Roof Extraction and Wall Axial Fan Manufacturer: Sizing Against Wind and Make-Up Air

NextAir Systems, Faridabad, India. Last reviewed October 2026.

A wall fan and a roof fan do the same job and meet different weather. A fan in a wall faces whatever wind blows on that face of the building. A fan on the roof meets suction more often than pressure. Neither is a surprise to a designer who has thought about it, and both are a surprise to a plant manager who has not.

NextAir builds axial fans for ventilation and exhaust up to 1,600 mm, 2,400 Pa and 185,000 m³/h, engineered to your duty and your mounting arrangement. This page shows what wind does to a wall fan, and the other thing that decides performance: where the air comes from.

At a glance

• A 10 m/s wind on the face of a building exerts a pressure of about 48 Pa on it, using an assumed pressure coefficient of 0.8. In our example fan, that cuts a wall fan’s airflow by 3%; at 20 m/s the cut is 13%.
• Wall fans are most exposed to wind on windward faces. Roof fans often work with wind suction, but are exposed to rain and need weather protection.
• A fan can only extract what can enter, and the make-up air path sets as much of the airflow as the fan does.
• Fans are engineered to your duty point and your mounting arrangement, and not sold from a fixed catalogue size.

Wind is a pressure on the fan

Wind pressure on a surface is the pressure coefficient times half the air density times the wind speed squared. For a windward face the coefficient is positive and roughly 0.8 in the open, and for a leeward face it is negative. A wall fan on a windward face must push against that pressure, which adds to the system resistance. The table uses the same simple fan and system as in the system effect article: a curve from 800 Pa at zero flow to zero at 20 m³/s, and a system designed for 14 m³/s. The coefficient of 0.8 is an assumption and the fan is an illustration.

The effect is negligible at 5 m/s, modest at 10 m/s and noticeable at 15 or 20 m/s, since it grows with the square of the wind speed. A fan chosen with no margin will meet its design airflow on a calm day and fall short in a windy one. Fan curves of real fans differ, and a steeper curve tolerates wind better, but the principle holds. The fixes are standard: put the extract fans on the leeward face or the roof, keep make-up air openings on the windward side so that wind helps instead of hinders, and select with margin for the pressure.

Wall or roof

Heat rises, so a roof fan removes the hottest air, which is why it suits hot process halls. A wall fan suits buildings where the roof is awkward or the fan needs to be reached. Our guide to ventilating a hot factory shows how to size the airflow from the heat load.

Gusts matter as well as the average. A fan that is fine in a steady 10 m/s wind may be pushed into an unstable part of its curve by a gust, and shutters or dampers on the outlet can flutter or slam. Ask for backdraught protection that suits wind and for shutters that stay shut when the fan is off, because an idle wall fan is just a hole in the building through which the wind blows.

The make-up air

Every cubic metre extracted must be replaced, and the air has to find a way in. If the openings are too small, the building runs at a negative pressure, the fans work harder and deliver less, and air is drawn in through leaks, doors and the wrong places. Size make-up air openings generously and place them to bring cool air across the work, not past it. Starved openings are one more form of system effect.

How we build and ship

Fans are engineered to your duty point and your mounting arrangement, so casing lengths and mounting arrangements can follow your installation. General ventilation fans are built in mild steel, with SS 304 or SS 316 for corrosive, humid or coastal sites, in direct drive and belt drive. They ship fully assembled, partially assembled or as CKD kits, to suit freight, site and handling. Motors and variable frequency drives are bought-out items. ATEX-certified and AMCA 99 spark-resistant fans are outside what we build.

To compare suppliers, see how to choose an axial fan manufacturer, and for several fans on one wall, parallel operation of axial fans.

What to send us for a quotation

• Airflow, from the heat load or the air change requirement, and the static pressure
• Building dimensions, the mounting position and the face orientation
• Prevailing wind and the make-up air arrangement
• Atmosphere: general, humid, corrosive or coastal
• Noise limit at the boundary
• Drive preference and delivery format

Send these to us through our contact page and we will confirm the duty is inside our range and propose a layout. A simple sketch of the building with the fan positions, the wind direction and the openings marked is worth more than a long description. See the axial fan range, our guide to industrial axial fans and our factory ventilation page. For cooling as well as ventilation, see our air washers.

Types and variants we build

• Belt-Driven Large-Diameter Axial Fan Supplier: Pulley Ratios, Tip Speed and Trimming Speed on Site
  Belt-driven axial fans up to 1,600 mm: how a pulley ratio sets speed, what it does to tip speed, noise and power, and why belts let you trim on site.
• Roof Extraction and Wall Axial Fan Manufacturer: Sizing Against Wind and Make-Up Air
  Roof and wall axial fans for factory and warehouse ventilation: how wind pressure cuts a wall fan airflow, with a calculated table, and why make-up air matters.
• Stainless Steel Axial Fan Manufacturer: SS 304 and SS 316 for Corrosive, Humid and Coastal Air
  Stainless steel axial fans in SS 304 and SS 316 up to 1,600 mm: choosing the grade, what pitting resistance numbers mean, and what to check on welds and finish.

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