Basic Documentation

Room Pressurization Control Application Guide
NOTE: In Chapter 5, Table 4. Velocity Pressure vs. Airflow Velocity shows that a
differential pressure of 0.02 inches w. c. would result in an airflow velocity from
the positive space to the negative space of about 566 feet per minute.
Given the chance, air in a positively pressurized room (higher static pressure) will flow out of
the room and into an area of lower static pressure. Conversely, air will flow into a room that is
negatively pressurized (lower static pressure) from a higher pressure (positively pressurized)
room or area. The potential direction of airflow is always from the higher pressure area
(positively pressurized) toward a lower pressure area (negatively pressurized).
Building Pressurization
A proper building ventilation system design ensures that all areas of the building are at a
slight positive pressure with reference to the outdoors to prevent outside air from entering the
building. Note that even rooms within a building that are negative with respect to the adjacent
corridor or another room, are often at a positive static pressure with respect to the outside of
a building. Without building pressurization, outside air (in accord with the positive-to-negative
airflow) will enter a building in several ways, including door clearance openings, construction
cracks, gaps, and even the porosity of the outer walls. However, by maintaining the inside of
the building at a slight positive static pressure with respect to the outside, this undesirable
inward airflow is prevented.
Without overall positive building pressurization, the inflow of outside air can pose many
problems:
• Unfiltered outside air can deposit airborne dirt wherever the air enters. For example,
around window frames, electrical outlets, etc.
• Unfiltered outside air can bring in harmful contaminants and unpleasant odors
creating health and inside air quality (IAQ) problems.
• Cold outside air can produce drafts and cold spots especially near the outer walls,
which adversely affect an otherwise good comfort control system.
• Humid outside air can condense on cooler interior surfaces of the building causing
dampness, wet spots and promoting growth of mold. This would very likely occur in
unseen places, such as inside wall spaces and above ceilings.
• Air entering through door clearances or window gaps can create annoying whistling
sounds.
• A building interior that is negative with respect to the outdoors will result in hard to
open entry and exit doors along with annoying inward drafts whenever a door is
opened.
4 Siemens Building Technologies, Inc.