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Controlling minimum outside air from mixed-air-plenum pressure offers a few advantages. For example, the method is simple. Static pressure is easy to measure with reasonable accuracy.

Additionally, small errors in static-pressure measurements affect minimum-outside-air quantity by the square root. If a set point of -0.25 in. wg is selected to achieve the desired minimum outside-air quantity, but the “real” pressure in the mixing box is 0.20 in. wg (a fairly gross error), outside airflow still is 89 percent of design.

Finally, unlike control schemes that use the return and spill/relief dampers to control minimum outside air, the risk that the return fan will “run away” and depressurize the building is not as great.

ECONOMIZER OPERATION

This article has described ways to maintain the minimum required outside-air quantity as supply airflow changes. If a system has an outside-air economizer, the outside-air quantity will exceed the minimum requirement. While maintaining the minimum outside-air quantity no longer is a concern, the return fan still needs to be controlled. The mixed-air-plenum-pressure control method also works when the economizer is active.

Figure 2 graphs flow across a parallel-blade damper at various damper positions. (Parallel-blade dampers can provide better mixing than opposed-blade dampers and, therefore, are preferred for mixing-box applications.) The graph represents a damper with 50-percent authority at constant pressure drop. Damper authority is the ratio of pressure drop across the damper to total pressure drop for the flow path that the damper controls.¹ The outside-airflow path runs from the outdoors to the mixed-air plenum. The intake louver and the outside-air damper usually are the only significant pressure drops in the flow path. If pressure drop across the wide-open damper is about the same as that through the louver (a not unlikely scenario), the pressure drop across the damper is 50 percent of the total and, therefore, damper authority is 50 percent.

When mixed-air-plenum-pressure control is utilized, the pressure in the mixed-air plenum is constant. A damper has nearly linear performance under these conditions (Figure 2). This linear relationship is important because it means the airflow relationships also remain roughly linear.

If the supply-fan speed increases at any given outdoor-air temperature (and resulting outside-air percentage), the airflow through the outside-air damper increases. The mixed-air-plenum pressure then becomes “more negative,” calling for the return fan to speed up and bring more return air into the mixed-air plenum. The reverse occurs as the supply fan slows. In other words, controlling the return fan via mixed-air-plenum pressure in the economizer cycle causes the return fan to track supply airflow.

When the outside-air temperature is between the supply-air temperature (typically 55°F to 60°F) and the economizer changeover temperature (when return air is less expensive to cool than outside air, typically 70°F to 75°F), the unit will operate on 100-percent outside air. The return-air damper will be closed, so controlling the return fan from mixed-air-plenum pressure no longer has feedback, meaning changes in return-fan speed will not change mixed-air-plenum pressure. Also, the wide-open outside-air damper is a fixed orifice. Pressure drop across it and the resulting pressure in the mixed-air plenum no longer will be constant and instead will vary with the square of supply airflow.

In 100-percent-outside-air mode, the return fan becomes a building exhaust fan. If the return fan is too slow, the building will be pressurized more than is desired. (This is not a terrible problem, but it could cause annoyances, such as doors that remain open against their closers.) If the return fan runs too fast, it can depressurize the building.

Depressurizing the building is a problem because it increases infiltration of unconditioned air and can draw rain water into the walls. One way to avoid this problem is to incorporate a building-pressure limit on the return-fan speed. At low supply airflow with a wide-open outside-air damper, the pressure in the mixed-air plenum will be “less negative” than the set point. The return fan, functioning as a building exhaust fan, ordinarily would speed up. The building pressure limit will keep the fan from speeding up so much that it pulls the building pressure below the desired set point (typically 0.02 to 0.05 in. wg).

CONCLUSION

This article explained how to use mixed-air-plenum pressure to control a VAV system's minimum outside air and why it works. The goal was to educate readers about constant mixed-air-plenum-pressure control, not advocate for or against it.

REFERENCE

1. Lizardos, E., & Elovitz, K. (2000). Damper sizing using damper authority. ASHRAE Journal, 42, 37-43.

A member of HPAC Engineering's Editorial Advisory Board, Kenneth M. Elovitz is an engineer and in-house counsel for Energy Economics Inc. His knowledge and experience with HVAC, electrical, and life-safety systems allow him to understand system function and performance, including interactions among disciplines.

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