Size a wall-mounted exhaust fan from the room's required air changes per hour, then confirm the total resistance stays under about 150 Pa — beyond that a wall fan with an auto shutter is the wrong machine; use a ducted centrifugal fan. Auto (gravity) shutters open on the fan's own discharge pressure, typically needing 15–30 Pa to swing fully clear, and close by gravity when the fan stops, preventing backdraught without any actuator. Common sizes run from 250 mm to 1,250 mm blade diameter, covering roughly 800 m³/h to 45,000 m³/h at 20–150 Pa.
Step 1: Choose the air change rate for the application
| Application | Air changes per hour | Typical fan diameter | Typical airflow | Notes |
|---|---|---|---|---|
| Toilets and washrooms | 10–15 | 250–350 mm | 800–2,500 m³/h | Run continuously or on occupancy plus run-on timer |
| Commercial kitchen back-of-house | 15–30 | 400–600 mm | 3,000–10,000 m³/h | Hood extract is separate and always ducted |
| Workshop, general factory bay | 6–12 | 600–900 mm | 8,000–25,000 m³/h | Site fans high on the leeward wall |
| Welding or paint-mixing room | 20–40 | 500–800 mm | 6,000–20,000 m³/h | Use spark-resistant construction where required |
| Generator or transformer room | 30–60 | 800–1,250 mm | 20,000–45,000 m³/h | Size on heat rejection, not air changes alone |
| Livestock housing | 20–60 (summer) | 900–1,400 mm | 20,000–45,000 m³/h | Staged fans plus variable speed on the first stage |
Convert air changes to airflow with Q = room volume × ACH. A 15 m × 10 m workshop with a 5 m ceiling holds 750 m³, so 10 ACH means 7,500 m³/h — comfortably one 600 mm fan, or two 450 mm fans if better distribution is wanted. For heat-driven rooms such as generator and transformer rooms, size instead on Q ≈ 3,000 × heat rejection in kW / allowable temperature rise in K; a 40 kW heat load with a 10 K allowable rise needs about 12,000 m³/h regardless of what the air-change table suggests.
Step 2: Check the pressure budget, including the shutter
Wall fans are low-pressure machines and every accessory eats into their capability. A typical resistance budget looks like this:
- Auto shutter, fully open: 15–30 Pa. This is not optional resistance; it exists whenever the fan runs.
- Weather louvre or hood, if fitted downstream: 20–50 Pa depending on free area.
- Insect or bird mesh: 10–40 Pa — fine anti-insect mesh is the most common cause of lost airflow.
- Wall guard and internal safety mesh: 5–15 Pa.
- Adverse wind pressure on the discharge face: 20–60 Pa in exposed locations.
Summing the middle of those ranges gives 90–120 Pa before any duct is added, which is why a fan rated for 45,000 m³/h at free delivery may only reach 32,000–36,000 m³/h once installed. Always select from the fan curve at the calculated system pressure, never from the free-delivery figure printed on the box.
Step 3: Get the make-up air path right
An exhaust fan can only move as much air as the room can admit. If the inlet path has less free area than the fan needs, the room simply goes negative and airflow collapses. Practical guidance is to provide inlet free area of at least 1 m² per 8,000–10,000 m³/h of exhaust, keeping inlet face velocity below about 2.5–3.0 m/s, and to locate inlets low and on the opposite side so the airflow sweeps the whole room.
- Keep room pressure within ±20 Pa of ambient; below −30 Pa, doors become difficult to open and combustion appliances may spill flue gas.
- Never rely on door undercuts for more than about 1,500–2,000 m³/h of make-up air.
- In rooms with open-flued gas or oil appliances, verify the depressurisation limit before selecting any exhaust fan.
- Provide a gravity or motorised inlet shutter of matched free area so the room does not lose heat when the fan is idle.
