Damper Positions and Occupied Mode Sequences
Off Mode
During unoccupied mode, the mode is considered ‘OFF’ if the space
temperature does not generate a heating mode or cooling mode based
on the unoccupied heating & cooling setpoints.
Damper Positions
The actual values for the minimum damper positions that are described
in the following paragraphs can be configured by changing the values in
setpoint screens 4 through 9 for the VAV/Zone Controller. These mini-
mums are expressed in damper open percentages for pressure dependent
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 VAV/Zone damper will move to the Cool
-
ing Minimum position.
Heating Minimum
When the HVAC unit is in the Supply Air Heating mode but the space
does not require Heating, the VAV/Zone damper will move to the Heat
-
ing Minimum position.
Vent Minimum
This is the position the VAV/Zone damper will move to when the HVAC
unit is in the Supply Air Vent mode.
Night Minimum
This is the position that the damper moves to during the Unoccupied
mode. When using non-fan powered terminal units, the VAV/Zone
damper will position itself in the Night Minimum position. In order for
fan powered terminal units to position the damper to the Night Minimum
position, the check for main fan status must be selected, and the HVAC
unit fan must be operating.
Reheat Minimum
This is the position that the damper moves to when Reheat is initiated.
If the VAV/Zone Controller is used on a terminal unit that has reheat, the
VAV/Zone damper will move to the Reheat position whenever a Space
Heating demand occurs and the HVAC unit is in Supply Air Cooling
or Vent mode. When the HVAC unit is in Supply Air Heating mode,
the VAV/Zone damper will modulate as required to maintain the Space
Heating Setpoint.
Occupied Mode Sequences
Space Vent Mode
This mode only applies to the Occupied Mode of operation. If the
equipment 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, you 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 demands. 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 VAV/Zone 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
position.
If the VAV/Zone 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 VAV/Zone
Controller has a cooling demand, the damper/airflow for the VAV/Zone
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 1.5 ºF above the setpoint. This is a 2
ºF proportional window starting 0.5 ºF below the cooling setpoint to 1.5
ºF above the cooling setpoint.
The damper/airflow is never allowed to modulate outside the user-
adjusted minimum and maximum setpoints. The maximum damper/
airflow setpoint applies to heating and cooling modes of operation only.
All of the modes have their own individual minimum setting.
Series Flow Fan Terminals
I
f the VAV/Zone Controller has been configured as a Series Fan Powered
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 deactivated.
21
Non-Modular ZCAP Technical Guide
Sequence of Operations
