Commercial Box Fans

Commercial box fans — also called inline box fans, box duct fans, centrifugal box fans or mixed flow box fans — are the standard fan type for ducted ventilation schemes that need high duty, low noise and a compact rectangular housing. These are commercial and industrial inline fans for M&E installations, not portable or window box fans for domestic cooling. eFans stocks 56 commercial box fans from 100mm to 650mm across three specialist ranges: Systemair (KVK Slim low-profile, KVK Silent acoustic, KV DUO twin-fan redundancy), Elta Multiflow SMB mixed flow, and Vent-Axia Quiet Pack QP and Slimpak SLP EC.

Single-phase AC, three-phase AC and EC energy-efficient motors, all speed controllable. Typical applications: commercial ventilation schemes, kitchen extract, office and retail supply-air, hotel and leisure, inline MVHR boosters, ceiling-void installations where axial fans don't fit. 

In-line Fans / Cased Axial Fans / MVHR Units / Roof Mounted Fans / Speed Controllers

What is a commercial box fan?

A commercial box fan is a centrifugal or mixed-flow impeller housed inside a rectangular steel or insulated box casing, with circular duct spigots on the inlet and outlet for inline ducted installation. They deliver high airflow at moderate-to-high static pressure in a compact footprint that fits above ceiling grids, in plant rooms or inline on exposed ductwork. Do not confuse with a domestic "box fan" — the portable plastic window or floor fan sold at Argos, B&M or similar for home cooling. Commercial box fans are inline ducted kit at £300–£5,000, specified by M&E consultants and installed by ventilation contractors.

What's the difference between a commercial box fan and a domestic box fan?

Domestic box fans are portable plastic units that sit on a floor or in a window, move air around a room for personal cooling, and cost £20–£60 from general retailers. Commercial box fans are permanently installed inline on ductwork, drive a whole ventilation scheme in offices, kitchens, retail or hotels, handle 300–13,000 m³/h at significant static pressure, and cost £300–£5,000 depending on size and motor type. They serve completely different purposes. If you need a portable cooling fan for a room, eFans is the wrong supplier — try a general retailer. If you need inline fans for a commercial ventilation design, this page is for you.

Centrifugal box fan vs mixed flow box fan — which do I need?

Centrifugal box fans use a backward-curved or forward-curved centrifugal impeller inside the box housing — high static pressure capability, ideal for long duct runs, filtration-heavy systems (grease filters, carbon, HEPA) and acoustically sensitive installations. Systemair KVK Slim and KVK Silent are centrifugal-type. Mixed flow box fans use a mixed-flow impeller that combines axial and centrifugal characteristics — higher airflow for the same size, compact, popular in general commercial ventilation. Elta Multiflow SMB is the market-leading mixed flow range. Specify centrifugal for pressure-demanding systems; specify mixed flow for airflow-led balanced schemes.

What is a slim box fan?

A slim box fan is a low-profile commercial box fan designed to fit above standard suspended ceiling grids (typically 600×600 tile) or in tight ceiling voids where a full-depth box fan won't fit. Systemair's KVK Slim range is the specified slim box fan on most UK commercial projects — available in 100mm, 125mm, 160mm, 200mm, 250mm, 315mm, 400mm and 500mm spigot diameters, with both AC and EC motor options. Vent-Axia's Slimpak SLP is the direct alternative. Performance matches full-depth equivalents at a shallower box height.

What is a twin box fan (KV DUO)?

A twin box fan houses two impellers in one casing. Specified for two reasons: (1) redundancy — if one impeller fails, the other continues running, critical for 24/7 commercial buildings, hotels, data rooms and life-safety systems; (2) variable duty — one impeller for low-load periods, both for peak, giving energy savings over a single oversized fan. Systemair KV DUO and KVK Duo are the main twin-fan ranges we stock, in 250mm, 315mm, 400mm, 500mm and 630mm spigots, EC motors throughout.

AC vs EC box fans — which should I specify?

AC box fans are the traditional option — single or three-phase induction motors, controlled via a 5-step or electronic speed controller, lower initial cost. EC box fans use electronically commutated motors — 30–50% more efficient than AC, soft-start, infinitely variable via a 0-10V BMS signal, better part-load performance, longer service life. On new commercial projects with Part L or BREEAM targets, EC is increasingly the default specification. Systemair's KVK Slim and KV DUO are available in both; the KVK Silent is EC-only. Vent-Axia Slimpak SLP is EC-only. Elta Multiflow SMB is AC-only. Retrofits usually match the existing spec for simplicity.

How do I size a commercial box fan?

Calculate the required airflow in m³/h or l/s for the application (air changes per hour × building volume, or specified grille diffusion rates), then sum the system's total static pressure — duct friction, grille loss, filter loss, attenuator loss, any heat-recovery cell, plus 20–30% margin. Plot the duty point on the fan's performance curve and select the fan where the point sits left of peak efficiency on the steeper curve. Box fans typically offer 200–13,000 m³/h at up to 800 Pa. For MVHR booster duty, match the existing MVHR unit's duct size and specify a fan with extractable-box-style installation for future service access.

What applications are commercial box fans used for?

Office and retail ventilation (ceiling-void installs), commercial kitchen extract (paired with a grease filter box upstream), hotel and leisure facilities, care homes, schools and universities, laboratories, inline MVHR boosters to overcome high duct pressure, dedicated smoke extract in refurb schemes, and any commercial scheme where the fan needs to sit inline on ductwork inside a building (not through a wall, not on a roof). Not typically specified for large warehouse or industrial extract — choose plate axial or cased axial for those.

Which brand — Systemair, Elta or Vent-Axia?

Systemair's KVK and KV DUO ranges are the specified default on most UK commercial ventilation projects — widest size range, broadest motor choice (AC, EC, silent, twin), excellent documentation and long service life. Elta Multiflow SMB is the market-leading mixed flow box fan and typically the best-value option on airflow-dominant schemes. Vent-Axia Quiet Pack QP is specified on noise-critical commercial installs (hotel rooms, offices); Slimpak SLP EC covers low-profile EC applications. All three brands are reputable; if you're not sure, send your duty and we'll match.

Are commercial box fans speed controllable?

Yes — every box fan eFans stocks is speed controllable. AC motors run via a 5-step or electronic variable speed controller sized to the fan's full-load current. EC motors accept a 0-10V signal from a BMS, thermostat, timer, CO₂ sensor or manual potentiometer, giving infinite variable speed and typically 30–50% lower energy use than AC at part load. For multi-fan office or hotel installs, specify BMS-integrated EC fans rather than AC with distributed controllers — simpler, more efficient and easier to commission.