WattMaster VAVBOX - Manual (Page 17)

VAVBOX Controller
Technical Guide
17
If the HVAC unit is in the Unoccupied Supply Air Heating mode be-
cause one or more of the VAVBOX controllers has a heating demand,
and another VAVBOX controller has a cooling demand, the damper/
airflow for the VAVBOX controller requiring cooling will position it-
self 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 Unoccupied Supply Air Cooling mode,
the damper will be held at the Night/Reheat minimum position until the
space temperature begins to rise above the cooling setpoint. As the
space temperature rises to within 0.5 ºF of the Unoccupied 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 and check for main status has been selected, the se-
ries fan relay will activate and run the series box fan continuously any-
time the HVAC unit fan is running. The damper will be held at the Nite/
Reheat minimum until the space temperature begins to rise above the
cooling setpoint. If check for main fan status has not been selected, the
series fan relay will activate and run the series box fan when in the
Space Cooling mode. The damper will be in the fully closed position
until the space temperature begins to rise above the cooling setpoint.
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
In the Unoccupied Cooling mode the parallel fan will be off whether
check for main fan status has been selected or not. If check for main fan
status has been selected the damper will be held at the Nite/Reheat
minimum until the space temperature begins to rise above the cooling
setpoint. If check for main fan status has not been selected the damper
will be held in the fully closed position until the space temperature
begins to rise above the cooling setpoint.
Space Heating Mode
During unoccupied mode the HVAC unit is normally off. Unoccupied
Space Heating mode is initiated by the temperature in the space falling
to within 0.5 ºF of the Unoccupied Space Heating Setpoint.
If the HVAC unit is in the Unoccupied Supply Air Cooling mode be-
cause one or more of the VAVBOX controllers has a cooling demand,
and another VAVBOX controller has a heating demand, the damper/
airflow for the VAVBOX controller requiring heating will position it-
self to provide the Night/Reheat setpoint amount of air into the space.
No modulation open will occur because the space does not want the
cold air currently being supplied by the air handler.
When the HVAC unit is in the Unoccupied Supply Air Heating mode,
the damper will be held at the Night/Reheat minimum position until the
space temperature begins to fall below the Unoccupied Heating Setpoint.
As the space temperature falls to 0.5 ºF below the Unoccupied Heating
Setpoint, the damper/airflow calculation causes the air valve to open
proportionally until the maximum setpoint is achieved at 3.5 ºF below
the setpoint. This is a 4 ºF proportional window starting 0.5 ºF above
the heating setpoint to 3.5 ºF below the heating setpoint.
As with the Occupied Mode of operation, two different configurations
are available for the Unoccupied Space Heating mode. If the box is
configured to allow reheat during Supply Air Heating mode, the reheat
relays can be activated even when the HVAC unit is in the Supply Air
Heating mode. If the box is configured not to allow reheat when the
HVAC unit is in Supply Air Heating mode, the box heat relays will be
de-energized when the HVAC unit is in Supply Air Heating mode. In
either configuration, when the HVAC unit is in the Supply Air Heating
mode, the damper will modulate open proportionally to the space de-
mand. The proportional window for the space temperature is 0.5 ºF
above to 3.5 ºF below the heating setpoint. This allows the space to take
advantage of the warm supply air in the duct.
If check for main fan status is not selected and the VAVBOX terminal
unit has auxiliary heat (baseboard heat etc.) that does not require the
HVAC unit fan to operate, reheat can be used without the HVAC unit
fan operating. If check for main fan status is selected, the reheat will
only operate when the HVAC unit fan is operating.
The VAVBOX Controller can activate auxiliary heating relays if the
relay expansion board has been connected and the correct number of
heating stages (1,2 or 3) has been configured. During demands for heat,
the first stage will activate whenever the space temperature drops be-
low the heating setpoint. The second stage will activate if the space
temperature falls 1.0 ºF below the heating setpoint. The third stage will
activate if the space temperature falls 2.0 ºF below the heating setpoint.
There is a two-minute delay between staging. This prevents stages from
activating at the same time. Once a heating stage has been activated, it
must remain on for at least one minute. Once it has been deactivated, it
must remain off for at least two minutes. The third stage relay will
deactivate when the space temperature rises to within 1.0 ºF of the heat-
ing setpoint. The second stage relay will deactivate when the space
temperature rises to the heating setpoint. The first stage relay will deac-
tivate when the space temperature rises above the heating setpoint by
1.0 ºF. See Table 3: Relay Staging - Unoccupied Mode for a com-
plete layout of the various fan & heat relay staging points.
Series Flow Fan Terminals
If the VAVBOX Controller has been configured as a Series Fan Pow-
ered terminal unit, the series fan will run continuously when the
VAVBOX controller is in the Space Heating mode no matter whether
check for main fan status has been selected or not. If the HVAC unit is
in Supply Air Heating mode the damper will modulate to maintain the
Space Heating Setpoint.
Any series fan terminal unit that has check for main fan status selected
will also operate its series box fan anytime the HVAC unit controller is
broadcasting that the HVAC unit fan is operating, regardless of whether
it is calling for heat or not. The damper will be held at the closed posi-
tion until the main fan status broadcast is received. Once the broadcast