Last updated: September 10, 2026
Room volume, air changes per hour, and the duct losses that make most wall fans underperform.
A wall panel exhaust fan is the simplest ventilation device on a construction site and the one most often specified wrong. The reason is almost never the fan — it is that the fan's catalogue figure is measured at free air, with nothing in front of it and nothing behind it. Once a shutter, a duct and a bend are added, the air that actually leaves the room is a different number. This guide walks through the four steps we use when a buyer sends us a floor plan.
A panel fan is an axial fan: a propeller pushing air straight through a square wall plate. That design is excellent at moving volume and poor at generating pressure. A cased axial or inline mixed-flow fan can push air down ten metres of duct; a wall panel fan cannot. So when you size a panel fan, you are really asking two questions at once — how much air does the room need, and how much of the fan's free-air rating will survive the installation?
Length × width × height, in metres. Use the finished ceiling height, not the structural slab, and measure the actual room rather than the floor plan — a 3.0 × 2.5 m bathroom at 2.4 m is 18 m³, not the 20 m³ a plan suggests.
Two cases catch people out:
Air changes per hour (ACH) is how many times the entire room volume is replaced each hour. It is the industry shorthand for "how dirty or how humid does this room get".
| Space | ACH | Why |
|---|---|---|
| Bathroom, en-suite, washroom | 8 – 10 | The baseline. Below 8 the mirror stays fogged and damp lingers. |
| Public toilet, high traffic | 10 – 15 | More occupants per m³ means more air changes, not fewer. |
| Domestic kitchen | 15 – 20 | Grease, moisture and odour all need a faster turnover. |
| Restaurant / commercial kitchen | 20 – 30 | Pair extract with make-up air or the fan will starve. |
| Utility room, laundry | 8 – 10 | Moisture-led, same band as a bathroom. |
| Office, shop, reception | 6 – 10 | Comfort ventilation rather than extract. |
| Workshop, light industrial | 6 – 10 | Add a local hood for welding or dust-generating work. |
| Warehouse, storage | 4 – 6 | Low occupancy, so the priority is condensation control. |
| Poultry / livestock shed | 8 – 12 | Ammonia and humidity drive the requirement, not comfort. |
Local codes override this table. The ranges above are standard ventilation practice for the Middle East, Europe and North America, but a building code, a hotel brand standard or a client specification always wins. If you are working to a tender document, send us the ACH figure and we will size to it.
The formula is two multiplications:
Required airflow (m³/h) = Room volume (m³) × ACH
Required airflow (CFM) = m³/h ÷ 1.699
Now compare that against the range. KLT-10B moves 150 m³/h at free air, which is below the requirement before any duct loss — so it is the wrong choice for this room. KLT-12B at 380 m³/h has comfortable headroom for the duct and the shutter. The gap between 150 and 380 m³/h is why the 8-inch model outsells the 6-inch model in hotel projects.
This is the step that decides whether the installation works. Every component between the fan and the outside air costs pressure, and pressure costs airflow.
| Installation | Practical allowance | Comment |
|---|---|---|
| Through the wall, shutter only, no duct | 0 – 5% | Best case for a panel fan. This is what the catalogue figure assumes. |
| Up to 1 m of straight duct | ≈ 10% | Typical bathroom with a short spigot. |
| 1 – 3 m of duct | 15 – 20% | Add more for every 90° bend and for flexible (ribbed) hose. |
| 3 – 5 m of duct | 25 – 30% | Flexible duct at this length can halve the airflow. Use rigid. |
| Over 5 m, or two or more bends | Not recommended | Specify an inline mixed-flow duct fan instead — see our inline duct fan range. |
Do not size a wall panel fan for a long duct run. A panel fan on ten metres of flexible hose will turn the impeller, make noise and move almost nothing. If the geometry forces a long run, the correct answer is a different fan, not a bigger one.
The table below applies the calculation above to the whole KLT series, assuming 10 ACH, a 2.5 m ceiling and a 15% allowance for the shutter plus a short duct. Treat it as a starting point, not a specification.
| Model | Airflow (free air) | Noise | Typical coverage* | Power | Detail |
|---|---|---|---|---|---|
| KLT-10B 6-inch | 150 m³/h | 35 dB | ≈ 5 m² | 30 W | View KLT-10B → |
| KLT-12B 8-inch | 380 m³/h | 38 dB | ≈ 13 m² | 30 W | View KLT-12B → |
| KLT-16B 9-inch | 396 m³/h | 40 dB | ≈ 13 m² | 40 W | View KLT-16B → |
| KLT-18B 10-inch | 432 m³/h | 42 dB | ≈ 15 m² | 40 W | View KLT-18B → |
| KLT-20B 12-inch | 792 m³/h | 45 dB | ≈ 27 m² | 80 W | View KLT-20B → |
| KLT-25B 14-inch | 1,500 m³/h | 48 dB | ≈ 51 m² | 120 W | View KLT-25B → |
| KLT-30B 18-inch | 1,800 m³/h | 48 dB | ≈ 61 m² | 120 W | View KLT-30B → |
*Derived from the published airflow at 10 air changes per hour, a 2.5 m ceiling and a 15% duct and shutter allowance. Kitchens need the next size up; long duct runs need an inline fan. Full series data is on the KLT series page.
Send those five things and you get a sized recommendation with a formal FOB quote, usually within one working day.
Multiply room volume in cubic metres by the air changes per hour for that room type, then add 10 to 30 percent for the duct and shutter. A 3 by 2.5 by 2.5 metre bathroom at 10 air changes needs about 188 m³/h, so an 8-inch panel fan rather than a 6-inch one.
Free air is the airflow with nothing attached — the number in a catalogue. Working airflow is what the fan delivers once the shutter, duct and bends are fitted. On a three-metre flexible duct run you should expect to lose 20 to 30 percent of the free-air figure.
No. A panel fan is a high-flow, low-pressure axial fan. Beyond about five metres of duct, or with two or more bends, specify an inline mixed-flow duct fan instead — it generates the pressure the duct run needs.
Eight to ten for a domestic bathroom, and ten to fifteen for a high-traffic public toilet. Kitchens need fifteen to twenty, and a commercial kitchen twenty to thirty. A local building code or hotel standard overrides these figures.
Persistent condensation, mould on cold surfaces, lingering odours and a fan that runs constantly without clearing the room. In commercial kitchens it also means grease and heat stay in the space, which affects both hygiene inspection and staff comfort.
Send your application, quantity and destination — our export team replies within one working day.
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