Changes in external factors such as wind and stack effect necessitate active control, even on constant volume systems. ^{1, 3} This is often overlooked by engineers and building maintenance supervisors, which generally rely on air balance measurements that at best assure proper airflow rates only at set up. As a result, systems that appear to be functioning properly may not operate properly at a later date. Control schemes that only modulate the outside air intake damper generally can not overcome the negative pressure induced by cross winds and/or changes in stack effect. ^{1, 2, 3} . By placing an airflow measuring station on the outdoor air intake of these smaller systems, the direct measurement and control of outside air intake flow rates will also help to assure compliance with ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air Quality. ²⁶

SYSTEMS WITH RETURN OR RELIEF FANS
Large air handling systems having either a return or a relief fan should ideally be setup to maintain a volumetric differential between the intake and relief airflow rates. Systems that have intake systems designed for airflow measurement, and also have adequate duct systems on the relief system, can maintain building pressurization while simultaneously assuring compliance with minimum outside air intake ventilation rates. Provision for conditions that allow the proper application of instrumentation will maximize the flexibility and efficiency of your HVAC system, when the ABS controls have valid inputs to work with.

Unfortunately, the typical outside air intake on a system with an economizer cycle is not usually designed for proper control of intake flow rates. Dampers are generally sized to turn down to very low velocities (< 100 fpm). Even though some airflow stations, such as those using thermal dispersion (TD) techniques, can accurately measure these low velocities ^{1, 2, 3} , the angular position of the damper blades at maximum turndown often results in a situation which is difficult, if not impossible, to control adequately.^{27, 28} Transient wind effects can also adversely affect control stability.^{1, 3}

An additional complication results from the lack of adequate space for airflow measurement in the relief air system. As with supply fan-only systems, the initial air balancing only assures proper operation at the time of balancing.^{1, 3} The dynamic nature of the economizer cycle adds to additional tracking errors and hence, lack of pressure control.

Figure #7 - Control Equivalents

If both the total outside air and relief air flow rates can not be measured, an alternate method for building pressure control must be used. Figure #7 shows the mathematical relationships and the equivalent for pressure tracking in systems with exhaust at the air handler. Controlling return or relief fans to maintain a differential between the supply air and return air flow rates into the building will achieve the same result as maintaining the differential between the outside air and relief air flow rates.

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