Combi Ducting

Combi ducting — also called combi flexible ducting, PVC flexible ducting or combi hose — is flexible ductwork built with a tough PVC outer jacket bonded over an aluminium inner duct and reinforced with a wire helix. It combines the durability and moisture resistance of PVC with the heat tolerance and smooth airflow of aluminium, giving a flexible duct that handles general-purpose extract and supply use with a harder outer surface than plain aluminium or fully laminated flex. We stock Systemair flexible combi ducting in 127mm, 160mm and 315mm diameters, all in 6m lengths. Typical applications include extract and supply runs in plant rooms, workshops, commercial kitchens (away from grease), loft and service voids where the duct may take knocks, and temporary or semi-permanent extract runs on construction and industrial sites. In stock with next-day delivery available.

All Flexible Ducting / Aluminium Ducting / Insulated Ducting / All Ducting

What is combi ducting?

Combi ducting is flexible ductwork made from two layers — a PVC outer jacket bonded over an aluminium inner duct, reinforced by a wire helix. The "combi" name comes from combining the two materials: the PVC outer gives abrasion resistance, moisture protection and a tougher external surface, while the aluminium inner gives smooth-ish airflow and tolerates warm extract air. It's sometimes called PVC flexible ducting or combi hose. It sits between plain aluminium flex and insulated flex in the range — more robust than aluminium, cheaper than insulated, and suited to general-purpose extract and supply where the airstream is below around 100°C.

What is combi ducting used for?

Combi ducting is used for general-purpose extract and supply runs where a tough, durable flexible duct is needed. The PVC outer jacket resists abrasion, moisture and minor impact better than plain aluminium or laminated combi ducting, which makes it the right choice in workshops, plant rooms, construction sites, and any install where the duct might get knocked, walked on, or exposed to damp. Also commonly used for temporary extract on industrial sites and for industrial fume extract where the airstream is cool and non-corrosive. The aluminium inner duct handles temperatures up to around 100°C continuously.

How is combi ducting different from aluminium or insulated ducting?

All three sit under flexible ducting but serve different use cases. Aluminium flexible is full aluminium construction — best for hot, greasy extract such as cooker hoods and tumble dryers, rated to around 250°C. Insulated flexible wraps a 25mm glass wool jacket over an aluminium inner duct — used where condensation risk or fan noise needs controlling, typically MVHR through cold lofts. Combi flexible (this page) has a PVC outer over aluminium inner — used where the duct needs to be robust and moisture-resistant, handles cooler airstreams up to ~100°C, and costs less than pure aluminium.

What sizes of combi ducting do you stock?

We stock Systemair flexible combi ducting in three diameters: 127mm, 160mm and 315mm, all in 6m lengths. 127mm is the standard domestic and light commercial extract size. 160mm covers larger kitchen, utility and small commercial extract. 315mm is specified for industrial extract, plant room connections, and large AHU takeoffs. Other sizes (100mm, 150mm, 200mm, 250mm) can usually be ordered to stock — contact us with your diameter and quantity for availability.

Can combi ducting be used for cooker hoods?

Only for domestic, lower-temperature cooker hoods where the airstream stays below around 100°C. For commercial kitchen hoods, high-output range cookers, or any sustained high-temperature greasy extract, specify aluminium flexible ducting (rated to 250°C) or rigid metal spiral instead. Always check the cooker hood manufacturer's installation specification — many require metal ducting regardless of airstream temperature. For standard domestic cooker hoods with moderate use, combi flexible is acceptable but aluminium is the safer default spec.

Can combi ducting be used outside?

Short external connections in sheltered positions are acceptable, but combi flexible ducting is not specified as a fully weatherproof external duct. The PVC jacket is UV-resistant to a degree but will eventually degrade with prolonged sunlight exposure. For any external run, transition to rigid PVC, galvanised or aluminium spiral ducting before the external wall, and terminate with a weatherproof cowl, louvre or wall kit. Indoor plant room, workshop and void installs are the correct use case.

How do I install and join combi ducting?

Push the combi duct over the spigot of the fan, rigid duct or terminal and secure with a worm-drive hose clamp. For airtightness on MVHR and building-regs extract, add a bead of duct sealant around the joint and foil-tape over the clamp. Pull the duct taut before clamping — slack reduces airflow and traps moisture. Support horizontal runs with banding straps or saddle clips every 600mm. Minimum straight runs of 300–500mm before and after fans and bends help maintain airflow.

What temperature can combi ducting handle?

Continuous operating temperature up to around 100°C. The aluminium inner duct handles the heat; the PVC outer jacket limits the top-end rating compared with pure aluminium flexible (250°C). For temperatures above 100°C — commercial kitchen extract, process exhaust, flue applications — specify aluminium or stainless steel spiral instead. For normal domestic and light commercial extract and supply, the 100°C rating covers all typical use cases.

Does combi ducting restrict airflow?

Yes, like all flexible ducting. The corrugated inner wall and any bends or slack add pressure loss. As a rule of thumb, 1m of flexible duct is equivalent to roughly 3–5m of smooth rigid duct in pressure loss, and every 90° bend adds another 1m-equivalent. Keep flexible runs short, pull them taut, avoid tight bends, and use rigid ducting for the main duct run wherever possible. If a fan isn't delivering its rated airflow, excess flexible ducting is almost always the cause.