What Is a Flexible Duct?
A flexible duct (also called a flexible duct pipe, flexible exhaust pipe, or flexi duct) is a corrugated, bendable tube used to carry air from one point to another within a ventilation system. Unlike rigid sheet metal ducting, it can bend around obstacles, change direction without fittings, and compress for tight installations.
Flexible ducts are used in:
- Bathroom and toilet exhaust systems – connecting a ceiling exhaust fan to a roof or wall vent
- Kitchen ventilation – connecting a range hood or kitchen fan to an external outlet
- HVAC and air conditioning systems – branching supply air from a main plenum to individual rooms
- Grow rooms and controlled environments – directing air through carbon filters and out of the space
- Industrial and commercial extraction – carrying fumes, heat, or particulates from work areas
Types of Flexible Duct
Not all flexible ducts are the same. The material, construction, and insulation level determine what each type is suited for.
1. PVC Flexible Duct
PVC flexible duct is the most widely used type for residential and light commercial applications in India. It consists of a spiral-wound PVC body reinforced with an internal helix wire, giving it both flexibility and structural strength.
Key advantages:
- Lightweight and easy to handle
- Resistant to moisture and mildew
- Available in a wide range of diameters
- Cost-effective for most home ventilation needs
- The thick grey outer PVC layer prevents light leaks — important for grow room applications
Astberg's PVC flexible duct range (the AFD series) uses a multi-layer construction. The heavy grey PVC exterior is more than cosmetic — it resists ultraviolet degradation and prevents light from penetrating the duct wall, which matters in cultivation environments.
2. Insulated Flexible Duct
Insulated flexible duct adds a layer of thermal insulation (typically fibreglass wool or polyester) sandwiched between an inner duct and an outer jacket. This serves two purposes:
- Thermal performance: Prevents the duct from sweating when carrying cool supply air through a warm roof space
- Noise reduction: The insulation absorbs some of the fan and airflow noise, reducing the amount transmitted along the duct
For air conditioning supply ducts in hot Indian climates, insulated flexible duct is essentially mandatory — uninsulated supply ductwork in a hot roof space gains heat rapidly, reducing cooling efficiency and causing condensation.
3. Aluminium Flexible Duct
Aluminium flexible duct uses a corrugated foil construction rather than PVC. It is lighter, has a slightly lower static pressure resistance (the smoother inner surface creates less friction), and handles higher temperatures. It is common in HVAC applications and higher-temperature exhaust situations.
How to Choose the Right Flexible Duct Size
Getting the diameter right is the single most important decision. Too small, and the duct creates excessive static pressure that reduces airflow and strains the fan motor. Too large, and you waste money and space.
Match the Duct to the Fan
Always start with the fan's recommended duct connection diameter, which is listed in the product specifications. Astberg fans, for example, specify the correct outlet diameter for each model. Connect the duct directly at this size.
Never reduce the duct diameter along the run. Stepping down from 150mm to 100mm midway through the duct will dramatically increase static pressure and reduce airflow. Upsizing is acceptable; downsizing is not.
Common Sizes and Their Applications
| Diameter | Common Applications |
|---|---|
| 100mm (4") | Bathroom fans, toilet exhaust, small utility rooms |
| 125mm (5") | Larger bathrooms, kitchen fans, some inline fans |
| 150mm (6") | Larger inline fans, kitchen extraction, grow rooms |
| 200mm (8") | Commercial extraction, larger grow rooms |
| 315mm (12") | Industrial, large commercial HVAC branches |
Astberg's flexible duct range covers 100mm and 315mm, with the AFD100 and AFD315 being the two most popular sizes for residential and commercial use respectively.
Keep Duct Runs Short
Every metre of flexible duct adds static pressure resistance. Corrugated flexible duct creates significantly more resistance per metre than smooth rigid duct — up to three to four times as much, depending on the duct and airflow rate.
As a practical rule:
- Keep flexible duct runs under 3 metres wherever possible
- If longer runs are unavoidable, use a larger diameter or a higher-static-pressure fan
- Never exceed the fan manufacturer's maximum recommended duct length
Why Duct Quality Matters
This is where most installations go wrong. Cheap flexible duct — the kind sold in rolls at general hardware stores — looks identical to quality duct on the outside. The differences only become apparent over time.
Wall Thickness
Thin PVC walls compress easily, especially at bends. A duct that kinks at a 90-degree bend has effectively zero airflow at that point. Quality ducting maintains its circular cross-section even through tight bends.
Astberg's AFD series uses a thick outer PVC layer specifically engineered to resist compression and kinking. You can bend it sharply without collapsing the airway.
Helix Wire Gauge
The internal steel helix wire is what keeps the duct from collapsing under negative pressure (suction). Thin, widely-spaced helix wire allows the duct walls to suck inward when the fan is running, choking the airflow. Heavy gauge wire, tightly wound, maintains the duct shape under the full range of operating conditions.
