Static Pressure Control Loop The purpose of the static pressure control loop is to maintain an optimal static pressure in the ductwork. The control loop senses the static pressure in the ductwork and compares it to a setpoint. The controller sends an output signal to the VSD which controls the fan speed. Adjusting the fan speed impacts the static pressure in the ductwork.

Static Pressure Control Loop Duct Static Temperature DDC Control Calculations Signal to VSD Controller Fan Speed Air flow through ductwork

Static Pressure Control Loop Air Flow DDC Controller Tuning Parameters Cooling Coil Static Pressure Sensor Variable Speed Drive

Static Pressure Control Loop Control Loop Setpoint

Static Pressure Control Loop Control Loop Setpoint Selecting the correct setpoint is very important for system performance. The optimal setpoint cannot be calculated in advance. It must be determined in the field. The current practice of fixing the setpoint wastes a lot of energy.

Static Pressure Control Loop Control Loop Setpoint If the setpoint is too low, some boxes will not be able to get enough air to provide comfort. Some boxes will have their damper 100% open but the actual flow will still be less than the desired flow. These boxes are “starved” and the VAV box will not be able to achieve the space temperature setpoint.

Static Pressure Control Loop Control Loop Setpoint If the setpoint is too high, fan energy is wasted. The higher the static pressure setpoint, the harder the fan must work to maintain the setpoint.

Static Pressure Control Loop Control Loop Setpoint If the setpoint is too high, the noise of the system will increase. If the static pressure setpoint is high, all of the VAV box dampers will only need to open a little bit to achieve the desired airflow. Airflow going through a small opening generates a lot of noise.

Static Pressure Control Loop Control Loop Setpoint If the setpoint is too high, VAV box control may go unstable. With a high static pressure setpoint, the throttling range of the damper to go from minimum flow to maximum flow is decreased. A small control adjustment to damper position has a large effect on airflow. The flow control loop is very sensitive and may cycle, depending on the control loop parameters.

Static Pressure Control Loop Control Loop Setpoint During air balancing, use a high setpoint to make sure that none of the boxes are starved.

Static Pressure Control Loop Control Loop Setpoint Choosing the optimal setpoint: 1.Enter a low setpoint so that some of the VAV boxes are starved with damper 100% open 2.Increase the static pressure setpoint until all of the VAV boxes are in control; damper less than 100% open

Static Pressure Control Loop Control Loop Setpoint Static Pressure Reset Energy Saving Strategy: 1.Every five minutes, check the position of all VAV box dampers. 2.If the damper which is open the most is more than 90% open, increase the static pressure setpoint by 50 Pascal. 3.If the damper which is open the most is less than 80% open, decrease the static pressure setpoint by 50 Pascal.

Static Pressure Control Loop Zone A “Hot” Zone B “Warm” Zone C “Cool” Static Pressure Setpoint = 100 Pascal Fan Speed of 40% Flow D 400Damper100% Flow A 300 Flow D 250Damper70% Flow A 250 Flow D 150Damper30% Flow A 150 Flow D = Desired Flow Flow A = Actual Flow Control Loop Setpoint

Static Pressure Control Loop Control Loop Setpoint Low Setpoint Fan runs at very low speed and uses little energy Zone “A” box is starved, even fully open the box cannot supply the desired air flow Zone “A” will get hotter because not enough cool air is being supplied

Static Pressure Control Loop Zone A “Hot” Zone B “Warm” Zone C “Cool” Static Pressure Setpoint = 200 Pascal Fan Speed of 60% Flow D 400Damper95% Flow A 400 Flow D 250Damper50% Flow A 250 Flow D 150Damper20% Flow A 150 Control Loop Setpoint Flow D = Desired Flow Flow A = Actual Flow

Static Pressure Control Loop Control Loop Setpoint Correct Setpoint Fan runs at low speed and uses some energy Zone “A” box is almost fully open but able to supply the desired air flow All zones will be comfortable as enough cool air is being supplied

Static Pressure Control Loop Zone A “Hot” Zone B “Warm” Zone C “Cool” Static Pressure Setpoint = 300 Pascal Fan Speed of 80% Flow D 400Damper65% Flow A 400 Flow D 250Damper30% Flow A 250 Flow D 150Damper10% Flow A 150 Control Loop Setpoint Flow D = Desired Flow Flow A = Actual Flow

Static Pressure Control Loop Control Loop Setpoint High Setpoint Fan runs at medium speed and wastes some energy (still better than CAV) All zones will be comfortable as enough cool air is being supplied Zone “C” will be noisy and it will be difficult to maintain stable control

Static Pressure Control Loop Damper Position 0%100% AirFlow Control Loop Setpoint Air flow through VAV box increases quickly as damper starts to open

Static Pressure Control Loop Damper Position 0%100% AirFlow 200 Pascal 500 Pascal 100 Pascal Static Pressure Control Loop Setpoint Characteristic curve of VAV box changes with the system static pressure.

