Key takeaways
- Ducting is the network of tubes that carries air through a ventilation system — from the fan or unit to the outside world
- The three main types are rigid round PVC, flat channel (rectangular), and flexible aluminium — each suited to different installations
- 100mm is the standard size for most domestic bathroom and kitchen extractor fan installations
- PVC ducting is the most common material for permanent installations; flexible aluminium is used for final connections and tight spaces
- Getting the duct size and type right matters — undersized or poorly routed ducting reduces airflow and makes fans work harder
Ducting is the system of pipes and channels that moves air from one place to another in a ventilation installation — from an extractor fan or MVHR unit, through the building fabric, to an external outlet. Without correctly sized and installed ductwork, even the best fan or ventilation unit won't perform as specified. This guide covers everything you need to know about choosing the right ducting for your project.
Types of Ducting
Rigid Round PVC Ducting
Rigid round PVC ducting is the most common type used in domestic ventilation. It comes in standard diameters — most commonly 100mm, 125mm, 150mm, and 200mm — and is sold in short lengths that connect via push-fit spigot connectors, bends, and adaptors. It's easy to install, airtight when properly jointed, and long-lasting. Used for bathroom extractor fans, kitchen extraction, and as part of MVHR duct runs.
Flat Channel (Rectangular) Ducting
Flat channel ducting — sometimes called rectangular ducting — is designed for installations where ceiling or wall void depth is limited. Instead of a round cross-section, it uses a flat rectangular profile (common sizes: 110×54mm, 204×60mm, 220×90mm) that fits neatly into shallow spaces above kitchen units or within studwork. Round-to-rectangular adaptors allow it to connect to standard round fans and terminal grilles. Domus Ventilation's flat channel systems are the most widely used in the UK for kitchen cooker hood extraction.
Flexible Ducting
Flexible ducting is a corrugated aluminium or plastic tube that can bend and flex around obstacles. It's used for final connections between rigid ductwork and a fan or terminal — the last short section where some movement or misalignment needs to be accommodated. It should never be used for long duct runs; its corrugated surface creates far more airflow resistance than rigid duct, and long flexible runs significantly reduce fan performance. Keep flexible sections as short and straight as possible.
Insulated Ducting
Insulated ducting has a layer of thermal insulation around the duct wall. This is important wherever ductwork passes through unheated spaces — loft voids in particular — to prevent warm moist air inside the duct from condensing on the cold duct wall. For MVHR systems and any extract ductwork running through a cold loft, insulated ducting is the correct choice.
Ducting Materials
PVC (rigid): The standard material for domestic ventilation ductwork. Lightweight, airtight, easy to cut and join, and resistant to moisture. Available in all standard diameters and fittings.
Aluminium (flexible): Used for flexible sections and some specialised applications. More heat-resistant than PVC, making it suitable where the duct passes close to heat sources such as a boiler flue.
Choosing the Right Duct Size
Duct diameter must match the connection size of the fan or unit at one end, and the external wall or roof terminal at the other. The most common domestic sizes are:
- 100mm (4"): Standard for most bathroom and utility room extractor fans
- 125mm (5"): Used for higher-flow bathroom fans and some kitchen applications
- 150mm (6"): Kitchen cooker hoods, higher-flow residential extraction
- 200mm (8") and above: Commercial extraction, MVHR main duct runs, and inline fan installations
Reducing the duct diameter from a fan's connection size — even by one step — significantly increases resistance and reduces airflow. Always size the ductwork to match the fan, not the other way around.
Ducting Fittings and Accessories
A complete duct run needs more than straight lengths. Common fittings include:
- Bends (45° and 90°): Change the direction of the duct run. Use 45° bends where possible — two 45° bends create less resistance than one 90°
- Connectors: Join two lengths of straight duct
- Adaptors: Change between diameters or from round to rectangular
- Tee pieces and Y pieces: Split a single duct run into two — used in MEV systems serving multiple rooms
- Jubilee clips and clamps: Secure flexible duct to spigot connections
Browse our full ducting and ductwork fittings range — 169 products from Domus Ventilation, Systemair, and our own eFans brand, available for next day delivery.
Frequently Asked Questions
What is the difference between ducting and ductwork?
Ducting refers to the individual pipes, channels, and fittings that make up a ventilation system. Ductwork is the broader term for the complete installed system — the planning, routing, and assembly of all the ducting components together. In everyday use the two terms are often used interchangeably.
What size ducting do I need for a bathroom extractor fan?
Most domestic bathroom extractor fans have a 100mm spigot connection, so 100mm rigid round PVC ducting is standard. Some higher-specification fans use a 125mm connection — always check the product specification before buying ducting. The duct run should be the same diameter all the way from fan to external outlet, with no reductions.
How long should a duct run be?
Building Regulations Part F provides maximum duct run lengths for different fan types and duct sizes. As a general guide, a 100mm duct run for a bathroom fan should not exceed around 4–6 metres with a straight run, less if bends are included. Each 90° bend is equivalent to approximately 1 metre of straight duct in terms of resistance. For longer runs, use a larger duct diameter or upgrade to an inline mixed flow fan, which generates higher static pressure to overcome the resistance.
Can I use flexible ducting for an entire duct run?
No — flexible ducting should only be used for short final connections, typically no more than 1–1.5 metres. The corrugated interior surface creates significantly more airflow resistance than smooth rigid duct, and a long flexible run will substantially reduce the fan's effective airflow output. Use rigid ducting for the main run and flexible only where needed at the connection point.
What ducting is needed for an MVHR system?
Whole house MVHR systems require a full network of rigid round ducting — typically 75mm or 90mm radial spurs from the rooms to a central manifold, or 125–200mm main duct runs depending on the system design. Insulated ducting is essential wherever the duct runs through unheated loft space. MVHR duct runs need careful planning to balance airflow across all rooms — refer to our MVHR buying guide for more detail.
How long does ducting last?
Rigid PVC ducting correctly installed and undisturbed will last the lifetime of the building — 30 years or more is typical. Flexible aluminium ducting has a shorter lifespan and should be replaced if it becomes kinked, torn, or compressed. The most common cause of duct failure is poor installation — joints not properly sealed, flexible duct left too long or too compressed, or runs with too many bends creating backpressure.