO-‘O’ PURE. Their construction and published performance values should be verified individually.

Uninsulated Flexible Duct for HVAC: Uninsulated flexible duct may be considered where thermal insulation is not required by the design or applicable code, such as certain installations within conditioned spaces.

SamcoFlex lists SAMCO-20 and SAMCO-30 as uninsulated flexible duct products. The product datasheets should be reviewed to confirm their construction, fire performance, pressure capability and intended applications.

The choice should be based on the project’s thermal, fire, airflow and installation requirements—not simply on product cost.

3. Size Flexible Duct Using Airflow, Velocity and Pressure Loss

Flexible duct diameter should be selected from the required airflow, acceptable velocity and allowable pressure loss.

A flexible duct should not be selected solely because its nominal diameter matches the neck size of a diffuser or grille. The complete connection, including the terminal device and transition, must be checked.

The design sequence is generally:

Airflow → Velocity → Duct Diameter → Pressure Loss → Fan and Terminal Performance

Increasing diameter may reduce velocity and friction loss, while an undersized duct may increase resistance and reduce delivered airflow. Actual performance also depends on length, bends, compression, sagging, surface condition and connection details.

Important parameters include:

Design airflow
Air velocity
Nominal and installed diameter
Equivalent length
Number and radius of bends
Pressure loss
Available static pressure
Terminal-device requirements
Manufacturer’s published performance data

SamcoFlex product ranges may include nominal diameters from approximately 3 inches to 20 inches, but available sizes differ by product. Confirm the current size range for the exact model before specifying it.

4. Check Flexible Duct Pressure and Velocity Ratings

The selected duct must be suitable for the system’s operating pressure and air velocity.

This is particularly important for:

High-static-pressure systems
Long flexible connections
Commercial air-distribution systems
Industrial ventilation
Large air-handling systems
Systems involving negative pressure

SamcoFlex publishes product-specific pressure and velocity information for its flexible duct ranges. Some published product information identifies a maximum air velocity of 5,000 FPM for certain insulated products, including SAMCO-GR and SAMCO-21. This value should not be applied automatically to every SamcoFlex product, diameter or installation condition.

Before approval, verify:

Positive-pressure rating
Negative-pressure or vacuum rating
Maximum air velocity
Applicable diameter range
Test method and test conditions
Whether the rating applies to the complete assembly
Any restrictions relating to temperature, length, bends or support

Maximum Rated Pressure Is Not the Same as Recommended Design Pressure. A maximum pressure value is a product limit or published rating under defined conditions. It is not necessarily the recommended continuous operating pressure for a particular HVAC system.

The design pressure should remain within the manufacturer’s stated limits and should also account for installation, safety factors, system transients and project requirements.

5. Specify Flexible Duct Fire Performance by Standard and Classification

Fire performance must be specified using the applicable test standard and classification. Depending on the jurisdiction and project requirements, relevant references may include:

UL 181
UL 723
ASTM E84
BS 476
EN 13501
NFPA 90A
NFPA 90B

These references do not all describe the same test, classification or regulatory requirement. The applicable standard must be determined from the governing building code, fire strategy, mechanical specification and authority requirements.

SamcoFlex publishes fire and performance information for different products and markets. Product literature may refer to standards such as UL 181, ASTM E84, BS 476, EN 13180 or other standards. Such references should be verified against the current certificate or test report for the exact product construction.

Do not describe a product generally as “fireproof” or “fire resistant” unless the claim is supported by the applicable documentation. A test result for one product, size, construction or configuration should not automatically be extended to another product.

Classifications from different standards are not automatically interchangeable. Class 1, Class 0 and European Euroclass classifications must be treated as separate classifications unless an applicable code or assessment specifically permits comparison.

The specification should identify:

The applicable test standard
The required classification
The exact product and construction tested
The certificate or test report
The test scope and limitations
Whether the documentation applies to the complete duct assembly

This approach helps prevent unsupported or overly broad fire-performance claims.

6. Evaluate Flexible Duct Insulation Thickness and Thermal Performance

For insulated flexible duct, insulation thickness should be considered together with thermal conductivity and the project’s design conditions. Relevant parameters include:

Insulation thickness
Thermal conductivity
Insulation density
Operating temperature
Vapour-retarder performance
Condensation-control requirements
Fire performance
Moisture resistance
Compression resistance

A thicker insulation layer is not automatically better. The required insulation should be determined by the project’s energy, thermal and condensation calculations and by applicable standards.

