What is a Flexible Duct?
A flexible duct (also called a flex duct, flexible duct pipe, or flexible air duct) is a bendable, tubular conduit used to channel airflow between HVAC equipment, fans, vents, and outlets. Unlike rigid sheet metal ductwork, a flexible duct can be routed around beams, walls, and tight corners — making it the go-to solution for both new installations and retrofits.
In India, flexible ducts are used across residential kitchens, commercial HVAC systems, grow tents, bathroom exhausts, dryer vents, and fresh air units. They are typically made from either aluminium alloy or PVC (polyvinyl chloride), each suited to different environments.
Aluminium vs PVC Flexible Duct — Which is Better?
Both aluminium and PVC flexible ducts serve the same core function — channelling air — but they're built for different conditions. Here's a side-by-side comparison:
| Feature | Aluminium Flexible Duct | PVC Flexible Duct |
|---|---|---|
| Material | Double-wall aluminium alloy + steel wire | Grey PVC + steel wire helix |
| Heat tolerance | ✓ High (up to 200°C) | ✗ Moderate (up to 60°C) |
| Light blocking | ✗ No | ✓ Yes (grow rooms) |
| Corrosion resistance | ✓ Excellent | ✓ Good |
| Best for | HVAC, AHUs, dryers, high-temp exhaust | Grow tents, ventilation, low-temp exhaust |
| Flexibility | Bend, compress, extend without leaks | Bend, compress, extend without leaks |
| Length available | 3 metres | 3 metres |
| Delamination risk | ✓ None (no delamination on heat aging) | ✓ None under rated temp |
Flexible Duct Sizes Explained
Choosing the right diameter is critical. A duct that's too narrow creates back pressure and noise; one that's too wide wastes space and costs more. Common flexible duct sizes include:
| Size (mm) | Imperial | Typical Airflow | Best Use |
|---|---|---|---|
| 100 mm | 4 inch | Up to 100 m³/h | Bathroom exhaust, small inline fans |
| 125 mm | 5 inch | Up to 160 m³/h | Kitchen exhaust, grow tent ventilation |
| 150 mm | 6 inch | Up to 250 m³/h | Range hoods, medium HVAC connections |
| 200 mm | 8 inch | Up to 500 m³/h | Large HVAC systems, commercial exhaust |
| 250 mm | 10 inch | Up to 800 m³/h | Air handling units, industrial ventilation |
Always match the flexible duct diameter to your fan or HVAC unit's outlet size. Using a reducer connector is acceptable but may slightly reduce airflow efficiency.
Top Applications of Flexible Duct
HVAC Systems
Connect air handling units and split systems to supply/return vents, routing air through ceiling cavities and wall spaces.
Kitchen Exhaust
Route cooking fumes from range hoods through walls or ceilings to external wall vents — flexible ducts handle the bends easily.
Grow Rooms & Tents
PVC flexible ducts with light-blocking grey PVC prevent light leaks while maintaining ideal airflow for indoor growing environments.
Dryer Vents
Aluminium flexible ducts safely exhaust hot, moist air from clothes dryers to the outside — heat-resistant and non-delaminating.
Commercial Ventilation
Used alongside inline fans and HRV/ERV units to distribute fresh air across offices, meeting rooms, and commercial kitchens.
Industrial & AHU
Larger 200mm–250mm ducts connect air handling units to zone diffusers in factories, hospitals, and large commercial buildings.
Flexible Duct Installation Tips
Do's
Keep the duct as straight and short as possible — every bend adds resistance. Fully extend the duct before securing; compressed duct reduces airflow significantly. Use proper duct tape or clamp rings to seal connections at both ends. Support longer horizontal runs with hangers every 1.5 metres to prevent sagging.
Don'ts
Never kink or crush a flexible duct — this restricts airflow and creates hot spots. Don't exceed the duct's temperature rating. Avoid tight 90-degree bends; use two 45-degree bends instead for better airflow. Don't leave duct connections untaped, as air leaks reduce system efficiency.
Astberg Flexible Duct Range
Astberg Ventilation offers a complete range of aluminium and PVC flexible ducts, all in standard 3-metre lengths, with free shipping on orders over ₹5,000 and PAN India delivery within 5 working days.
