![]() ![]() Most manufacturers, as well as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), 1 recommend installation of the devices 10 to 15 ft above a building to further minimize wind effects. Designed to minimize the effects of wind speed, these devices typically consist of two plates, with the sensing element in the middle of the inside surface of one of the plates. Many outside-air-pressure-sensing devices are available for installation at low-port-tubing terminations. This light breeze can have a significant impact on pressure control, as pressurization systems typically operate to maintain a slightly positive building pressure in the same range (0.05 in.). For example, a 10-mph outside breeze translates to wind pressure of 0.05 in., per Bernoulli's Equation. The termination of low-port tubing is important, as the wind can have a significant impact on measured outside-air pressure. It is worth mentioning that neither of the buildings was commissioned originally. There was intent to extend the tubing to the outside, as evidenced by a hole in the adjacent exterior wall, but the tubing never was installed. ![]() At this location, with the low-side port open to the panel and, thus, the plenum, the pressure sensed by the low-side port was higher than the pressure sensed by the high-side port. In the other building, the control panel containing the sensor was located in the return-air plenum, on the discharge side of the return fan. While that location was good for minimizing wind effects and protecting the sensor from rain, readings likely were not representative of true outside-air static pressure because the panel was in a room containing equipment (boiler burner fan, exhaust fans, etc.) that could have a significant effect on the pressure in the space. In one building, the control panel was in the penthouse mechanical room, which was open to the outside via boiler outside-air-intake louvers. In both retro-commissioned buildings, the low port of each pressure sensor was open to the control panel in which the sensor was installed, not extended to the outside via tubing. This article will discuss two such opportunities: outside-air-static-pressure-measurement termination and control sequences. The recent retro-commissioning of two office buildings-one 25 stories and 500,000 sq ft, the other 24 stories and 200,000 sq ft-served by central-station variable-air-volume air handlers in California revealed substantial opportunities for optimizing pressurization-control systems in large commercial buildings. Typically, this is done as part of a strategy to keep interior-space pressure slightly positive relative to outdoor pressure, reducing infiltration that could lead to comfort and/or indoor-air-quality (IAQ) issues. The sensor determines the difference in static pressure between the two ports, which it can report to a building-automation system. Commonly, low-port tubing is extended to the outside of a building, while high-port tubing is extended to an interior space. Commonly, the return fans are controlled with variable-frequency drives (VFDs), which receive a speed signal based on building differential static pressure-the difference between interior-space static pressure and outside-air static pressure.īuilding differential static pressure commonly is measured with a sensor with two ports: a high-pressure port and a low-pressure port. Often, these units include return fans, which draw air from occupied spaces for recirculation or exhaust. In many large commercial buildings, central-station air handlers are used to maintain occupant comfort. ![]()
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