Static Pressure Control Loop Damper Position 0%100% AirFlow 200 Pascal 500 Pascal 100 Pascal Static Pressure Minimum Flow Maximum Flow  VAV box controller modulates airflow between minimum and maximum airflow. Control Loop Setpoint

Static Pressure Control Loop 0%100% AirFlow 200 Pascal 500 Pascal 100 Pascal Static Pressure Minimum Flow Maximum Flow 50%  With system static pressure of 200 Pascal, damper is modulated 2% to 50%. Control Loop Setpoint

Static Pressure Control Loop 0%100% AirFlow 200 Pascal 500 Pascal 100 Pascal Static Pressure Minimum Flow Maximum Flow 15%  With system static pressure of 500 Pascal, damper is modulated 1% to 15%. Control Loop Setpoint

Static Pressure Control Loop 0%100% AirFlow 200 Pascal 500 Pascal 100 Pascal Static Pressure Minimum Flow Maximum Flow  With system static pressure of 100 Pascal, VAV box can never achieve maximum flow. Control Loop Setpoint

Static Pressure Control Loop Control Loop Setpoint In this example, the VAV box flow control loop is three times as sensitive when the static pressure is 500 Pascal as compared to when the static pressure is 200 Pascal. An overly sensitive control loop will cycle.

Static Pressure Control Loop Control Loop Setpoint Does dynamically resetting the static pressure setpoint really save energy as compared to having a fixed setpoint? Fan SizeOperationsSavings AHU-1125 hp 24 hours 42% AHU-2100 hp 24 hours 37% AHU-350 hpOccupied 19% AHU-425 hpOccupied 20% Source : ASHRAE Journal April 1993

Static Pressure Control Loop Variable Speed Drive

Static Pressure Control Loop Variable Speed Drive The Variable Speed Drive (VSD) Controller is typically provided with the fan and accepts a setpoint signal (0-10VDC or 4-20mA). The VSD provides voltage free contacts indicating a tripped motor.

Static Pressure Control Loop Variable Speed Drive Voltage Frequency 100% Nominal Frequency Variable Voltage Variable Frequency Based on the control signal input, the VSD will adjust the voltage and frequency to the fan motor. VSD reacts to changes in setpoint by changing the voltage and frequency. Changes are applied slowly based on acceleration and deceleration parameters.

Static Pressure Control Loop Static Pressure Sensor

Static Pressure Control Loop Static Pressure Sensor The conventional approach requires the static pressure sensor to be located at the point of minimum static pressure in the ductwork. For one long run of ductwork, this point is 1/2 to 2/3 of the way along.

Static Pressure Control Loop Static Pressure Sensor Conventional Approach Static Point of Lowest Pressure in System

Static Pressure Control Loop Static Pressure Sensor If the ductwork has two branches, the static pressure in each branch is taken and the lower of the values is used to control the fan.

Static Pressure Control Loop Static Pressure Sensor Conventional Approach Low Signal Select

Static Pressure Control Loop Static Pressure Sensor For example, if branch “A” serves the east side of the building and branch “B”serves the west side of the building then in the morning, the fan would be controlled by sensor “A” and in the afternoon, the fan would be controlled by sensor “B”.

Static Pressure Control Loop Static Pressure Sensor If the static pressure setpoint is continuously reset based on VAV damper position:  Energy will be saved as fan only works as hard as is necessary  Placement of static pressure sensor does not matter - as long as airflow is not turbulent : away from fan, 5 duct diameters away from bends, dampers, reducers, etc.

Static Pressure Control Loop Converts differential pressure between H and L into a signal. When H is inside duct and L is outside duct static pressure of duct is measured. Static Pressure Sensor

Static Pressure Control Loop Static Pressure Sensor Calibrate a static pressure sensor by:  Disconnect static pressure sensor.  Inject a 2.00 VDC voltage and verify that the DDC Controller reads zero.  Inject a VDC voltage and verify that the DDC Controller reads maximum.  Replace static pressure sensor.