WattMaster VAVBOX - Manual (Page 15)

VAVBOX Controller
Technical Guide
15
Damper Positions
The actual values for the minimum damper positions that are described
in the following paragraphs can be user configured by changing the
values in setpoint screens 5, 6 and 7 for the VAVBOX controller. These
minimums are expressed in damper open percentages for pressure de-
pendent terminal units or in CFM for pressure independent terminal
units.
Cooling Minimum
When the HVAC unit is in the Supply Air Cooling mode but the space
does not require cooling, the VAVBOX damper will go to the Cooling
Minimum position.
Heating Minimum
When the HVAC unit is in the Supply Air Heating mode but the space
does not require Heating, the VAVBOX damper will go to the Heating
Minimum position.
Vent Minimum
This is the position the VAVBOX damper will move to when the HVAC
unit is in the Supply Air Vent mode.
Nite/Reheat Minimum
This setpoint has two different functions depending on whether the
HVAC unit is in Occupied or Unoccupied mode.
Occupied Mode
If the VAVBOX controller is used on a non-fan-powered terminal unit
that has reheat, the VAVBOX damper will move to the Nite/Reheat
position whenever a Space Heating demand occurs and the HVAC unit
is in Supply Air Cooling or Vent modes. When the HVAC unit is in
Supply Air Heating mode the VAVBOX damper will modulate as re-
quired to maintain the Space Heating setpoint.
Unoccupied Mode
When using non-fan powered terminal units, the VAVBOX damper will
position itself in the Nite/Reheat minimum position. In order for fan
powered terminal units to position the damper to the Nite/Reheat mini-
mum position, the check for main fan status must be selected and the
HVAC unit fan must be operating.
Occupied Mode Sequences
Space Vent Mode
This mode only applies to the Occupied Mode of operation. If the equip-
ment is in the Unoccupied Mode, then a lack of heating or cooling
demand would generate the Off Mode.
If the HVAC unit is in Supply Air Vent Mode, the user can adjust the
damper position on pressure dependent terminal units and the airflow
on pressure independent terminal units to provide a fixed amount of
ventilation air into the space when there are no heating or cooling de-
mands. During this time, the damper does not modulate on pressure
dependent terminal units. On pressure independent terminal units, it
only modulates to the extent required to maintain the vent minimum
airflow setting.
If the VAVBOX Controller detects that the HVAC unit is in Supply Air
Heating mode, indicating that the air handler has activated its heat, the
heating airflow minimum will be substituted for the vent minimum po-
sition.
If the VAVBOX Controller detects that the HVAC unit is in Supply Air
Cooling mode, indicating that the air handler has activated its cooling,
the cooling airflow minimum will be substituted for the vent minimum
position.
.
Space Cooling Mode
Occupied Space Cooling mode is initiated by the temperature in the
space rising to within 0.5 ºF of the Occupied Cooling Setpoint.
If the HVAC unit is in the Supply Air Heating and another VAVBOX
controller has a cooling demand, the damper/airflow for the VAVBOX
controller requiring cooling will position itself to provide the heating
minimum setpoint amount of air into the space. No modulation open
will occur because the space does not want the warm air currently being
supplied by the air handler.
When the HVAC unit is in the Supply Air Cooling mode, the damper is
normally held at the minimum cooling position until the space tempera-
ture begins to rise above the cooling setpoint. As the space temperature
rises to within 0.5 ºF of the Occupied Cooling Setpoint, the damper/
airflow calculation causes the air valve to open proportionally until the
maximum setpoint is achieved at 3.5 ºF above the setpoint. This is a 4
ºF proportional window starting 0.5 ºF below the cooling setpoint to
3.5 ºF above the cooling setpoint.
The damper/airflow is never allowed to modulate outside the user ad-
justed minimum setpoint and the maximum setpoint. The maximum
damper/airflow setpoint applies to heating and cooling modes of opera-
tion only. All of the modes have their own individual minimum setting.
Series Flow Fan Terminals
If the VAVBOX Controller has been configured as a Series Fan Pow-
ered terminal unit, the series fan relay will activate and run the series
box fan continuously anytime the HVAC unit fan is running.
In all cases, before the series box fan can be activated, the air damper is
driven fully closed and held that way for 30 seconds to make sure the
series box fan hasn’t inadvertently started to spin backwards. Once the
series box fan starts, it waits an additional 10 seconds to allow the fan
to spin up before it starts to open the damper and introduce airflow from
the HVAC unit fan.
Parallel Flow Fan Terminals
During normal cooling or vent mode and adequate air supply, the paral-
lel fan will be off. During the occupied cooling mode the fan will only
activate if the damper/airflow is below a user defined low limit setting.
This causes it to be used as a make-up air source. When the damper/
airflow rises 15% above the low limit setpoint, the fan will be deacti-
vated