Supply air VAV box, pressure dependent control (VavSu15)

Required elements and how to configure them:

Function

VAV supply heating, cooling, or ventilation request (VavSuHReq / VavSuCReq / VavSuVntReq) is received and processed into the output signal (in percent) for VAV supply damper position (VavSuPos).

Basic function: Accept request signal from associated room controller and map it to damper position setpoint; Pass result through a device mode logic switch prior to outputting as a command to the object that controls the supply damper position.

Damper position setpoint selection: In control mode (VavSuDevMod = 2) the damper position setpoint (VavSuSpPos) is the largest of the values needed to satisfy the requests for:

• Cooling
• Heating
• Ventilation
• Air flow support for cooling coil
• Air flow support for heating coil

Each request is expressed as a percent value, from 0 to 100%. The percent values are separately mapped to their respective damper positions for heating, cooling, and ventilation. Since damper limits (minimum and maximum) can be / are different for each of the request types, a comparison must be made to select the actual maximum demand (selecting the highest initial percent value among request signals will not necessarily yield the maximum demand signal).

Example:

• Assume damper position limits for cooling as min flow 10% and max flow 80%, resulting in a scale range of 70 (80 - 10 = 70). In this case, a cooling request signal of 50% results in a mapped damper position of 45%. (70 * 0.5) + 10 = 45
  In the equation, "70" represents the scale range for damper excursion; "0.5" is the multiplier for a 50% cooling request signal; "10" represents the start position (minimum flow 10%) of the scale range.
• Assume damper position limits for ventilation as min flow 15% and max flow 80%, resulting in a scale range of 65 (80 - 15 = 65). In this case, a ventilation request signal of 48% results in a mapped damper position of 46.2%. (65 * 0.48) + 15 = 46.2
  46.2% is larger than the mapped cooling position of 45% above, despite what appears to be a smaller initial request signal for ventilation (48%) than for cooling (50%).

The mapping includes a step-up at the low end, with a switch-on point of 4% and a hysteresis of 2%.

Potential uses of the step-up include:

• Keeping the damper position setpoint high enough to support heat transfer from a coil

If VavSuSpPos is in normal priority and receives a signal of less than 2%, the value of VavSuSpPos will be 0%. The value will stay at 0% until the signal to VavSuSpPos rises above 4%, at which time the value of VavSuSpPos will equal the signal. Between 2% and 4% (hysteresis) VavSuSpPos retains the last value.

Damper position rate change limiter: A rate change limiter function is provided to minimize rapid damper repositioning if the damper is changed by a large amount (for example, manually by a user). The rate limiter delay (default 60sec) is only applied when VavSuSpPos is in normal runtime priority – not protection or emergency priority.

Saturation signal: The saturation signal VavSuPosStrtn is a binary object that is True ("Starved") when the VAV supply air terminal cannot get enough air for a time exceeding a built-in time delay.

Note
VavSuPosStrtn is always Off if parameter EnStrtnCal = 0 (No). This allows the user to exclude a particular terminal from the saturation pressure reset system.

Relief input signal: A VAV box’s binary air flow relief object (VavSuAirFlRlf) can be set to True by the air handling unit. This is done to relieve flow resistance in the duct system in order to maintain proper static pressure.

When device mode = control mode and both EnRlf (Enable relief) and VavSuAirFlRlf equal On, VavSuSpPos will not go below the value of the damper relief parameter (PosRlf).

Note
Relief has no effect on the supply chain VAV air demand signal VavSuAirDmd.

The output signal VavSuPosPvdVnt is the amount of air flow available for ventilation. When VavSuAvlVnt is True, a maximum value (VavSuPosMaxVnt) is transferred to VavSuPosPvdVnt. VavSuPosPvdVnt is the largest max flow (damper position) of the supply VAV boxes. These values are set in each supply VAV for the maximum flow in the segment.

Minimum ventilation flow: In an installation without DCV (demand control ventilation), the minimum primary air flow is applied whenever ventilation is required. This effectively replaces minimum cooling and minimum heating values. In an installation with DCV, the primary air flow value is applied when the calculated ventilation demand is at minimum and ventilation is required.

