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RT-SVX34R-EN

second before energizing the first stage electric heat
contactor (AH). A 10 seconds minimum “off” time
delay must elapse before the first stage heater is
activated. When the heating cycle is completed, the
RTRM de-energizes the Fan and the heater contactor
(AH) at the same time.

The RTRM cycles the first stage of heat as required to
maintain zone temperature. If the first stage cannot
satisfy the heating requirement, the RTRM energizes
the second stage electric heat contactors (BH) and (CH)
providing first stage has been on for at least 10
seconds or the second stage has been off for at least 10
seconds. (CH contactor is used on 54KW and larger
heaters.)

The RTRM cycles the second stage electric heat as
required to maintain the zone temperature.

Variable Air Volume (VAV)

During Unoccupied heating, Morning Warm up, or
Daytime Warm up, the VHR relay will be energized for
at least 6 minutes and the VFD output will go to 100%.
The heaters will stage on and off to satisfy the zone
temperature setpoint.

Variable Air Volume
Applications (Single Zone VAV)
Sequence of Operation

Occupied Cooling Operation

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.

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.

Economizer Enabled at Cooling Start

Once the unit has a request for economizer cooling and
the unit has met all Cool mode transition requirements,
the Economizer will open beyond minimum position
with the Supply Fan Speed at 45% in order to meet the
calculated discharge air setpoint value. If the
economizer at 100% alone cannot meet the active
discharge air setpoint, the Supply Fan Speed will
increase to 100% for 3 minutes. Once the 3 minute
compressor inhibit delay has expired, compressors will
be allowed to energize to meet the space demand. The
supply fan speed output will continue to modulate in
order to meet the zone cooling requirements.

Once compressors are being utilized for additional
cooling capacity, the economizer will be forced to 100%
if enabled. As the cooling capacity begins to stage back
(less cooling load) the economizer will remain at 100%,
if enabled, until all compressors have de-energized.

Economizer Enabled to Disabled

If the economizer is enabled and the unit is actively
cooling with the economizer, if the economizer
becomes disabled the economizer will be closed to the
active minimum position and compressors will be
allowed to stage without delay if the minimum off
timers have expired and there is a Cooling demand
requesting compressor operation. During this
transition, the fan will continue to modulate in order to
meet the space demand.

Economizer Disabled to Enabled

If compressors are energized for Cooling and the
economizer was disabled, but becomes enabled due to
desirable ambient conditions, the economizer will be
forced to 100% as on traditional VAV units.

Compressor Cooling

Compressor output control and protection schemes
will function much like on non-SZ VAV units. Normal
compressor HPC and LPC control will remain in effect
as well as normal 3-minute minimum on, off, and inter-
stage timers. Also, the condenser fans will be
controlled as on non-SZ VAV units and compressor
staging sequences will be as described in

Table 23, p.

and

Table 24, p. 47

based on unit tonnage

configuration and lead/lag status.

Cooling Sequence

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 accordingly.
Note that a 5 second delay will be enforced between
the command for supply fan output operation and the
command for compressor output operation. This delay
is enforced to ensure that the supply fan is energized
and ramping up to operating speed prior to energizing
compressors.