Gravity ventilation (also known as natural ventilation) requires no powered ventilators to achieve air movement. It is most useful in buildings with open floor plans requiring no air conditioning, and where the primary motive for ventilation is heat removal or contaminant dilution. Gravity ventilation is achieved by strategically placing openings in the building to take advantage of prevailing winds and thermal buoyancy or stack effect.

Wind pressure
Wind moving past the building creates areas of high and low pressure. The windward side of the building is an area of high pressure. The leeward side & roof are areas of low pressure. Placing openings in both high and low pressure areas of the building causes air to move through the building.

Thermal buoyancy (stack effect)
Heat generated within buildings rises up. In buildings where the structure is of significant height, this natural tendency of warm air to rise (known as thermal buoyancy or stack effect) can generate air movement in the building. By placing openings in both the highest and lowest levels of the building, hot air is allowed to rise out of the upper part of the building and pull cooler air into the lower level of the building.

Opening placement
In order to take advantage of both wind pressure and stack effect, gravity ventilators are generally used in conjunction with open doors and windows to ventilate the structure. Gravity vents are placed on the roof, being the highest part of the structure, to provide for exhaust of building air. Windows and doors at lower elevations provide the intake portals of the gravity ventilation system.
Use with mechanical systems
Gravity ventilators may be used with mechanical ventilation systems, where power ventilators force air in or out of the building through the gravity vents. The power ventilator may be directly connected to the gravity ventilator via ductwork or the building may constitute a plenum, the air induced to move through the gravity ventilator merely by the pressure differential created between the interior of the building and the ambient atmosphere.

ILLUSTARION of Natural cooling & contaminant
Ducted relief
This application is the most simple of all gravity ventilator applications. The ventilators are selected based on the air volume and acceptable pressure drop along with any physical limitations which may affect the dimensions of the product.

ILLUSTARION of Natural cooling & contaminant
Ducted intake
Ducted intake applications are similar to ducted relief with the additional consideration of intake velocity. Moisture carryover begins to occur at approximately 500 FPM intake velocity and gravity ventilator selections should, therefore, be kept below that threshold unless moisture containment measures are added to the system.

ILLUSTARION of Natural cooling & contaminant
Non-ducted relief
Non-ducted relief gravity ventilator applications are very similar to ducted relief applications with the exception that the pressure drop of the ventilator should be kept to a minimum. The building will be pressurized to approximately the selected pressure drop of the gravity ventilator. Selecting the ventilator for pressure drops in excess of 1/8” wg will result in problems with exit doors standing open and unwanted infiltration into any adjacent conditioned areas.

ILLUSTARION of Natural cooling & contaminant
Non-ducted intake
Although the 500 FPM intake velocity will usually limit the intake static pressure, the designer should double-check gravity ventilator pressure drop to ensure that it does not exceed 1/8” w.g. If the negative pressure is excessive, it can result in difficulty opening exit doors and dangerous slamming of already open doors.