Precedent & Voyager II VAV and MZVAV Daytime Warm-Up sequence of operation
Caution: Because DWU (MAX Heat) is a constant volume mode, the system’s VAV boxes must be driven to a maximum position to allow for maximum unit airflow. A six-minute air valve “drive time” is allowed to pass after entering DWU mode before the IGV, VFD or supply fan is driven to full airflow.
Daytime Warmup – Stand Alone Application (Variable Air Volume Units Only)
For VAV units with heat installed, DWU can be used to supply heat to the conditioned space. This feature is enabled by setting the RTAM SW1, dip switch 5 to the on position.
If DWU is enabled, a temperature sensor must be designated as the DWU sensor along with an Occupied heating setpoint, a DWU initiate temperature and a DWU terminate temperature. The DWU initiate temperature is at least 3° lower than the DWU terminate temperature. The DWU terminate temperature is the RTAM MWU Setpoint R31 potentiometer setting.
RTAM Module
When the Unit is Occupied, DWU is enabled and the space temperature as measured by the DWU sensor is lower than the DWU initiate temperature, the unit switches from VAV to constant volume mode and the economizer controls to its minimum position. The unit remains in DWU until the temperature in the space reaches the DWU terminate temperature setpoint.
The Unit terminates DWU when the space temperature reaches the DWU terminate setpoint or the unit is Unoccupied. If the Unit is Occupied when DWU is terminated, all heat outputs are disabled, and the unit returns to a VAV cooling mode.
Setting the DWU terminate setpoint above the Occupied heating setpoint “locks” the RTU in DWU. The reason for this is that the Unit will heat and maintain the space at the Occupied heating setpoint. Unless internal heat sources such as lights, computers and people raise the space temperature to the DWU terminate setpoint, the Unit remains in DWU until the unit transitions to the Unoccupied mode.
(See Figure 2)
In contrast, setting the DWU terminate setpoint below the Occupied heating setpoint causes the Unit to heat until the space temperature reaches the DWU terminate setpoint. After the DWU terminate temperature has been achieved, all heat outputs are disabled. (See Figure 3)
VHR Relay
When the unit transitions from occupied mode to morning warm-up, daytime warm-up, or unoccupied mode, the VHR relay is energized, as long as the unit has a valid zone temperature input at RTRM J6 1&2, or through a zone sensor.
The VHR relay has a set of contacts on LTB3 for the purpose of commanding the VAV boxes to drive open to maximum airflow position. If the unit has heat, the VAV boxes must be driven open. If the boxes are allowed to stay in control, the boxes may try to close instead of open when the warm air enters the room.
- When RTAM DIP SW1-1 is on, the output voltage range is approximately 0 to 10.5VDC.
- When RTAM DIP SW1-1 is off, the output voltage range is approximately 2.5 to 8.5VDC.
- IGV: When the supply fan is on and the output is 0%, the IGV are closed.
- VFD: When the supply fan is on and the output is 0%, the VFD runs at 35hz.
- If the supply static pressure goes below -0.2" (0.2VDC) the IGV / VFD output will stay at 0% and the diagnostic COOL FAIL + SERVICE FAIL will be present.
- If the static pressure exceeds 3.8" WC the supply fan will stop and the diagnostic HEAT FAIL + COOL FAIL +SERVICE FAIL will be present.
- During the heating modes; Daytime Warm-up, Morning Warm-up, and Unoccupied, the IGV or VFD output is always 100%.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.