Table 17. Programmable Parameters
Keypad/Display ID
Menu Item Parameter Name
5. Group #1 Press Calc= Min Group 1 DSP Calculation Method Press Spt= 1.50"WC Group 1 DSP Set point
Speed/Vane= ___% Group 1 Speed/Vane Position Set point Dead band= 0.08"WC Group 1 DSP Dead band
Mod Limit= 0.40"WC Group 1 DSP Mod Limit Wait Time= 60 Sec Group 1 DSP Wait Time Max Step= 4% Group 1 DSP Max Step 6. Group #2 (Same as menu 5) (Similar to Group 1 parameters) 7. Group #3 (Same as menu 5) (Similar to Group 1 parameters) 8. Group #4 (Same as menu 5) (Similar to Group 1 parameters)
When two or more VAV units feed a common supply duct system, their supply fan air volumes must modulate in unison to maintain a common duct static pressure set point.
The RMC Panel assures that this happens by (1) reading the duct static pressure value from each unit in the group; (2) conditioning these values with a minimum, maximum, or average function; (3) calculating a common airflow set point based on the resultant conditioned pressure value and a common duct static pressure set point, and (4) distributing the resultant airflow set point to each unit in the group via network communications. Each unit then controls its supply fan’s inlet vane position or speed to maintain the airflow set point. (This set point is expressed as a percentage of supply fan capacity.) The Group [ DSP Calculation Method parameter defines the type of
pressure set point for the group. The Group [ Speed/Vane Position Set point is the common airflow
set point that is sent to each unit in the group.
To calculate the Group [ Speed/Vane Position Set point, the RMC controller uses four Change-and-
Wait parameters that are dedicated to duct static pressure control for the group: (1) Group [ DSP
Dead band, (2) Group [ DSP Mod Limit, (3) Group [ DSP Wait Time, and (4) Group [ DSP Max
Step. For most applications, the default values for these parameters will provide the best control. For more information, see the “Change-and-Wait Algorithm” section in the “MicroTech DDC Features” portion of this manual.
As an example, assume that Group 1 contains two units: one with a 1.00"WC duct pressure reading and one with a 1.25"WC reading. If the Group 1 DSP Set point is 1.20"WC and the Group 1 DSP Calculation Method is “Minimum,” the Group 1 Speed/Vane Position Set point used by both units will be rising since the RMC Panel uses the 1.00"WC (minimum) reading in its control process. Note: If a unit has two duct static pressure sensors connected to it, the unit controller automatically selects the minimum of the two readings as its working duct static pressure value. This is the value that the RMC controller reads and uses in its duct static pressure conditioning function (minimum, maximum, or average).
Unit Controller Setup
The unit duct static pressure control parameters, which are summarized in Table 18, can be set to any values desired as long as they are set the same in each unit—except for the Duct Pressure Reset Type. The Duct Pressure Reset Type parameter must be set to “Position” in any VAV unit that is (1) part of a multiple-unit control group and (2) connected to a common supply duct. At the
keypad/display, you can find the duct static pressure control parameters under applied rooftop controller menu 18 or SCAC controller menu 17.
Table 18. Duct Static Pressure Parameters
Unit Controller Parameter Keypad/Display Item Name
Duct Static Pressure Set point Duct Spt= 1.00"WC Duct Pressure Reset Type Reset= Position Duct Pressure Dead band Dead band= 0.08"WC Duct Pressure Mod Limit Mod Lim= 0.40"WC Duct Pressure Wait Time Wait Time= 20 Ten Duct Pressure Max Step Max Step= 20 Ten
Note: When the Duct Pressure Reset Type is “Position,” a unit controller will ignore the other parameters shown in Table 18 if communications exist between it and the RMC Panel. However, if communications are lost, a unit controller will use these parameters to control the unit’s supply fan airflow. Therefore, it is important that these parameters be set identically for every unit within a particular common duct static pressure control group.
How Unit Start-Up Affects Operation
If one or more units in a group are operating when another unit in the same group starts up, the operating units will be affected in the following ways:
• The Group [ Speed/Vane Position Set point will be forced to minimum position (17%), causing
the supply fan air flow to go to minimum in the units.
• The cooling and heating equipment will be disabled in the units. As a result, each unit’s cooling status and heating status will change to Network Disable.
The airflow is forced to minimum to prevent an overpressurization condition that might otherwise occur when the new unit’s supply fan starts. The cooling and heating equipment is disabled since there may be a significant loss of airflow—and thus load—through the coils or heat exchangers. (Since the change in air volume is sudden, the units’ normal temperature control processes would not be able to react quickly enough to prevent overcooling or over heating.)
Operation will return to normal when both of the following conditions are true: 1. The unit status of the units that were operational is Fan Only or Recirculate. 2. The unit status of the unit that just started is Recirculate.
The following table summarizes this operation:
When this condition is true The RMC Panel
All units in an Off state Sets the group supply fan speed/vane position to 0% At least one unit in Start-Up Initialization Sets the group supply fan speed/vane position to 17% and
disables unit heating and cooling All operational units in Fan Only or Recirculate; all
nonoperational units in an Off state
Allows the group supply fan speed/vane position to modulate and returns heating and cooling control to the units
For more information on Off, Start-Up Initialization, Recirculate, and other unit operating states, refer to the appropriate MicroTech unit controller operation manual (see Table 2.)
Note: If a new unit starts while others in a group are operating at or near full capacity, the
reduction in airflow and cooling or heating capacity that will result may be noticeable. Therefore, all units in a common duct static pressure control group should be scheduled to start at the same time. How Loss of Communications Affects Operation
If the RMC Panel loses communications with a unit in a group, it will ignore that unit when it calculates the common duct static pressure.
The unit that lost communications will continue to use the last supply fan vane position or speed set point it received from the RMC Panel for approximately five minutes. If communications are not restored in this time, the unit’s Duct Pressure Reset Type parameter will automatically change to “No Reset.” As a result, the unit will start controlling its supply fan airflow to maintain its own Duct Static Pressure Set point.
When communications are restored again, the unit’s Duct Pressure Reset Type parameter will automatically change back to “Position” within one minute.
Note: Although a unit that has lost communications remains operational, its supply fan airflow control may be erratic since the RMC Panel is no longer coordinating it. Communications should be
restored as quickly as possible. If communications to a unit cannot be restored quickly, that unit
should be shut down and isolated from the system until the problem is corrected.
u
u To set up the common duct static pressure control feature
1. Assign the units to a group and set up the common Control Temperature source feature (see “Assigning Units to Groups” and “Common Control Temperature Source” above).
2. Set the Group [ DSP Calculation Method parameter in the RMC controller to “Min,” “Max,” or
“Avg.”
3. Set the group’s common duct static pressure control parameters as desired in the RMC controller.
5. Set the remaining duct static pressure control parameters as desired in each unit (see Table 18). Be sure the settings are the same for all units within the group. (You may want to use the group’s set point, dead band, and mod limit values.)
MicroTech DDC Features
The following section provides detailed information on an intrinsic MicroTech direct digital control (DDC) feature used by the RMC Panel: the Change-and-Wait algorithm. Change-and-Wait is used to calculate a common supply fan inlet vane position or speed setting for multiple VAV units connected to a common supply duct.