Auto shutter types and their limits
Gravity shutters use lightweight aluminium blades hinged at the top; discharge pressure lifts them and gravity closes them. They are cheap and reliable but leak: typical closed leakage is 5–15 % of the fan's rated airflow under a 25 Pa wind pressure. Where tight shut-off is needed — cold stores, cleanroom boundaries, fire compartments — a motorised or spring-return damper with edge seals is required instead, achieving leakage classes an order of magnitude lower.
- Mount blades horizontally with the hinge at top; vertical-blade gravity shutters do not close reliably.
- Verify the fan can develop the shutter's opening pressure at its lowest operating speed — a fan run at 40 % speed on a variable drive may not open a 30 Pa shutter at all.
- In exposed or coastal locations specify aluminium or 304 stainless blades with nylon bushes; galvanised blades seize within a few seasons.
- Do not fit gravity shutters where the fan is on the windward face without a wind hood; gusts will chatter the blades and destroy the bushes.
When to choose which arrangement
- Choose a wall fan with an auto shutter when the wall is directly on the outside, the total resistance is under 150 Pa, and no ductwork is needed — workshops, toilets, plant rooms and agricultural buildings.
- Choose a wall fan with a motorised damper when tight shut-off matters, when the fan cycles frequently in a heated space, or when a variable-speed drive will run the fan below the shutter's opening pressure.
- Choose a ducted centrifugal or mixed-flow fan when resistance exceeds about 150 Pa, when the exhaust point must be remote from the room, or when the discharge must be raised above the roof for dispersal.
- Choose two or three smaller fans instead of one large fan when the room needs staged ventilation, when redundancy matters, or when a single large fan would create a draught above 0.5 m/s at a work station.
- Choose spark-resistant construction, class A or B, whenever flammable vapours, dust or solvent mists may be present in the airstream.
Installation and motor specification
Motor ratings for wall fans typically run 0.09–3.0 kW, and single-phase units above about 0.55 kW should be avoided in favour of three-phase for starting torque and balanced loading. Specify at least IP54 for indoor dry rooms and IP55 with class F insulation for washdown, agricultural or coastal service. Fit the fan so the blade centre is at least 2.3 m above the floor, or add a guard to a recognised finger-safety standard, and use a resilient mounting frame with a gasketed flange so structure-borne noise does not transmit into the wall — typical noise levels are 55–75 dB(A) at 3 m. Keruite manufactures wall-mounted axial fans and shutters at an 80,000 m² plant in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every impeller is balanced to G2.5 and performance-tested before shipment.
FAQ
How do I size a wall-mounted exhaust fan?
Multiply room volume by the required air changes per hour, then select from the fan curve at the calculated system pressure rather than free delivery. For heat-driven rooms use airflow of about 3,000 times the kilowatts of heat divided by the allowable temperature rise in kelvin.
How much resistance does an auto shutter add?
A gravity shutter needs roughly 15 to 30 pascals of discharge pressure to open fully, and that resistance is present whenever the fan runs. Adding weather louvres, insect mesh and guards commonly brings the total installed resistance to 90 to 120 pascals before any ductwork.
Do gravity shutters seal completely when the fan stops?
No. Typical closed leakage is 5 to 15 percent of rated airflow under about 25 pascals of wind pressure. Where tight shut-off is needed, such as cold stores or heated spaces with frequent fan cycling, use a motorised or spring-return damper with edge seals instead.
How much make-up air does a wall exhaust fan need?
Provide inlet free area of at least one square metre per 8,000 to 10,000 cubic metres per hour of exhaust, keeping inlet velocity below roughly 2.5 to 3.0 metres per second. Room pressure should stay within about 20 pascals of ambient so doors and flues work normally.
When should I use a ducted fan instead of a wall fan?
Whenever total system resistance exceeds about 150 pascals, when the extract point must be remote from the wall, or when the discharge has to be raised above roof level for dispersal. Wall axial fans lose airflow rapidly once pressure rises beyond their flat curve region.