Startup
RT-SVX34F-EN
45
Position Target setpoints based on the supply fan speed
and space CO2 requirements.
This new scheme will require the setting of 5 OA Damper
position setpoints; 3 more than on non-SZ VAV. These new
setpoints are located on the RTVM module:
1.
Design Min Position @ Minimum Fan Speed
Command (RTVM R130)
2. Design Min Position @ Middle Fan Speed Command
(RTVM R136)
3. Design Min Position @ Full Fan Speed Command
(RTEM Design Min Position)
4. DCV Min Position @ Minimum Fan Speed Command
(RTVM R41)
5. DCV Min position @ Full Fan Speed Command (RTEM
DCV Min Position)
As the supply fan speed command varies between
minimum and maximum, the Building Design and DCV
Minimum Position Targets will be calculated linearly
between the user selected setpoints based on the
instantaneous supply fan speed. The Bldg. Design and
DCV Minimum Position Targets will be used to calculate
the Active OA Damper Minimum Position Target, as on
traditional units, based on the Space CO
2
relative to the
active Design and DCV CO
2
setpoints. Refer to
Figure 40,
p. 45
for additional details on the design.
The Design Minimum and DCV Minimum OA Damper
Position setpoints at Minimum Fan Speed Command and
the Design Minimum OA Damper Position setpoint at
Middle Fan Speed Command will have a range of 0-100%
while the Design Minimum and DCV Minimum OA
Damper Position setpoints at Full fan speed will have a
range of 0-50%. Note that as on non-Single Zone VAV
units, a 10% offset will be enforced between the Design
and DCV Minimum Positions throughout the fan speed
range.
By default, the Design Minimum Position schedule (red
line below) will be a linear line through all user selectable
Design Minimum Position setpoints. The user will have the
ability to set the Design Minimum Position at Middle fan
speed command to a point that would be lower than the
calculated linear line between the Design Minimum
Position setpoints at 0% and 100% fan speed command in
order to compensate for the non-linear outside airflow
through the fan and damper modulation range. However,
if the Design Minimum Position at Middle fan speed
command is set to a point that would be higher than the
calculated linear line between the Design Minimum
Position setpoints at Minimum and Full fan speed
command, the minimum position will be limited to the
point that would make the Design Minimum Position
schedule linear.
Provisions have been made in Service Test Mode to allow
for proper damper minimum position setup:
1.
To set the Design and DCV Minimum Position setpoints
at Minimum Fan Speed, set the unit to operate at Step
1 (Fan ON) or Step 2 (Economizer Open) and make the
proper adjustments.
2. To set the Design Minimum Position setpoint at Middle
Fan Speed, set the unit to operate at Step 3 (Cool 1) and
make the proper adjustment.
3. To set the Design and DCV Minimum Position setpoints
at Full Fan Speed, set the unit to operate at Step 4 (Cool
2) and make the proper adjustments.
Outside Air Damper Minimum Positions
without DCV
For units not configured with DCV (no CO
2
sensor value
available), additional minimum position setpoints to
increase outdoor airflow accuracy will be supported. The
operation will be similar to OA CFM Compensation on
Traditional VAV units with the addition of a Design
Minimum Position setpoint at Middle Fan Speed
Command. The following setpoint potentiometers will be
used on the RTEM:
1.
Design Min at Minimum Fan Speed Command (RTEM
DCV Min)
2. Design Min at Middle Fan Speed Command (RTEM
DCV Setpoint LL)
3. Design Min at Full Fan Speed Command (RTEM Design
Min)
The controller will calculate the active OA Damper
Minimum position linearly between the user-selected
setpoints based on the supply fan speed command. The
range for the Design Min setpoints at Minimum and
Middle Fan Speed Command will be 0-100% while the
range for the Design Min at Full Fan Speed Command
setpoint will be 0-50%.
By Default, the Design Minimum Position schedule (red
line below) will be a linear line through all user selectable
Design Minimum Position setpoints. As with Demand
Figure 40.
SZVAV DCV with OA CFM compensation
Full Fan Speed
Minimum Fan Speed
Design @ Middle Fan Speed
Middle Fan Speed
OAD
Posion
Setpoints
Corresponds to Design
CO2 (DCV UL) Setpoint
Corresponds to DCV CO2 (DCV
LL) Setpoint
OAD Target
Setpoint
Increasing CO2
Design @ Minimum Fan Speed
Design @ Full Fan Speed
DCV @ Minimum Fan Speed
DCV @ Full Fan Speed