UV and Moisture Resistance
Flexible duct installed in roof spaces is exposed to wide temperature swings, some UV light, and often high moisture levels. PVC that is not properly formulated becomes brittle in these conditions and cracks within a few years. Premium ducting uses UV-stabilised PVC that maintains flexibility and integrity over a long service life.
Airtightness
Poorly manufactured flexible duct can leak air through microscopic pores in the PVC or through poorly sealed end connections. Every litre of air that leaks out of a supply duct or into an exhaust duct is air that does not reach its intended destination — and moisture that enters a roof space or wall cavity.
Flexible Duct Installation: Best Practices
A quality flexible duct installed badly performs worse than a cheap one installed correctly. Follow these guidelines for every installation.
Avoid sharp bends. Flexible duct should never be bent tighter than the manufacturer's minimum bend radius. As a rough rule, the bend radius should be at least equal to the duct diameter. Where sharper turns are needed, use a 90-degree rigid elbow fitting and short sections of flexible duct either side.
Do not compress or crush the duct. Flexible duct performs best when fully extended to its working length. A compressed, accordion-folded duct creates dramatically higher resistance than the same duct stretched out. Never squash it behind a ceiling panel to take up the slack — cut it to length instead.
Secure the ends properly. Use proper duct clamps (not just tape) to secure the flexible duct to fan outlets and external vent grilles. Aluminium duct tape can be used as a secondary sealant over clamps but should not substitute for mechanical fastening.
Support long runs. Flexible duct sags under its own weight if left unsupported. Use duct hangers or saddle straps every 1–1.5 metres on horizontal runs to keep the duct in shape and prevent low points where condensation can pool.
Insulate in cold conditions. In areas that experience cold winters, uninsulated flexible duct carrying warm moist bathroom air through a cold roof space will condense moisture on the inside of the duct. This moisture can drip back into the fan or the room. Use insulated flexible duct in any installation where the duct passes through an unconditioned space.
Common Flexible Duct Mistakes to Avoid
Using the wrong diameter. As covered above, diameter mismatches destroy system performance. Always match the duct to the fan outlet size.
Running the duct uphill. Warm, moist exhaust air contains water vapour that will condense somewhere. If the duct runs uphill, condensate collects in low spots — usually near the fan — and can drip back into the room or damage the fan. Run exhaust ducts with a slight downward pitch toward the external outlet wherever possible.
Forgetting the backdraft damper. Flexible duct on its own does not prevent cold air, wind, insects, or vermin from entering through the external outlet when the fan is off. Always terminate the duct with a grille or cowl that incorporates a backdraft damper — a flap that opens when the fan runs and closes when it stops.
Blocking the duct with insulation. In insulated roofs, loose-fill or batt insulation sometimes gets laid over flexible ductwork and compresses it flat. Make sure the duct route is clear and marked before roof insulation is installed.
Flexible Duct for Grow Rooms
Grow room ventilation is one of the most demanding applications for flexible duct. Systems must move large volumes of air continuously, often through carbon filters and heat exchangers, while maintaining total light exclusion.
For grow room use, the thick grey PVC construction of Astberg's AFD series provides the light-blocking properties that thin foil duct cannot. The duct must also be rated for continuous high-volume airflow without deteriorating — a consideration that rules out cheap general-purpose duct.
Sizes of 150mm and above are typically used in grow room applications, paired with inline fans matched to the room size and target air change rate.
Why Choose Astberg Flexible Duct?
Astberg Ventilation specialises in ventilation systems for Indian homes and commercial spaces. The AFD flexible duct range is designed to work seamlessly with Astberg's inline fan and ceiling exhaust fan products, ensuring a matched system from the fan outlet to the external vent.
- AFD100 (100mm / 4"): Ideal for bathroom fans, toilet extraction, and small inline fan systems. 10 metres per roll. Thick PVC construction with light-blocking outer layer.
- AFD315 (315mm / 12"): Designed for large commercial HVAC applications, industrial extraction, and high-volume grow room systems.
All Astberg flexible duct products are available online with free shipping on orders over ₹5,000, with PAN India delivery within 5 working days.
Final Thoughts
Flexible duct is one of those components that rewards careful selection and proper installation far more than the initial cost difference justifies. A ₹500 saving on cheap duct that kinks, collapses, or fails in three years will cost far more in investigation, remediation, and the damp damage that results.
Spend a few minutes matching the diameter to the fan, planning a clean route without sharp bends, and choosing a duct with the wall thickness and construction quality the application demands. Your ventilation system will perform as designed from day one — and continue to do so for many years.
Browse Astberg's PVC flexible duct range at astbergventilation.com/collections/accessories-1
