Ducting is the network of pipes and channels that moves air through a ventilation system, carrying it from an extractor fan or MVHR unit, through the building fabric, to an outlet outside. It is the part of the system that actually delivers the air, and if it is the wrong size, the wrong type, or badly routed, even the best fan will not perform as specified. This guide explains what ducting is, the main types and materials, the standard sizes, and how to choose the right duct for your project.
Key takeaways
- Ducting is the tubing that carries air through a ventilation system, from the fan or unit to the outside world.
- The main types are rigid round PVC, flat channel (rectangular), flexible aluminium, and insulated, each suited to different installations.
- 100mm is the standard size for most domestic bathroom and kitchen extractor fans; commercial and MVHR runs go larger.
- Rigid PVC is the default for permanent runs; flexible is for short final connections only; insulated is for cold loft spaces.
- Size the duct to match the fan, not the other way around. Undersized or over-bent ducting throttles airflow and makes the fan work harder.
In this guide
What is ducting?
Ducting is the system of pipes and channels that moves air from one place to another in a ventilation installation. In a typical setup, a fan or ventilation unit draws air in, and the ducting carries that air through walls, ceilings and voids to an external outlet, or brings fresh air the other way. It is the delivery network of the system: the fan provides the push, and the ducting decides how much of that air actually arrives.
Because the ducting sets the resistance the fan has to work against, it has as much influence on real-world performance as the fan itself. A powerful fan connected to undersized, over-bent or unnecessarily long ductwork will move a fraction of its rated air. That is why sizing and routing the ducting correctly is the single most important part of a ventilation install.
What ducting is used for
Ducting appears in almost every kind of building ventilation. The most common applications are:
- Extract ventilation: carrying stale, damp air out of bathrooms, kitchens and utility rooms to an external wall or roof terminal.
- Supply ventilation: bringing fresh air into a space, either on its own or as part of a balanced system.
- Heat recovery (MVHR): a full network of supply and extract ducting connecting every wet room and habitable room back to a central heat-recovery unit.
- Commercial and industrial extraction: larger-diameter duct runs serving commercial kitchens, workshops, plant rooms and warehouses.
In every case the job of the ducting is the same: move a specified volume of air with as little resistance as possible.
Fire safety note: any duct passing through a fire compartment wall or floor needs a fire damper at the penetration to stop fire and smoke travelling through the ductwork. See our complete guide to fire dampers, or browse the fire dampers range, if your run crosses a compartment line.
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 is easy to install, airtight when properly jointed, and long-lasting. It is used for bathroom extractor fans, kitchen extraction and as part of MVHR duct runs. Browse rigid ducting and the diameter-specific ranges: 100mm, 125mm and 150mm.
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 110x54mm, 204x60mm, 220x90mm) that fits neatly into shallow spaces above kitchen units or within studwork. Round-to-rectangular adaptors let it 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 around obstacles. It is used for the final connection between rigid ductwork and a fan or terminal, the last short section where some movement or misalignment needs to be absorbed. It should never be used for long 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. See the flexible ducting range.
Insulated ducting
Insulated ducting has a layer of thermal insulation around the duct wall. This matters wherever ductwork passes through unheated spaces, loft voids in particular, to stop warm moist air inside the duct condensing on the cold duct wall and dripping back. For MVHR systems and any extract ductwork running through a cold loft, insulated ducting is the correct choice. See the insulated ducting range.
Duct type comparison
| Type | Best for | Avoid for |
|---|---|---|
| Rigid round PVC | Main permanent runs, best airflow | Shallow voids where depth is tight |
| Flat channel (rectangular) | Shallow voids above units, kitchen hoods | High-flow or long commercial runs |
| Flexible | Short final connections only | Long runs (huge resistance) |
| Insulated | Cold lofts, MVHR, condensation risk | Nothing, but costs more than plain duct |
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, which makes it suitable where the duct passes close to a heat source such as a boiler flue.
Ducting sizes
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:
| Size | Typical use |
|---|---|
| 100mm (4") | Standard for most bathroom and utility room extractor fans |
| 125mm (5") | 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, inline fan installations |
Reducing the duct diameter below the fan's connection size, even by one step, significantly increases resistance and cuts airflow. Always size the ductwork to match the fan, not the other way around.
Fittings and accessories
A complete duct run needs more than straight lengths. Common fittings include:
- Bends (45° and 90°): change the direction of the run. Use 45° bends where possible, as 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 and Y pieces: split a single run into two, used in MEV systems serving multiple rooms.
- Jubilee clips and clamps: secure flexible duct to spigot connections.
Browse the full ducting and ductwork fittings range, with 169 products from Domus Ventilation, Systemair and our own eFans brand, available for next-day delivery.
Ducting vs ductwork
The two terms are often used interchangeably, but there is a distinction. Ducting refers to the individual pipes, channels and fittings, the physical components. Ductwork is the broader term for the complete installed system: the planning, routing and assembly of all those components into a working network. In everyday trade use you will hear both, and usually it does not matter which you say.
Common ducting mistakes
Most underperforming ventilation comes down to the ductwork, not the fan. The usual culprits are: reducing the duct below the fan's outlet size, running long lengths of flexible duct instead of rigid, packing in too many 90° bends, and leaving joints unsealed so air leaks out before it reaches the outlet. For a deeper look at specification errors and how they interact with Building Regulations, see our guide to ducting in construction: specification mistakes and building regs.
FAQs
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, so always check the product specification before buying. 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 sets 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 to 6 metres on a straight run, less if bends are included. Each 90° bend is worth roughly 1 metre of straight duct in resistance. For longer runs, use a larger diameter or 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 to 1.5 metres. The corrugated interior creates far more airflow resistance than smooth rigid duct, and a long flexible run will substantially reduce the fan's effective output. Use rigid ducting for the main run and flexible only at the connection point.
What ducting is needed for an MVHR system?
Whole-house MVHR systems need a full network of rigid round ducting, typically 75mm or 90mm radial spurs from the rooms to a central manifold, or 125 to 200mm main runs depending on the design. Insulated ducting is essential wherever the duct runs through unheated loft space. MVHR runs need careful planning to balance airflow across all rooms; see our MVHR ducting size guide for detail.
How long does ducting last?
Rigid PVC ducting correctly installed and undisturbed will last the lifetime of the building, 30 years or more. Flexible aluminium ducting has a shorter lifespan and should be replaced if it becomes kinked, torn or compressed. The most common cause of failure is poor installation: unsealed joints, flexible duct left too long or compressed, or runs with too many bends creating backpressure.
Not sure what you need? Browse the full ducting range, or call the eFans team on 0161 818 2050 and we will help you size the run and pick the right fittings.
Last updated: July 2026. Guidance is general; always check current Building Regulations Part F and the manufacturer's specification for your fan or unit. For sizing help, get in touch.