SamcoFlex offers different insulated constructions, including conventional insulated flexible duct products and the SAMCO-‘O’ PURE system. Claims concerning recyclability, emissions or environmental performance should be confirmed from current manufacturer documentation and any relevant third-party evidence.

7. Review Indoor Air Quality and Emissions Requirements

Indoor-air-quality requirements may be especially important in:

Hospitals
Laboratories
Pharmaceutical facilities
Food-processing facilities
Schools
Hotels
Commercial buildings
Clean environments

The consultant should determine whether the duct construction has documented information relating to:

Fibre shedding
VOC emissions
Odour
Chemical emissions
Microbial growth
Cleanability
Surface contamination
Compatibility with the project’s air-quality requirements

The inner surface is particularly relevant because it is exposed directly to the airstream.

SAMCO-‘O’ PURE for Sensitive HVAC Applications: SamcoFlex positions SAMCO-‘O’ PURE for applications where fibre shedding and material emissions are important considerations. The manufacturer’s current literature should be reviewed to determine the precise basis and scope of claims such as “fibre-free,” “zero-emission” or “non-irritating.”

These terms should not be treated as universal regulatory classifications unless supported by a specific test, certification or assessment.

For healthcare and other sensitive applications, product selection should also be coordinated with the project’s infection-control, filtration, cleaning and ventilation requirements.

8. Assess Moisture, Vapour Control and Condensation Risk

Condensation can occur when a duct surface or adjacent component falls below the surrounding air’s dew-point temperature.

Risk increases when:

Cold supply air passes through warm, humid spaces
Ducts are installed above ceilings
Vapour retarders are damaged
Insulation is insufficient
Joints are not sealed
Supports compress the insulation
Moisture enters through damaged outer layers

An insulated flexible duct with a continuous vapour retarder may help reduce condensation risk when correctly selected and installed. It cannot compensate for inadequate insulation design, damaged jackets or poor sealing. The consultant should ensure that:

The vapour retarder remains continuous
Joints are sealed using approved materials
Insulation is not excessively compressed
The outer jacket is protected from damage
Hangers and supports do not crush the duct
Penetrations and connections are treated according to the manufacturer’s instructions

9. Account for Flexible Duct Installation Conditions

Flexible duct is often selected because it can simplify routing and connection. Flexibility does not eliminate the need for proper support, alignment and clearance. A flexible duct should generally be:

Routed as straight as practical
Fully extended
Supported at the spacing specified by the manufacturer
Installed with bends of suitable radius
Protected from sharp edges and construction damage
Free from unnecessary compression
Connected securely to the terminal and rigid ductwork
Installed with sufficient access for inspection and maintenance

Avoid Excessive Flexible Duct Bends

Bends, compression and deformation can increase resistance and reduce the effective air passage. The effect depends on the product, bend radius, length, support and operating conditions.

A flexible duct that is compressed, sharply bent or sagging may perform differently from the same product installed fully extended and correctly supported. Pressure-loss calculations should reflect the intended installation condition where appropriate.

10. Limit Flexible Duct Length in the HVAC Design

Flexible duct should not normally replace the main rigid-duct distribution network unless the design and product documentation specifically permit that use.

Rigid duct is commonly used for main distribution, with flexible duct used for shorter terminal connections. The permitted length depends on the system design, pressure budget, product and installation method. Long flexible runs may result in:

Higher pressure loss
Greater sensitivity to bends and sagging
More installation variability
Reduced airflow
Increased difficulty in maintaining design performance
Greater inspection and maintenance challenges

Do not state a universal maximum length unless it is supported by the manufacturer’s current installation instructions or the project’s engineering calculations. The consultant should define the permitted length, routing and support requirements in the project specification.

11. Review Flexible Duct Inner Core Construction

The inner core affects the duct’s mechanical and airflow characteristics. The consultant should review:

Wire reinforcement
Core material
Layer construction
Dimensional stability
Tear and puncture resistance
Pressure capability
Temperature capability
Compatibility with the intended joining method
Published air-performance data, where available

SamcoFlex product literature describes corrosion-resistant steel wire helix constructions for its flexible duct range. This description should be confirmed for the exact product being proposed, because construction can vary between models.

The outer appearance alone is not sufficient for technical approval. The inner core, reinforcement, material layers and connection details should be reviewed against the product documentation.

