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©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 2, February 2017

By Robin J. Rader, P.E., Member ASHRAE

About the Author
Robin J. Rader, P.E., is a senior mechanical engineer with Design Alaska, Fairbanks.

Measurement and control of static pressure within a building is an important, if not vital, control function in many facilities. Electronic differential pressure sensors, called transducers here, have become increasingly more accurate and reliable since the introduction of direct digital controls (DDC). However, the accuracy of this measurement can be affected by weather, either through wind or by changes in ambient outdoor air temperature. Static pressure within a building requires comparing the ambient outdoor pressure to the pressure inside the building. The amount of influence wind or ambient temperature has on the measurement accuracy greatly depends on the selection of the locations for the indoor and outdoor pressure sensing reference points.

Wind can affect the static pressure measurement if the outdoor reference is located in an area subject to wind stagnation forces that occur on the windward side of a building face. To mitigate these effects, the reference point is often connected to a wind attenuating head, which is usually located as high as practical above the roof line to move the sensing point above the wind stagnation zone.

This head consists of two plates with the sensing orifice in the center of one of the plates. The air driven by the wind is straightened as it passes through the plates so the airflow is always normal to the sensing orifice, mitigating wind turbulence effects.

This article was in the process of peer review as a similar article, “Impact of Stack Effect on Building Static Pressure Measurement,”1 was published in the October 2016 ASHRAE Journal. In response to that article, the focus of this article shifted from describing the measurement error caused by changes in ambient air density to a discussion on how best to mitigate this measurement error, as well as how to mitigate the effects of negative pressure at the ground floor.

 

Building Pressure Control

If building static pressure is out of control, exterior doors may stand open or slam shut, automatic exterior doors or interior elevator doors can malfunction, and occupants may experience drafts. In cold climates, operation of a building in a negative pressure relationship can cause significant occupant discomfort.
The pressure relationship of a building is somewhat of an ephemeral quantity that can drop immediately to ambient outdoor pressure upon opening a single exterior door. It is affected by wind pressure and stratified by stack effect due to the buoyancy of hot air surrounded by cooler air.

The most typical building static pressure setpoint is 0.05 in. w.c. (12.5 Pa) positive pressure. Some operators and commissioning agents are pushing the control setpoint as close to neutral pressure as practical to reduce energy use and increase control stability. The practical upper limit for positive pressure is not far beyond 0.05 in. w.c. (12.5 Pa). Automatic door closures lose their ability to be safely and reliably adjusted above 0.10 in. w.c. (24.9 Pa), while still operating reliably at neutral pressure when the fan systems are off. At pressures as high as 0.20 in. w.c. (49.8 Pa), a standard 3 ft (0.9 m) wide door is physically difficult to operate and can require a shoulder to push it open or two hands to pull it closed. Wider doors have less tolerance to pressure variation.

Significant static pressure variation is imposed on a structure simply by keeping it warm. Using the stack effect equation from ASHRAE Handbook—Fundamentals2 for a building with a roof height of 75 ft (23 m), an indoor temperature of 70°F (21°C), and an outdoor temperature of – 40°F (– 40°C), the internal static pressure varies nearly 0.30 in. w.c. (74.7 Pa) between ground floor and underside of the roof, with the roof elevation at a positive 0.15 in. w.c. (37.4 Pa) and with the ground floor at – 0.15 in. w.c. (– 37.4 Pa). This relationship can be shifted by drawing in excess outdoor air compared to exhaust or relief air. The necessary amount of excess outdoor air depends on the tightness of the building. To achieve a 0.05 in. w.c. (12.5 Pa) positive pressure at grade level in this example, the pressure at the roof level would have to be increased to 0.35 in. w.c. (87.2 Pa).

 

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Comments

  • 13 Dec 2017 | Jean Provost
  • What type of outdoor probe would you recommend to measure the static pressure. Wouldn't wind traveling between the two plates create a venturi effect at the sensing orifice? Fluctuation in wind speed would create variable suction at the orifice therefore the DP signal would be very noisy. Thank you,
  • Reply
  • 21 Dec 2017 | Robin Rader
  • We recommend using two outdoor references on opposite faces of a symmetrical portion of the building so that if one reference is windward the other is leeward. These references are then tubed together so that the transducer senses the average. The ends of tubes simply stick out the wall and should be covered with a suitable vented enclosure. This allows the outdoor references to be located at the same elevation as the indoor reference so that both wind effects and ambient temperature change effects are mitigated. The wind attenuating head referenced in the comment uses two plates to straighten flow perpendicular with the reference orifice mitigating the effects of wind velocity pressure. The problem with a wind attenuating head is that it must be located above the wind stagnation pressure zone which typically imposes a height difference which is then subject to ambient temperature change effects.
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