Differential pressure sensors have shown dramatic improvements over the past decade. However, many of the devices used for static pressure control in HVAC systems display instability at differential pressures of 0.05 in. WG.²⁷ Static pressure control points should ideally be between 0.010" - .0.015" WG, much lower than that which the typical non-temperature compensated, differential pressure sensor can be accurate and reliable. Besides sensor instability, controlling return fans to maintain such a small differential pressure is difficult in practice and in most cases requires a significant amount of signal dampening to achieve control. As a result, there are numerous opportunities for the building pressure to become negative.

Figure #4 - Effect of Wind on Building Pressure Control

The graph illustrates the range of building pressures that would result from either a 15 MPH direct wind or a 15 MPH cross wind. Note that the building pressures range from more than double the setpoint to less than zero (negative building pressure).^{1, 27} Direct control of the equivalents in "pressurization flow" would provide a much more stable and reliable input. In our example in Figure #4, maintaining the pressurization flow to +/- 20% of set point, will maintain the building pressure at a level better than +/- .025" WG.

The location of the outside pressure tap will also result in significant sources of error. Placing a static sensor tap on exterior walls will expose the tap to wind induced increases or decreases in static pressure (Figure #4), even if guards are used that mitigate the velocity pressure component of the wind. Changes in wind direction will result in significant pressure errors between the building and its ambient environment. Placing an outside pressure tap on a roof to avoid the directional effect of the wind may help, but even the best shields can not completely remove the pressure changes induced by winds ²⁷ .

The ability to control to a specific input should not deter one from using that same input to "monitor" critical building functions or the status of control variables. For example, where direct static pressure control of a large building is near impossible with typical commercial products, we should not forget to compare direct DP measurement with alternate, but equivalent measures. Doing so can only improve your ability to identify problems and help verify proper functioning. By logging this data over time, the control system will have the data necessary to optimize the set points for Pressurization Flow (Fp) to compensate for any shifts or changes in the building's Fp curve.

A MORE EFFECTIVE CONTROL STRATEGY
Since pressurization flow is equal to the difference between the airflow into and out of a building (Figure #5), maintaining a positive differential (as indicated in Equation #1) will develop a positive pressurization flow.

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