Sequence of Operation
46
RT-SVX26R-EN
Single Zone Variable Air Volume
(Single Zone VAV) Control
Note:
Single Zone VAV is designed to be used with a zone
sensor. If a unit is configured for single zone VAV
operation but is connected to a thermostat, the
control will revert to multi-speed (2-speed) indoor
fan control.
For normal Cooling operation, available cooling capacity
will be staged or 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 stages in order to meet the discharge air
requirements, once supply fan proving has been made,
the unit will begin to stage 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 that the supply fan speed will
remain at the compressor stage's 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 stage and control the
compressor outputs accordingly. As the compressors
begin to de-energize, the Supply Fan speed will fall back to
the Cooling Stage'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.
Minimum Fan Speeds for Cooling Stages
As the unit begins to stage compressors to meet the
cooling demand, the following minimum Supply Fan
Speeds will be utilized for each corresponding Cooling
Stage. Note that the Supply Fan Speed will be allowed to
ramp up beyond the minimum speed in order to meet the
zone cooling demand.
2-Stage Cooling Units
The minimum fan speed for units with 2 stages of DX
Cooling will be 63% of the unit's full airflow capacity. At
Stage 1 of DX Cooling the Fan Speed will be at a minimum
of 63% and at Stage 2 of DX Cooling the Fan Speed will be
at a minimum of 83%.
3-Stage Cooling Units (High Efficiency Units with
Short Orifice)
The minimum fan speed for units with 3 stages of DX
Cooling will be 63% of the unit's full airflow capacity. At
Stages 1 and 2 of DX Cooling the Fan Speed will be at a
minimum of 63% and at Stage 3 of DX Cooling the Fan
Speed will be at a minimum of 83%.
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:
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 CO
2
relative to the active Design and DCV CO
2
setpoints.
