14
RT-SVF69C-EN
Sequence of Operation
Single Zone Variable Air Volume
(Single Zone VAV) Control
For normal Cooling operation, available cooling capacity
will be modulated in order to meet the calculated
discharge air setpoint between the user selected upper
and lower limits. If the current active cooling capacity is
controlling the discharge air within the deadband no
additional Cooling capacity change will be requested. As
the Discharge Air Temperature rises above the deadband
the control will request additional capacity as required
(additional compressor operation or economizer). As the
Discharge Air Temperature falls below the deadband the
algorithm will request a reduction in active capacity.
Cooling Operation
If the control determines that there is a need for
compressor operation in order to meet the discharge air
requirements, once supply fan proving has been made,
the unit will begin to modulate compressors. As the zone
cooling demand continues to increase, if additional
capacity is required, the supply fan output will be
modulated above minimum speed in order to meet the
zone requirements.
Note:
The supply fan speed will remain at the activated
compressors’ associated minimum value until the
control requires additional capacity to meet the
zone demand.
As the cooling load in the zone decreases the control will
reduce the speed of the fan down to minimum per
compressor operation and control the compressor
outputs accordingly. As the compressors begin to de-
energize, the Supply Fan speed will fall back to the Cooling
Capacity's associated minimum fan speed but not below.
As the load in the zone continues to drop, cooling capacity
will be reduced in order to maintain the calculated
discharge air setpoint.
Economizer Cooling
During normal Economizer Cooling, the fan speed will
operate at its minimum. However, if the economizer is able
to meet the demand alone, due to desirable ambient
conditions, the supply fan speed will be allowed to
increase above the minimum prior to utilizing mechanical
cooling. Note that Economizer Enable/Disable decisions
will be made based on the previous sections; however, the
economizer control point will now be variable based on
the zone cooling demand.
Demand Controlled Ventilation
Units configured for SZVAV and Demand Controlled
Ventilation (CO
2
sensor value available) require a new
control scheme comprised of 2 existing schemes that have
been traditionally mutually exclusive; DCV and OA CFM
Compensation. Units configured with DCV will invoke the
new Demand Controlled Ventilation scheme which allows
variable Bldg. Design and DCV Minimum Positions and OA
Damper Position Target setpoints based on the supply fan
speed and space CO
2
requirements.
Economizer Damper Position Set-Up with
DCV
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 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 CO2 relative to the active Design and DCV CO2
setpoints.
By default, the Design Minimum Position schedule 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.