Basic Documentation

Chapter 7–Room Ventilation, Makeup Air, and Pressurization Control Systems
It is useful to keep the following points in mind with reference to room pressurization:
• Room static pressure is the result of the force imposed on the room air
mainly from the HVAC system fans.
• Room static pressure can only be measured as the difference in static
pressure with respect to another location. This is typically another room or an
adjacent corridor, but it can also be the outside of the building.
• The location with the higher static pressure is said to be positive with respect
to the other location. Likewise, the lower pressure location is said to be
negative with respect to the higher pressure location. (Either means of
expression is equally correct and acceptable.)
• Given the chance, air will flow from the positive (higher) pressure location to
the lower (negative) pressure location.
• A static pressure difference of 0.01 in. WC (where the laboratory room is
0.01 in. WC negative with respect to a corridor) is a generally acceptable
level of negative pressurization for normal laboratory room applications.
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• For isolation rooms in medical facilities, a static pressure difference of at
least 0.001 in. WC is recommended.
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The room should be negative when
the intent is to prevent airborne pathogens from migrating out from the room,
and the room should be positive when the intent is to prevent airborne
pathogens from entering into the room from adjacent areas.
Room Pressurization Control by Pressure Sensing
There are various ways to maintain a desired level of room pressurization by
controlling the room ventilation. One method is to simply apply a static pressure
sensor between the room of concern and the reference area. Then the ventilation
control system controls the room’s ventilation airflow to create the desired static
pressure difference. Figure 12 illustrates a laboratory and an adjacent corridor with
the essential components of a room ventilation and the associated system. Aside
from the ventilation system’s room supply and exhaust provisions, a room controller
and a differential pressure sensor in the wall are used. Note that this laboratory
contains constant air volume containment units which in this case are shown as
chemical fume hoods.
With reference to Figure 12, the room controller must continuously measure the
static pressure of the laboratory with respect to the adjacent area by means of the
differential pressure sensor located in the wall between the laboratory and the
adjacent area. The room controller then modulates the supply terminal damper to
provide the required amount of incoming air to maintain the required negative room
static pressure.
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A differential of 0.01 inches of water will normally produce an airflow velocity of between 50 and 100 fpm through an
opening such as the clearance space between the door and floor. However, it should be noted, that when a door is
open, the static pressure difference will likely decrease substantially and the air velocity through the door will also
appreciably decrease.
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Source: U.S. Department of Health and Human Service, Centers for Disease Control. Experienced health facility
designers and safety practitioners would more likely call for a minimum of 0.01 inches of static pressure difference for
isolation applications.
64 Siemens Building Technologies, Inc.