All too often, when we ask for the “ventilation design basis” (1910.119(d)(3)(i)(E)) we are given some calculations from a 3rd party engineering group. These calculations demonstrate the fan sizing for the necessary air exchanges and flows; however, what these calculations do NOT provide us is the full design basis for the ventilation system. For me, the “design basis” begins with the ventilation intakes (i.e. where is the “fresh air” coming from). I turn to the International Mechanical Code (IMC) as my RAGAGEP for a baseline of any ventilation system. We can see the IMC even begins with “intake opening location” and some of you may be surprised as to what this code actually requires:
(emphasis by me)
401.4 Intake opening location. Air intake openings shall comply with all of the following:
- Intake openings shall be located NOT LESS THAN 10 feet (3048 mm) from lot lines or buildings on the same lot.
- Mechanical and gravity outdoor air intake openings shall be located NOT LESS THAN 10 FEET (3048 mm) horizontally from ANY HAZARDOUS OR NOXIOUS CONTAMINANT SOURCE, such as vents, streets, alleys, parking lots and loading docks, except as specified in Item 3 or Section 501.3.1. Outdoor air intake openings shall be permitted to be located less than 10 feet (3048 mm) horizontally from streets, alleys, parking lots and loading docks provided that the openings are located NOT LESS THAN 25 FEET (7620 mm) vertically above such locations. Where openings front on a street or public way, the distance shall be measured from the closest edge of the street or public way.
- Intake openings shall be located NOT LESS THAN 3 FEET (914 mm) BELOW contaminant sources where such sources are located within 10 feet (3048 mm) of the opening.
- Intake openings on structures in flood hazard areas shall be at or above the elevation required by Section 1612 of the International Building Code for utilities and attendant equipment.
This section addresses intake openings and Section 501.3.1 addresses EXHAUST openings. These two sections must be applied in harmony because they both can affect the separation between intakes and exhaust openings.
To prevent the introduction of contaminants into the ventilation air of a building, Item 1 requires a minimum separation of 10 feet between outdoor air intake openings AND any lot lines or buildings on the same lot.
Item 2 addresses powered intakes, (mechanical) and gravity (nonmechanical) intakes and requires separation from potentially harmful contaminant sources including chimneys and vents, plumbing vents and areas where motor vehicles operate. In this item, the 10-foot distance is measured from the closest edge of a street, alley, parking lot and loading dock. The last sentence of Item 2 addresses intake openings that face a street or public way.
Item 3 addresses those cases where the required 10-foot separation cannot be met. For example, if the 10-foot horizontal separation required in Item 2 cannot be achieved, the intake could be located at least 3 feet below the contaminant source. The code assumes that the contaminants likely to be present are buoyant in air because of their temperature or specific gravity and they will rise ABOVE AND AWAY from the intake opening.
Item 4 intends to prevent floodwaters from entering a building through an air intake opening. In addition, this prevents water from floods up to the base flood from entering a building through an air intake openings and causing damage to equipment.
See Sections 501.3 and 501.3.1 and consider the fact that an existing intake opening can be impacted by the installation of a new exhaust termination, as well as the fact that an existing exhaust termination can affect the location of a newly installed intake opening.
