RT-SVX34R-EN
47
Table 24. Compressor staging sequence - high
efficiency units and std eff. 40–50T CV units)
Sequence 1 Sequence 2
Sta-
ge
CPR
1
CPR
2
CPR
3
Sta-
ge
CPR
1
CPR
2
CPR
3
1 ON OFF OFF 1
Start at Stage 2
2 OFF ON OFF 2 OFF ON OFF
3 ON ON OFF 3 ON ON OFF
4 OFF ON ON 4 OFF ON ON
5 ON ON ON 5 ON ON ON
Sequence 3
Sta-
ge
CPR
1
CP-
R2
CP-
R3
1
Start at Stage 2
2 OFF OFF ON
3 ON OFF ON
4 OFF ON ON
5 ON ON ON
Figure 40. Compressors
40 Tons Std Eff. VAV Only27.5, 30, 35 Tons Std Eff.
50 Tons Std Eff. VAV Only 27.5-50 Tons High Eff. & 40-50 Tons
Std Eff. CV
Zone Temperature - Occupied Cooling
(CV and SZVAV)
When the unit is in the cooling mode and the zone
temperature raises above the cooling setpoint control
band, the economizer and the compressor stages will
be cycled as required by the zone sensor, remote panel,
or Tracer®. For SZ VAV control, the fan capacity will
also be controlled in order to meet the zone cooling
demand.
Zone Temperature - Occupied Heating
(CV and SZVAV)
When the unit is in the heating mode and the zone
temperature falls below the heating setpoint control
band, the necessary stages of heat will cycle to raise
the temperature to within the setpoint control band.
For SZ VAV, the fan capacity will also be controlled in
order to meet the zone heating demand.
Supply Fan (CV and SZ VAV)
When the Fan Selection Switch is in the “AUTO”
position and a call for cooling is initiated, the supply
fan will delay starting for approximately one second on
traditional CV units. For SZ VAV units, the supply fan
will be controlled ON based on the zone cooling
demand. Once ON, the unit will begin staging cooling
capacity (economizer and/or compressors) in order to
meet the discharge air requirements. Once the zone
has been satisfied, the supply fan will be controlled
OFF. When the Fan Selection Switch is in the “ON”
position, the supply fan will run continuously. If airflow
through the unit is not proven by the differential
pressure switch (factory setpoint 0.15“w.c.) within 40
seconds nominally, the RTRM will shut off all
mechanical operations, lock the system out, send a
diagnostic to ICS, and the SERVICE LED output will
pulse. The system will remain locked out until a reset is
initiated either manually or through ICS or a mode
transition from OFF to a non-OFF mode.
Supply Air Tempering (CV and SZ VAV)
CV Units with Staged Heat
This function allows the supply air temperature to be
maintained within a low limit parameter during
minimum ventilation periods. For CV units configured
with a Staged Heat design (Electric or Gas) and Supply
Air Tempering operation enabled, if the following items
are true, the unit will enter Supply Air Tempering
mode:
• The supply fan is ON.
• The unit is in Occupied mode.
• Zone Temp. is less than the active Cooling setpoint.
• The unit is in Heat mode but is not actively heating
OR
• The unit is in AUTO-COOL mode but not actively
cooling and cooling capacity has been OFF for 5
minutes.
Once the above conditions are met, if the supply air
temperature drops to 10°F BELOW the Occupied
Heating Zone Temperature Setpoint, the SA Tempering
function will bring ON one stage of gas or electric heat.
Once SA Tempering is active, heating will be turned
OFF if the Supply Air Temperature rises to 10°F ABOVE
the Active Occupied Zone Heating Setpoint, or the
Zone Temperature rises to the Active Zone Cooling
Setpoint. Also, if the Zone Heat Control function is
calling for 1 or more stages of Heat, Tempering will be
discontinued and the unit will stage additional heating
to meet the current demand.
When an economizer is installed, air tempering is
allowed with ICSTM when the fan system switch is in
the “ON” position with no call for heating. The same
SSttaarrttuupp
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