General Industrial Ventilation Part 2: Powered Forced Supply/Natural Exhaust

Posted by Gary Leseman on October 25th, 2016

Last month, we covered powered forced supply/powered forced exhaust in our first installation in the general industrial ventilation series. This month, we are looking at a powered forced supply/natural exhaust ventilation system, which uses motorized supply fans and natural exhaust vents (often called gravity vents).

Typically, supply fans are mounted in the perimeter wall of the building about 8 to 10 feet above the floor with a diverter to force the air back down to the floor level. Natural exhaust vents are mounted in the roof of the building. Natural exhaust vents can take the form of a hood style ventilator, a continuous ridgeline vent, or a low profile thermal vent. The type of vent you choose will depend on what works best to allow a low pressure drop and high volume flow of air out of the building while at the same time preventing rain or snow from entering the building.

Roof mounted natural exhaust vents are located in strategic areas above high heat generating processes. Fresh air is forced to the areas of highest heat by supply fans and the natural rise of hot air is vented out of the building through the roof mounted natural exhaust vents. When combined with column-mounted fans, the fresh outside air can be kept moving through critical work areas hundreds of feet away from the wall supply fans.?

For winter ventilation, direct heated air make-up units or space heaters can be used in order to add heat to the work area. However, typically the powered forced supply/natural exhaust system is used when there is an excessive amount of heat in the building. Thus, winter heating is often achieved just by turning off a few supply fans and closing a damper on the natural exhaust vents.?

Under Pressure

This type of system should only be used to provide a positive pressure within the building. If there are other sources of process exhaust, then the supply fans volume must offset that additional exhaust. An example of this would be an engine room or a boiler room where combustion air is taken from the space. The supply fans must provide the adequate amount of combustion air to the space plus an adequate amount of air for ventilation, otherwise the system could go into a negative pressure which would cause the natural exhaust vents to become natural supply vents and possibly draw in rain water.

Care must also be taken to have adequate natural exhaust available so as to not overpressure the building, which would create problems with operation of man doors.

It becomes a tricky balancing act, but this system is commonly and successfully used in industrial buildings with extremely high heat sources where a forced air system is desirable such as engine rooms, boiler rooms, foundries, forge shops, steel manufacturing, and even some food process industries.

Fresh air can be provided through supply fans fitted with filters to maintain interior building cleanliness and a positive building pressure as required for food processing or where high outside dust conditions exist.


As for other advantages:

  • A powered forced supply/natural exhaust system is not dependent on prevailing winds in order to operate effectively all the times.
  • The weight load and wind load on the roof structure of the building is reduced with natural exhaust.
  • An overall building air change rate and positive building pressure can be guaranteed.
  • Depending on the air change rate, the temperature throughout the building will be as close to the outside ambient temperature as possible.
  • Workers feel airflow, which provides a sensible cooling effect in the summer.
  • With the addition of some supply fans as direct fired, heated air make-up, people can stay warm and dry even in the wintertime for improved productivity and health.


Yes, it is very common to acoustically treat the supply fans so that the combined noise from the fan and from equipment inside the building meets a specified noise level at some distance from the building. Also, with the right type of natural exhaust equipment, acoustical treatment is not a problem.


Powered forced supply/natural exhaust is an excellent way to ventilate an industrial building. The system is designed so that the ventilation rate is not contingent on exterior conditions like wind, outside ambient temperature, the height of the building, temperature differentials, etc.

The ability to always maintain a positive building pressure with this type of ventilation system is of particular value to many industries.

The goal of a properly ventilated facility is achieved: to reduce inside building temperature, prevent the buildup of internal heat, create an interior work environment as close to the outside temperature as possible, and optimize air movement for personnel comfort.

A properly ventilated industrial facility will have a fresh air change rate conducive for people’s comfort and equipment requirements.

With a forced supply/natural exhaust system, the initial equipment costs, operating costs, and long-term maintenance costs are all significantly less than with forced supply/forced exhaust or air conditioning.

In the next installation in this series, we will be discussing natural supply/powered forced air exhaust.

If you have questions about your own ventilation system, don’t hesitate to contact the ventilation experts at Eldridge.

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