230923 DDC Control Revised 7 24 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 the automation of 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 modifications to the existing Johnson Controls software programming to accomplish the control sequences noted in this specification. In addition, provide setpoint modifications when directed in the field as part of the ongoing TAB work to accomplish optimum operating conditions. Provide new devices such as sensors/switches, wiring and other materials if existing controls are discovered to be either defective or where none exist to achieve the noted sequences.

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 accepted by the VA shall be reused or modified to the new requirements if desired to revise. Other control sequences not mentioned in this specification and currently used 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 and DDC controls work of Air Handling Systems shall be planned to occur on a weekend after hours when a equipment shutdown can be scheduled.

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

Units and DOAS Units controls for baseline of systems.

E. The Temperature Controls Contractor shall work together with the VA facility personnel, CEP BAS technicians 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 Campus Air Handling Units and DOAS Units.

TEMPERATURE CONTROL

The chilled water coil discharge air temperature (not unit leaving air temperature), shall be set at 51 degrees maximum, to maintain consistent building space moisture levels. Disable any existing programming referencing temperature reset to values higher than this.

CARBON DIOXIDE CONTROL

The outside air supply values from the DOAS shall remain constant (volume) to maintain building pressures. Disable any existing programming referencing CO2 control and the modulation of the DOAS supply air fan. The existing CO2 sensor located in the return air ductwork shall remain for BAS information only.

AHU SUPPLY AIR FLOW CONTROL

The Air Handling Unit supply air fans shall modulate per the existing programming control sequences. This shall be either by maintaining a duct static pressure setpoint tested and established by the TAB firm to deliver required air to the worst case supply VAV box or by static pressure reset optimization based on supply VAV box damper positions.

DOAS AIR FLOW CONTROL

The DOAS supply air fans shall maintain a fixed air volume required to balance the building air pressures. Supply fans shall not modulate based on the existing programming of duct static pressure setpoint.

The DOAS exhaust fans shall always operate at 100% capacity to serve building general exhaust.

The DOAS heat recovery wheel shall be programmed and set to disable when outside air temperatures are between 75 degrees and 38 degrees.

B. Where this sequence is utilized at the existing Campus BAS for VAV Air Handling Units, provide verification and correction where needed.

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.), 23 09 23 - 3 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.

C. Provide this sequence for all VAV Air Handling Unit systems that utilize a modulating return air fan. This occurs in IP, OP, TLR, Pan Am and CEP Buildings.

RETURN AIR FAN CONTROL

The BAS shall monitor the Air Handling Unit mixed air section/plenum to always maintain the pressure in this section neutral to the Mechanical Room space by increasing or decreasing the return air fan speed. As the supply air quantities vary with fan modulation (supply fan negative pressure to the section) and the DOAS outside air supplies constant volume (positive pressure to the section), the return air fan shall modulate up or down to make up this differential (positive pressure to the section). This shall be accomplished with a new differential pressure sensor installed in this section.

D. Provide this sequence for all VAV Air Handling Unit systems that utilize an existing unit relief air damper. This occurs in DNT, IP, OP and Pan Am Buildings.

RELIEF AIR DAMPER CONTROL

The BAS shall monitor the Air Handling Unit mixed air section/plenum pressure with a new static air pressure sensor. If the air pressure increases beyond neutral by +0.01” (adj.), the unit relief air damper shall start to open to relive air exterior to the unit. As the air pressure begins to return to neutral, the damper shall begin moving to the closed position.

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