12. Check the Flexible Duct Outer Jacket and Vapour Retarder

For insulated flexible ducts, the outer jacket may provide mechanical protection and, where designed for that purpose, vapour-retarder functionality. The consultant should verify:

Outer-layer material
Vapour-retarder properties
Moisture resistance
Tear and puncture resistance
Temperature limitations
Compatibility with tapes, mastics and clamps
Repair requirements
Exposure limitations

SamcoFlex lists multi-layer polyester and other composite constructions for different products. The exact material description and performance should be taken from the current technical datasheet for the specified model.

The outer jacket should remain intact during transport, installation and subsequent work above ceilings or in plant areas.

13. Confirm Flexible Duct Operating Temperature

The consultant should establish:

Minimum continuous operating temperature
Maximum continuous operating temperature
Short-duration exposure, if relevant
Temperature at the inner core
Temperature at the insulation and outer jacket
Any limitations associated with pressure, velocity or installation

Published temperature ranges are product-specific and may depend on the test method and component being evaluated.

Some SamcoFlex product information identifies an approximate operating range of -26°C to 150°C for certain standard insulated flexible duct products. This range should not be applied to the entire product portfolio without confirmation from the current datasheet for the exact model.

SamcoFlex aluminium semi-rigid products may have different and potentially wider published temperature limits. The applicable value must be verified for the exact product, construction and service condition.

A temperature rating is not, by itself, evidence that a product is suitable for every high-temperature, fire, exhaust or industrial application.

14. Compare Flexible Duct and Semi-Rigid Duct

Not every HVAC connection should use the same type of duct. A semi-rigid duct may be considered where the application requires greater dimensional stability, mechanical protection or temperature capability than a conventional flexible duct can provide. These advantages must be confirmed from the product documentation and should not be assumed solely from the term “semi-rigid.”

SamcoFlex product categories include:

Conventional Flexible Duct: Used where a flexible air connection is required, subject to the product’s pressure, temperature, velocity, fire and installation limits.

Insulated Flexible Duct: Used where flexibility and thermal insulation are both required, subject to the product’s published construction and performance data.

Aluminium Semi-Rigid Duct: Used where an aluminium semi-rigid construction is appropriate for the application. Mechanical strength, pressure, temperature and connection requirements should be verified for the selected model.

Insulated Aluminium Semi-Rigid Duct: Combines an aluminium semi-rigid construction with insulation. The thermal, pressure, temperature and fire properties must be confirmed from the current product documentation.

15. How to Select a SamcoFlex Flexible Duct Product

Important: Final selection must be based on the latest manufacturer’s technical datasheet, applicable certificates, project specifications, governing codes and actual operating conditions.

16. Compare Flexible Duct on Total Installed Cost, Not Price Alone

Price per metre is not a meaningful comparison when products have different constructions, ratings or installation requirements.

A broader comparison may include:

Total installed cost = Product cost + Installation cost + Pressure-loss impact + Energy impact + Maintenance + Expected service life

This is a decision framework, not a guaranteed life-cycle-cost result. The actual outcome depends on the system design, operating hours, energy prices, installation quality and maintenance.

Consultants should compare:

Material construction
Insulation
Fire classification
Pressure and velocity ratings
Air leakage information
Temperature range
Certifications
Durability
Installation requirements
Warranty terms
Technical support
Availability of replacement or repair materials

17. Verify Flexible Duct Certifications and Test Reports

Before approving a flexible duct, obtain and review the manufacturer’s current documentation. Depending on the project, this may include:

Product technical datasheet
Installation instructions
Fire-test certificate
UL certification, where applicable
ASTM E84 report, where applicable
BS 476 report, where applicable
EN classification or test report, where applicable
Product construction details
Temperature rating
Pressure rating
Velocity rating
Air-leakage information, where published
Warranty documentation
Environmental or emissions documentation, where required

SamcoFlex publishes certification and test information for different products and markets, including references to standards such as UL 181, ASTM E84, BS 476 and EN 13180. These references should not be treated as applying to every product. Confirm the exact product, construction, size range, test scope, certificate number and validity.

A certificate or test report for one product should not automatically be used to substantiate claims for another product.

18. Follow Flexible Duct Installation Requirements

Even a product with suitable published ratings can perform poorly if installed outside the manufacturer’s instructions.

Recommended installation practices include:

Fully extend the duct
Minimise bends
Avoid sharp bends
Provide proper support
Seal connections properly
Avoid excessive sagging
Protect the vapour retarder
Follow the manufacturer’s installation instructions