In many large commercial buildings, central-station air handlers are used to maintain occupant comfort. Often, these units include return fans, which draw air from occupied spaces for recirculation or exhaust. 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.

Building differential static pressure commonly is measured with a sensor with two ports — a high-pressure port and a low-pressure port. Tubing extends from both ports. Commonly, low-port tubing is extended to the outside of a building, while high-port tubing is extended to an interior space. The sensor determines the difference in static pressure between the two ports, which it can report to a building-automation system. 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 recent retrocommissioning 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. This article will discuss two such opportunities: outside-air-static-pressure-measurement termination and control sequences.

Outside-air-pressure-tubing termination

In both retrocommissioned 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.

In one building, the control panel was in the penthouse mechanical room, which was open to the outside via boiler outside-air-intake louvers. 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 pressure in the space.

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. 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. 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 (Photo A).

It is worth mentioning that neither of the buildings was commissioned originally.

The termination of low-port tubing is important, as wind can have a significant impact on measured outside-air pressure. For example, a 10-mph outside breeze translates to wind pressure of 0.05 in., per Bernoulli's Equation. 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.).

Many outside-air-pressure-sensing devices are available for installation at low-port-tubing terminations. 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. 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.

Building-Pressurization-Control Scheme

The building-pressurization-control scheme for the two retrocommissioned buildings is similar to the one employed in many buildings with central-station air-handling systems: Return-fan speed is varied to maintain a set differential between interior-space and outside-air pressure. The two buildings also employ an economizer-damper-control scheme that is fairly typical: The economizer-damper controller sends a signal to control outside-, return-, and relief-air dampers, with the return-air dampers operating opposite of the outside- and relief-air dampers (Figure 1).

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