P12-230923 DDC Control System for HVAC 4-7-20.pdf

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SECTION 23 09 23

DIRECT-DIGITAL CONTROL SYSTEM FOR HVAC

PART 1 - GENERAL

1.1 DESCRIPTION

A. The VA Campus has an existing Johnson Controls DDC System for automation of existing HVAC systems. The front end is located at

Level 3.5 of the CEP building. The scope of work for this project is to provide as required modifications to the existing Johnson

Controls software programming to accomplish the control sequences noted in this specification. In addition, provide setpoint modifications as required when directed in field as part of the ongoing TAB work to accomplish optimum operating conditions.

B. The Temperature Controls Contractor shall arrange to visit the VA site to become familiar with the content of the existing Johnson

Controls software programming, existing points and existing control sequences. Existing sequences that are currently working and acceptable shall be reused or modified to the new requirements where possible. Other control sequences not mentioned in this specification are to remain as existing. Any new or modified control sequence programming may be done or troubleshooted offsite before uploading to the VA front end (CCT) during actual day of TAB work.

C. The actual TAB, Commissioning and DDC controls work of Air Handling

Systems shall be planned to occur on a weekend after hours when a equipment shutdown is scheduled.

D. Refer to Johnson Controls flow diagrams and sequence of operation for typical existing DNT and OP Building Air Handling Units and DOAS

Units controls.

E. The Temperature Controls Contractor shall be work together with the

Commissioning Agent and TAB contractor to coordinate all activities to complete the project.

F. The Temperature Controls Contractor shall be a subcontractor of the

General Contractor and shall report to and be paid by the General

Contractor.

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PART 2 - PRODUCTS

2.1 NONE

PART 3 - EXECUTION

3.1 SEQUENCE OF OPERATION

A. Provide this sequence for all VAV Air Handling Unit supply air fan (20) and return air fan (5) systems that have connected space supply and return air terminal VAV boxes. Return VAV terminals are utilized in DNT Building only.

DUCT STATIC PRESSURE RESET OPTIMIZATION

The building automation system (BAS) shall continuously monitor the damper position of all VAV terminal units. The existing duct static pressure shall be sensed directly at the ductwork near the fans.

When any VAV damper is more than 95% (adj.) open, the fans duct static pressure setpoint shall be reset upward by 0.1 in W.C. (adj.), at a frequency of 1 minute (adj.), until no damper is more than 95% open or the static pressure setpoint has reset upward to the system maximum duct static pressure setpoint or the variable-frequency drive is at the maximum speed setting.

When all VAV dampers are less than 85% (adj.) open, the fans duct static pressure setpoint shall be reset downward by 0.1 in W.C.(adj.), at a frequency of 1 minute (adj.), until at least one damper is more than 85% open or the static pressure setpoint has reset downward to the system minimum duct static pressure setpoint or the variable-frequency drive is at the minimum speed setting.

B. Provide this sequence for all VAV Air Handling Unit return air fan systems that do not have space terminal VAV boxes and are ducted directly from ceiling grilles to duct mains. This occurs in IP, OP, TLR, PanAm and CEP Buildings, 26 total AHU’s.

Existing Air Handling Units (12) not modified with this sequence shall remain as currently controlled, maintaining fixed static pressure set point.

RETURN AIR FAN CONTROL

The return air fan shall track the supply air fan and outside air intake to always maintain the required total offset differential (RA = SA-OA). The return air fan shall modulate speeds to maintain an offset based on either reading the supply duct, return duct and outside air duct air measuring stations or reading the supply, return and outside air fan VFD Hertz. The differential offset of either CFM or Hertz shall be for the entire operating range of the Air Handling Units.

C. Provide this sequence for (9) DOAS or Air Handling Units in buildings outside of DNT that have existing CO2 sensors in the return air ductwork.

The DNT building shall maintain the existing DOAS sequence of operation. This is to keep the outside air supply fixed at design

23 09 23 - 3 values, non-modulating, due to maintaining continuous building pressure requirements.

CO2 OUTSIDE AIR DEMAND CONTROL VENTILATION

The DOAS outside air fan or damper shall modulate based on maintaining a CO2 setpoint (adj.) of 700 ppm as sensed in the main return air ductwork. When the CO2 rises above setpoint, outside air shall increase upward by 5%, by either fan speed or damper position, as required and not exceed the VA specified maximum (adj.) air volume (cfm). When the CO2 drops below setpoint, outside air shall decrease downward by 5%, by either fan speed or damper position, as required and not exceed the VA specified minimum (adj.) air volume (cfm).

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