Priority #1: Optimize the Setpoints
For the static pressure setpoint, I use about 0.3 for the unoccupied setting and 1.4 for the maximum setting. Using OAT, my system operates below 1.4 inches most of the time. If your ductwork is tight and you need more cooling, it’s cheaper to increase fan speed than to decrease the supply temperature.
The control of the cold deck temperature is based on demand, time of day, and OAT. In all cases, the supply temperature should be adequate to maintain relative humidity below 60%. On a dry, cold day, the setpoint might be 60 degrees F. If indoor humidity reaches 60%, the setpoint might go below 55 degrees F. The supply temperature should be at least 10 degrees below wet bulb. If the cold deck can’t make temperature, I lower the fan speed until the chiller can supply more water or water with a lower temperature. I do the same for the hot deck.
The hot deck is also controlled by demand, time of day, and OAT. I tracked setpoints, time of day, OAT, and room temperatures until I had built a history adequate to develop my minimum setpoints. I don’t use the preheat deck and precool decks except when needed. When the OAT is near freezing, the preheat deck should open to keep it and the precool deck from freezing. When the hot deck or cold deck can’t make setpoint, then the predecks help out.
The outside air damper (OAD) is usually set for minimum flow. If CO2 builds up, the damper opens. Based on a function of OAT and humidity, the controls open up the outside air as much as possible when the OAT is less than my return air. The outdoor enthalpy should be lower than the return-air enthalpy. Occasionally, someone microwaves the popcorn for 20 minutes instead of 2. Then I turn on the point I named POPCORN, the outside air dampers go to full open, the fan speed goes to maximum, and the pressure relief fans come on.
Air filters need regular checking. To check mine, I set all my room dampers to 30% and the AHU’s static pressure at 1.1 inches. If the pressure drop doubles across the filters, I change them.
When the chillers stop on cold days, I open the outside air dampers to 100% and open the precool deck to 60%. The outside air cools the chilled water several degrees. The chilled water warms the outside air up to roughly 55 degrees F. On these cold days, humidity is not usually a problem, and the chillers are off until the outside air temperature reaches 60 degrees F.
When the OAT is really warm and the boilers are off, I circulate the heating water through the preheat deck to remove some of the heat in the outside air. At night I circulate the heating water to cool it. The result can also be a small reduction in peak load.
If your AHUs have return air fans, ensure that they don’t pull too much air during low demand. Even at their lowest speed, their action may make your building pressure go negative.