A flexible duct connector is a short section of flexible fabric installed between HVAC equipment (fans, air handling units, VAV boxes) and rigid ductwork. It serves two critical functions: isolating mechanical vibration so fan noise does not transmit through the duct system, and accommodating thermal expansion and minor misalignment between equipment and ductwork.
Without a flex connector, a direct rigid connection transmits every vibration from the fan motor into the sheet metal duct — which acts as a speaker membrane, amplifying the noise throughout the building. A properly installed flex duct connector reduces transmitted vibration by 85-95%, making it one of the most cost-effective noise control measures in any HVAC system.
We manufacture flexible duct connectors in both round and rectangular configurations from our Brookshire, TX facility. This guide covers material selection, sizing, installation, and common mistakes based on 30 years of production and field experience.
Types of Flexible Duct Connectors
1. Canvas Duct Connector
Canvas connectors are the most economical option. Made from woven cotton or polyester fabric, sometimes with a vinyl or acrylic coating for moisture resistance. Temperature rating is typically 180°F (82°C) continuous. Canvas has good flexibility and is easy to work with, but it has the shortest lifespan (5-10 years) and the lowest fire resistance of the three common materials.
Best for: Standard commercial HVAC in dry environments. Supply and return air connections where temperatures stay below 150°F. Budget-conscious projects where replacement access is available.
2. Neoprene-Coated Fiberglass Connector
The industry standard for commercial HVAC. A fiberglass fabric base is coated with neoprene rubber, providing excellent flexibility, fire resistance (UL 181 Class 1), chemical resistance, and durability. Temperature rating is 200-250°F (93-121°C) continuous. Lifespan is 15-20+ years in normal conditions.
Best for: Nearly all commercial applications. Supply, return, and exhaust air connections. Projects requiring UL 181 listing. Humid environments where moisture resistance matters.
3. Silicone-Coated Fiberglass Connector
High-temperature flexible connectors use silicone-coated fiberglass fabric rated to 500°F (260°C) or higher. Silicone maintains flexibility at extreme temperatures where neoprene would harden and crack. These connectors are also resistant to ozone, UV, and most chemicals.
Best for: Kitchen exhaust hoods, industrial process exhaust, boiler connections, and any application where duct air temperature exceeds 200°F. Required by code for grease-laden exhaust systems in commercial kitchens.
Sizing & Dimensions
Flexible duct connectors must match the duct size exactly. Round connectors are specified by inside diameter; rectangular connectors by width × height. Standard dimensions:
| Parameter | Round | Rectangular |
|---|---|---|
| Diameter / Size range | 4" - 60" | 6×6" to 96×48" |
| Exposed length | 4-6" (standard) | 4-6" (standard) |
| Overall length (with flanges) | 6-10" | 6-10" |
| Flange width | 1-2" clamp band | 1-1.5" standing seam |
| Pressure rating | ±10" w.g. | ±6" w.g. |
Critical dimension: The exposed flexible length (the distance between the two rigid connections) should be 4-6 inches. This provides sufficient movement for vibration isolation without introducing excessive sag or turbulence. For large rectangular connectors (over 48 inches wide), an internal fabric stiffener may be needed to prevent the connector from collapsing under negative pressure.
Installation Guide
Correct installation is essential — an improperly installed flex connector can actually increase noise, create air leaks, or fail prematurely.
Step 1: Align equipment and duct. The rigid duct and equipment flanges should be aligned within 0.5 inches in all directions before installing the connector. The flex connector absorbs minor misalignment, but it is not designed to bridge large offsets. Forcing the connector to span a 2-3 inch gap creates a permanent stress that shortens its lifespan.
Step 2: Attach to equipment side first. Slide the connector over the equipment discharge (or intake) flange. Secure with a band clamp (round) or screws through the standing seam (rectangular). The fabric should lay flat against the flange — no wrinkles, folds, or bunching.
Step 3: Attach to duct side. With the equipment-side secure, connect the opposite end to the rigid duct. Allow the fabric to hang naturally between the two connections with a slight droop — about 0.5-1 inch of slack. Pulling the connector taut defeats the vibration isolation; too much slack creates turbulence and noise.
Step 4: Seal and inspect. Apply mastic sealant at both connections if the system requires Seal Class A (most commercial projects). Inspect that the fabric is not twisted, kinked, or in contact with any sharp edges that could puncture it. Ensure no screws protrude through the fabric.
Common Installation Mistakes
Painting the connector. Never paint a flex connector. Paint stiffens the fabric, reducing flexibility and vibration isolation. It also voids the UL listing. If aesthetics matter, specify a factory-colored connector or install a metal band cover.
Installing too tight. A taut connector transmits vibration directly — it becomes a rigid connection with extra weight. Always leave 0.5-1 inch of slack.
Using duct tape for sealing. Cloth duct tape fails within months in HVAC environments. Use foil tape or mastic for sealing connector-to-duct joints. The connector fabric itself provides the air barrier; the sealant prevents leakage at the clamp or flange joint.
Ignoring the return side. Return air connections need flex connectors too. Return fans and return air plenums generate vibration just like supply fans. Skipping the return-side connector is a common shortcut that leads to noise complaints.
Need flexible duct connectors for your project? Browse our connector product page or request a quote for custom sizes and materials.
Related Guides
- Duct Boot Guide — register boot types, sizing, and how they connect to flex duct
- Duct Fittings Guide — elbows, wyes, tees, reducers, and static pressure data
- Standard Duct Sizes Chart — dimensions 3" to 24" with CFM capacity
- Duct Sealing Guide — mastic, foil tape, and Aeroseal methods for airtight connections
- Flex Duct Installation Guide — step-by-step installation instructions