Startup
42
RT-SVX34F-EN
Mechanical Cooling without an
Economizer (CV and SZ VAV)
Mechanical cooling is used to maintain the zone
temperature. The RTRM is designed to limit the
compressor cycle rates to within 10 cycles per hour based
on the minimum compressor “on” and “off” times.
It stages the mechanical cooling to control the zone
temperature to /- 2°F of the sensor setpoint at the
sensed location.
Table 21
lists the compressor stepping
sequence.
Zone Temperature - Occupied Cooling (CV
and SZ VAV)
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 SZ VAV)
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:
1.
The supply fan is ON.
2. The unit is in Occupied mode.
3. Zone Temp. is less than the active Cooling setpoint.
4. The unit is in Heat mode but is not actively heating OR
5. 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 conditions
must be met as described above for entering and leaving
Tempering operation.
Table 21.
Compressor staging with lead/lag disabled
“ON” “OFF”
Unit Model Step 1 Step 2 Step 3 Step 3 Step 2 Step 1
27.5 - 35
CPR 1
1
CPR 1, 2 N/A
N/A
CPR 1, 2 CPR 1
1
40 CPR
1
2
CPR 2
3
CPR 1, 2 CPR 1, 2 CPR 2
3
CPR 1
2
50
CPR 1
2
CPR
2,3
4
CPR 1,
2, 3
CPR 1,
2, 3
CPR 2,
3
4
CPR 1
2
Notes:
1.
Single circuit, dual manifolded compressors
2.
Number one refrigeration circuit, Standalone compressor, is “On”.
3.
First stage is off. Number two refrigeration circuit, standalone
compressor, is “On”
4.
First Stage is “Off”, Number two refrigeration circuit, manifolded
compressor pair operating simultaneously, is “On”.
Figure 39.
Compressors
