Respect the VAV System Static Pressure Sensor!

Assuming design and installation are correct (a huge assumption), there are two system Commissioning tasks that massively impact VAV system operational efficiency, indoor air quality (IAQ) and energy use;

1. Setting up the minimum outside air flow rates and their control loops;
2. Setting the system static pressure sensor that modulates the primary fan speed. 

Setting up the minimum outside air will be subject of another post, this post will deal with locating and setting the system static pressure sensor that modulates the primary VAV fan speed. Why is this so important? Because the system static pressure sensor is a critical control point and potential single point of failure. If it is not located and set correctly;  

• the VAV system operates inefficiently and consumes more energy than it should;
• IAQ is not achieved, impacting occupant comfort and even health;
• ductwork leakage is increased via the system operating at higher pressures than required;
• equipment life can be shortened via the system operating at higher pressures than required;
• if this single device fails the system can run up to VMax with no control and damage ductwork and equipment.

The VAV system static pressure sensor is a system mission critical item. IMHO this $250 item of equipment, deserves much respect!

Design

1. Nominate a suitable range for the sensor in the controls specification (IMHO ~0-250 Pa).
2. Show an indicative location (~2/3’s along the ductwork distribution network) on the design drawings and controls schematic.
3. Explicitly state in the specification and drawings that deriving this important set point is an empirical task requiring the Controls, and TAB specialists working together. 

None of the above 3 points are the responsibility of the installation or controls contractor, they are the MEP design engineers job. 

Installation

1. If the specification and drawings do not state items 1 and 2 above then issue an RFI to have them confirmed 
2. Install the system static pressure sensor in an accessible place to enable Commissioning, maintenance and replacement on failure

On a side note, system ductwork leakage is a big issue on VAV systems so please pressure test (this should be specified by the design engineer) the high/medium pressure ductwork and install with appropriate sealing at all joints / connections.

Commissioning VAV System Static Pressure Sensor

1. Calibrate and verify the static pressure sensor.
2. Verify the static pressure sensor back to the BMS head end graphic.
3. Start up and commission the VAV fan VFD.
4. Controls point to point testing – VAV boxes and AHU.
5. Commission all VAV boxes – see previous post #36.
6. Apply system diversity.
7. Test and determine the optimum set point, with diversity applied, for the system static pressure sensor (located ~2/3’s along the ductwork distribution network).
8. Set parameters and alarms for “set point out of range” on the BAS.

Note: Items 1 thru 8 above require the Installer, Controls, TAB specialists & CxA working together and not in isolation.  

Facilities Maintenance 

1. Check and verify calibration once a year. 
2. Verify the static pressure sensor back to the BMS head end graphic once a year. 
3. Know and monitor the least favoured VAV box (the index) to ensure it is under control (“floating”) on a daily basis.

How many projects have you experienced where this has been done correctly?