Inline Duct Fans

Frequently asked questions

What is an inline duct fan?

An inline duct fan, also written in-line fan or inline extractor fan, is a ventilation fan designed to fit inside a round or rectangular duct run rather than at the terminal point. Air is drawn in one end of the fan housing and pushed out the other, so the fan can sit anywhere along the duct between the extract grille and the discharge point. This lets you position the fan away from the occupied space for noise reasons, or closer to the discharge to keep the duct under negative pressure. Inline fans are the most common fan type in commercial M&E installations because they're flexible, compact and easy to service.

Centrifugal, mixed flow or circular: what's the difference?

Centrifugal inline fans (S&P VENT-NK, Hydor HIT, Vent-Axia SDX, Systemair K) use a forward-curved impeller and are best where the system has high static pressure (long duct runs, bends, filters or carbon units), because they maintain airflow against resistance. Mixed-flow inline fans (S&P TD Mixvent, Systemair KVK) combine axial and centrifugal characteristics, giving good airflow with lower noise and lower power draw, and are ideal for offices, toilets and residential extract. Circular duct fans (Systemair RVK, Elta VL) are axial-type fans in a tubular housing: high airflow, lower pressure, cost-effective for simple short-run extract. If the duct run is under 5 metres with one or two bends, mixed flow or circular is usually fine; over that, step up to centrifugal.

How do I size an inline duct fan?

Three numbers decide the fan: duct diameter (match the fan to the existing duct: 100mm, 125mm, 150mm, 160mm, 200mm, 250mm, 315mm, 355mm or 400mm), required airflow in m³/h or l/s (from your ventilation design or Building Regulations Part F for the application), and the static pressure the fan has to overcome (duct length, bends, grilles, filters and dampers each add resistance). Manufacturers publish fan curves showing airflow against pressure. The operating point needs to sit on the curve above your design pressure, not at the fan's free-air rating. If you've only got duct size and target airflow, send us the details and we'll match it.

Which brand: Systemair, S&P, Elta, Hydor or Vent-Axia?

Systemair K and RVK are the default specified brand in UK commercial projects: robust, well-documented, and you can buy matching controllers and accessories off the shelf. S&P VENT-NK and JETLINE are strong on centrifugal performance and value, and the TD Mixvent is one of the best mixed-flow ranges on the market. Elta VL is a UK-made circular duct fan with straightforward pricing and good availability. Hydor HIT is a budget centrifugal alternative that matches S&P VENT-NK performance at a lower price point. Vent-Axia SDX fits where a recognised domestic/light-commercial brand is specified. If you want quiet, go Systemair; if you want value, go Hydor or Elta; if it's specified, match the spec.

Kitchen extract: which inline fan handles grease and heat?

Domestic kitchen extract (hoods, single-dwelling) is fine on a standard inline centrifugal like the S&P VENT-NK or Systemair K. Commercial kitchens with a canopy are a different category. The fan needs to handle continuous elevated temperatures and grease-laden air, which means either a roof-mounted kitchen extract fan or a properly-rated inline fan like the Systemair KBR kitchen fan range. Standard inline duct fans on this page are not rated for grease extract. If you're specifying for a commercial kitchen, talk to us first and we'll point you to the right product.

What's the difference between an inline duct fan and a box fan?

Inline duct fans are usually circular and sit in round duct with matching connections either end. Commercial box fans (Systemair KVK Slim/Silent, Elta Multiflow SMB, Vent-Axia QP) do the same job but have a rectangular box-shaped housing with acoustic insulation built in, and are typically used where a lower noise profile or a specific installation footprint is needed. Functionally they overlap, with a centrifugal or mixed-flow impeller moving air along a duct run, but if you need quiet, a box fan usually wins; if you want simple and compact, a circular inline fan wins.

Can I control the speed of an inline duct fan?

Every AC inline fan on this page is speed-controllable via a TRIAC or thyristor controller (we stock Systemair RE-1.5 / RE-4 / RTRD, S&P REB and Elta matching controllers). EC fans (anything with "EC" in the model name) are controlled via a 0–10V signal from a BMS, PIR sensor, CO₂ sensor or humidistat, and use roughly 30–50% less energy than AC equivalents at part load. For toilets and offices where the fan runs all day, EC pays back quickly; for short-run extract it's often not worth the uplift.

Duct booster fans: where do I use one?

An inline duct fan used as a booster sits mid-run to lift airflow when the primary fan is at the limit of its pressure capability, or when a long duct run has starved the far end. The 100mm and 125mm circular fans on this page (Elta VL100/125, Systemair RVK 100E2/125E2, S&P TD-100) are the most common duct boosters for residential or small commercial applications. For larger systems the Systemair K XL range is the typical booster. Install with matching flexible connectors to isolate vibration, and wire through the primary fan's controls so the booster runs with the main fan rather than independently.

Do I need an EC fan or is an AC fan fine?

AC is still the right choice for short-run, on-demand extract (toilets, cupboards, small shops) where total runtime is low. EC makes sense where the fan runs more than a few hours a day (offices, restaurants, MVHR, server rooms), because the 30–50% energy saving pays back within a year or two, and EC gives you proper modulation via a 0–10V signal rather than fixed-step speed control. If the project is chasing a low Specific Fan Power (SFP) rating for Building Regulations Part L or a BREEAM credit, EC is almost always mandated. Systemair K EC and S&P TD EVO publish SFP figures in their datasheets so you can match to the design target.

What accessories do I need with an inline duct fan?

At minimum: matching flexible ducting or rigid duct, two jubilee clips or duct tape for connections, and a speed controller. Most installations also include a flexible rubber connector either side of the fan to isolate vibration and noise, and a non-return damper (shutter) on the discharge to stop backdraught. For kitchen and toilet extract, add a wall louvre or cowl at the discharge point. We stock everything you need alongside the fan, so order the fan and we'll flag the matching accessories.