See the room level ventilation control AF (VavVntCtl.xx) for detailed information on configuring minimum ventilation levels.

VavSuDevMod supports the following states:

When device mode = Min.air vol.flow:

• If VavSuChovrCnd (Supply air VAV changeover condition) is Neither,
  then VavSuSpPos = VavSuPosMinVnt (Supply air VAV min.position for vent)
• If VavSuChovrCnd is Heating, then VavSuSpPos = VavSuPosMinH
• If VavSuChovrCnd is Cooling, then VavSuSpPos = VavSuPosMinC
• If VavSuChovrCnd is Neutral, then VavSuSpPos = greater of VavSuPosMinH or VavSuPosMinC

Fan coasting feature: The fan coasting feature prevents premature closing of the room supply damper while the AHU fan is coasting down. This might occur during a switch between operating modes (for example, switching from Comfort to Economy).

The fan coasting feature is active when:

• The room supply air damper is the only damper that is open – all other VAV supply air dampers are closed.
• The AHU supply fan has been turned off and is coasting down.
• The air flow setpoint signal (VavSuSpAirFl), while on its way to zero, falls below the air flow demand hysteresis setting (HysAirFlDmd).

When these conditions are true, the boolean supply fan coasting signal (FanSuCstVal) will equal 1 (True) and the damper is reset to its last (previous) operating position / value. This keeps the damper open while the fan coasts down to prevent supply duct over-pressure.

In addition to the fan coasting signal freezing the damper in place, an internal timer function was activated when VavSuSpAirFl initially dropped below HysAFlSta. The fan coasting signal and the timer (60 seconds) must both finish (return to False) before the room supply damper can close. See the figure below.

Note
The internal timer operates independently and regardless of FanSuCstVal. Once the timer is activated, the room supply air damper is frozen in place until the counter reaches zero, after which the damper may close.

 

Supply chain interface: There are four demand output signals for air.

• VavSuCDmd - Supply air VAV cooling demand
  When cooling is available, VavSuCDmd is the percentage value of the cooling request from the room (otherwise it is 0).
• VavSuHDmd - Supply air VAV heating demand
  When heating is available, VavSuHDmd is the percentage value of the heating request from the room (otherwise it is 0).
• VavSuVntDmd - Supply air VAV ventilation demand
  When ventilation is available, VavSuVntDmd is the percentage value of the ventilation request from the room (otherwise it is 0).
• VavSuAirDmd - Supply air VAV air demand for plant mode
  VavSuAirDmd is a multistate value that equals PltOpMod (VavSuSpPos must be greater than the switch-on point for air flow demand. If PltOpMod is "Depressurize" or "Emergency off", VavSuAirDmd will equal PltOpMod regardless of VavSuSpPos value.

AHU changeover condition signal: The multistate VAV supply changeover condition input signal (VavSuChovrCnd) comes from the room coordinating function (RCoo). It indicates the current heating / cooling available status for central AHU. Four states are supported:

1. Neither
2. Heating
3. Cooling
4. Neutral

"Neutral" means that heating and cooling PID controllers are both enabled and that the system is providing air that is not hot or cold.

Configuration

Objects

Parameters

Engineering and commissioning

- Check for correct damper actuator installation. Actuator mis-wiring or improper installation is a major cause of common problems.

- VavSuSplyAir Group Member – provides information to the central AHU for duct static pressure reset:
RHvacCoo_xx contains a GroupMember (SplyAir) that monitors/receives data from the central function SplyAir group manager. The central function should be located in the room automation station that contains ALL central coordination functions that come from rooms (e.g., child) serviced by the central system (e.g., parent). Verify the assignment of the group numbers in the group manager objects and in the assigned group member objects. Make sure that the group numbers of the group managers in the same group category are unique on the network.

 

See the room level ventilation control AFs (VavVntCtl, FcuVntCtl) for detailed information on configuring minimum ventilation levels.