Static Pressure Control and VAV Operation

Andrew BuchananStatic pressure is a key ingredient in a successful Variable Air Volume (VAV) system. It ensures that air flow will be available to enable the VAVs in a system to achieve their designed Cubic Feet per Minute (CFM) at maximum demand. Simply put, the push of the air must be greater than the resistance to the flow or no air will circulate through the ducts. Typically the setpoint for static pressure is determined through the course of the testing, adjusting, and balancing (TAB) at the end of a job.

Without proper TAB, problems such as condensation and undue stress on your HVAC system are bound to arise. During the TAB phase, the VAVs are set to their maximum cooling CFM setpoint after each VAV has been calibrated. A static setpoint is determined by driving the fan motor to the necessary static required for all VAVs to hit their maximum CFM target. This static pressure setpoint is what is known to be required to achieve maximum cooling airflow.

This is sometimes where the story ends. Maximum cooling can be achieved. The VAVs will modulate their air flow and be able to hit their maximum CFM requirements. As they meet their temperature setpoints, they will back down airflow and the static in the duct will go up. As the static goes up, the fan will slow down to hit the established static pressure setpoint from the TAB, saving fan energy.

This is not, however, the end to what can be done to promote optimum energy savings. At this point, we know we can achieve our maximum demand output. The fixed static setpoint will accommodate this. A floating static setpoint can be used instead to allow the system more flexibility and greater efficiency.

A reset schedule based on static setpoint can be used to allow the system to back off further when temperatures in the field are being met. It stands to reason that if we have one static pressure needed for the VAVs to hit their maximums, a lower static setpoint would also allow the VAVs to hit their minimum CFM targets. These two numbers would be the range of a reset schedule for the static setpoint. There are many ways to establish what drives the reset schedule up and down.

Two of the ways to guide a system’s static pressure reset schedule are face/bypass damper position and by monitoring the greatest of, or an average of each VAV’s CFM deviation. The colder the air temperature required, the greater the cooling demand of the VAVs. The further under a VAV’s CFM is from its target, the more static pressure is required for it to hit max.

Working in Crawl SpaceIn newer DDC systems, the VAV’s CFM deviation can be monitored and used to swing the air handling unit’s (AHU) static setpoint reset schedule. As the system’s VAVs go from lower to peak demand, their CFM deviations would increase. The static setpoint would then ramp up with the fan speed behind it. As the VAVs meet their temperature setpoints, the CFM target goes down and duct static increases and so on. This is a very direct way to maintain just the airflow needed for the VAVs to do their job. Throttling the leaving air temperature in similar fashion can be used to further promote energy efficiency.

In older systems, with pneumatic or stand alone VAV controls, being able to track the face/bypass damper position is another way to monitor cooling demand. As a face/bypass damper modulates higher and higher towards full open to cooling, it can be an indication that demand from the field is increasing. The higher the demand, the more fan or static is needed. A static reset schedule can be used to increase the static required as the face/bypass damper position goes to full cooling and back down as the dampers go closer to full bypass.

By utilizing these methods and applying these same principles to regulate the leaving AHU air temperature, your system can achieve greater efficiency. Applied building wide, this can lead to greater energy savings. This can allow a system to provide enough cooling potential to maintain comfort while at the same time, reducing energy consumption.

This story appears in the March 2017 issue of Automate, the Computrols newsletter.