As its name suggests, a bleed sensor allows for a small positive flow to be maintained at all times. In this case, we are measuring across the relief air damper, to maintain ultra-low differential pressures and positive flow. The bleed sensor provides a precision functionality to an independently controlled damper that can assure positive control during both the economizer and minimum outside air intake cycles.

During periods when the relief damper is closed, the return fan should be controlled to maintain a positive bleed-flow rather than a differential CFM, since the differential CFM control loop will conflict with the minimum outside air intake control loop. By selecting the bleed-flow setpoint with the relief damper closed, with any control point less than the setpoint goes to open, the relief damper control loop will remain independent.

The flexibility and power of today's digital control systems have enabled users to write software to accomplish tasks, where they were previously limited to hardware solutions. In the past 10 years, software techniques have been developed to allow buildings to better optimize their energy usage. One in particular deserves mention, due to its unique ability to properly size intake, recirculation and exhaust dampers; to trend log data points; and to reset damper positions, minimizing energy requirements while optimizing ventilation rates. ³¹

Several projects across the country will implement this software in the near future. Schools, offices and government buildings will be able to: insure that building control components are operating as intended; continuously verifying that building and space pressurization set points are achieved; and continuously verify outside air intake rates; all while optimizing the energy required to accomplish their automated control functions.

Combining some of these hardware components with the software and methods mentioned will provide solutions to many existing comfort, energy and liability issues. But that is the subject for another paper.

CONCLUSIONS
Indoor air quality can be detrimentally influenced, to the point of toxicity and a threat to human safety, by the presence of microbial activity within building walls. In humid climates, air condensing in the walls of negatively pressurized buildings is conducive to microbial development and growth. The product of these growths can be spread throughout the building by the mechanical ventilating system, due to the typical lack of effective filtration or air treatment.

Positive building pressurization flow is achieved by the action of more air being supplied to the building's interior space than is mechanically exhausted. Traditional HVAC control strategies do not assure positive pressure and ventilation control. More current and innovative airflow control strategies help to assure a more healthful, positive building pressure, verify and control air intake rates and can be designed as independent control loops. The resulting control methodology aids in compliance with ventilation codes and national standards. These methods are one of the primary components in a comprehensive strategy to control dilution ventilation, positive building pressurization, sources of contaminants, and required for acceptable indoor air quality in today’s office buildings, schools and hospitals.

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