Pressurization

Although pressurization is not the sole source of moisture problems in a building, it is a major and often overlooked cause of mold growth in the building envelope. Pressure is a significant water transport path across the building envelope. Wind pressure cannot be compensated for all of the time without damage to the building envelope (if you develop enough pressure on one exterior wall to compensate for wind pressure you will inevitably over-pressurize another wall). The objective in exterior wall pressurization is to develop a net pressure (positive during cooling, neutral during heating). A net positive pressurization will result in a drying effect during humid summer months.

Define Pressure Zones!

Regardless of whether or not you are concerned about pressure from a moisture (IAQ) perspective or for containment of contaminants (labs, processes) you must define pressure zones within the building. Many buildings have pressurization problems because the designers treated the whole building as a single pressure zone.

Buildings with multiple air handling systems have complex dynamics that effect pressurization. A common method to control building pressure is to control fan systems to space static pressure. Unfortunately, static pressure control can not ensure that net pressurization is achieved. All it can do is assure that one or more pressure points within the building is positive or negative to a reference point outside of the building. The reference point is influenced by both wind and stack pressures.

A more significant problem is the interaction of multiple pressure zones when static pressure control is used. This problem becomes apparent when multiple handlers are controlled on space static pressure. These systems often result in very poor and sometimes catastrophic building pressurization.

The Solution

Define pressure zones and control the supply and return airflow rates into and out of each zone (D cfm). Monitor “bleed” airflow rates between critical spaces and reset the differential as required. In addition, monitor local exhaust flow rates if changes in flow rates during operation will significantly impact pressurization.

Typical Pressure Zones

• Single air handler serving a single floor: control supply and return airflow rates at the air handler.
  
• Single air handler serving multiple floors: control supply and return airflow rates at each floor to compensate for stack effect.
  
  
• Single air handler serving multiple pressure zones on one or more floors: control supply and return airflow rates at each pressure zone.

• Maintain Outside Airflow
  when there is no relief on a single pressure zone system
  

Maintain Outside Airflow
when there is no relief on a single pressure zone system