Carrier WeatherMaker 48TC**04, Product Data

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SEQUENCE OF OPERATION
General
The sequence below describes the sequence of operation
for an electromechanical unit with and without a factory
installed EconoMi$er IV and X (called “economizer” in
this sequence). For information regarding a direct digital
controller, see the start--up, operations, and
troubleshooting manual for the applicable controller.
Electromechanical units with no economizer
Cooling (Single speed indoor fan motor) —
When the thermostat calls for cooling, terminals G and Y1
are energized. As a result, the indoor fan contactor (IFC)
and the compressor contactor (C1) are energized, causing
the indoor fan motor (IFM), compressor #1, and outdoor
fan to start. If the unit has 2 stages of cooling, the
thermostat will additionally energize Y2. The Y2 signal
will energize compressor contactor #2 (C2), causing
compressor #2 to start. Regardless of the number of
stages, the outdoor fan motor runs continuously while unit
is cooling. When SAV system is utilized, indoor fan motor
runs at design CFM (full speed) during the heating
operation.
Cooling (2--speed indoor fan motor) —
Per ASHRAE 90.1 2010 standard section 6.4.3.10.b,
during the first stage of cooling operation the VFD will
adjust the fan motor to provide 2/3rd of the total cfm
established for the unit. When a call for the second stage
of cooling is required, the VFD will allow the total cfm
for the unit established (100%).
Heating (Single or 2--speed indoor fan motor) —
NOTE
: WeatherMaker (48TC) units have either 1 or 2
stages of gas heat. When SAV system is utilized, indoor
fan motor runs at design cfm (full speed) during the
heating operation.
When the thermostat calls for heating, power is sent to W
on the Integrated Gas Controller (IGC) board. An LED
(light--emitting diode) on the IGC board turns on and
remains on during normal operation. A check is made to
ensure that the rollout switch and limit switch are closed.
If the check was successful, the induced--draft motor is
energized, and when its speed is satisfactory, as proven by
the “hall effect” sensor, the ignition activation period
begins. The burners will ignite within 5 seconds. If the
burners do not light, there is a 22--second delay before
another 5--second attempt. This sequence is repeated for
15 minutes or until the burners light. If, after the 15
minutes, the burners still have not lit, heating is locked
out. To reset the control, break 24--v power to the
thermostat.
When ignition occurs, the IGC board will continue to
monitor the condition of the rollout switch, the limit
switches, the “hall effect” sensor, as well as the flame
sensor. 45 seconds after ignition occurs, assuming the unit
is controlled through a room thermostat set for fan auto,
the indoor fan motor will energize (and the outdoor air
dampers will open to their minimum position). If, for
some reason, the over--temperature limit opens prior to the
start of the indoor fan blower, the unit will shorten the
45--second delay to 5 seconds less than the time from
initiation of heat to when the limit tripped. Gas will not be
interrupted to the burners and heating will continue. Once
the fan--on delay has been modified, it will not change
back to 45 seconds until power is reset to the control.
On units with 2 stages of heat, when additional heat is
required, W2 closes and initiates power to the second
stage of the main gas valve. When the thermostat is
satisfied, W1 and W2 open and the gas valve closes,
interrupting the flow of gas to the main burners.
If the call for W1 lasted less than 1 minute, the heating
cycle will not terminate until 1 minute after W1 became
active. If the unit is controlled through a room thermostat
set for fan auto, the indoor fan motor will continue to
operate for an additional 45 seconds then stop. If the
over--temperature limit opens after the indoor motor is
stopped, but within 10 minutes of W1 becoming inactive,
on the next cycle the time will be extended by 15 seconds.
The maximum delay is 3 minutes. Once modified, the fan
off delay will not change back to 45 seconds unless power
is reset to the control. A LED indicator is provided on the
IGC to monitor operation.
Electromechanical units with an economizer
Cooling —
When free cooling is not available, the compressors will
be controlled by the zone thermostat. When free cooling is
available, the outdoor air damper is modulated by the
EconoMi$er IV and X control to provide a 50
_
F (10
_
C) to
55
_
F (13
_
C) mixed air temperature into the zone. As the
mixed air temperature fluctuates above 55
_
F (13
_
C)or
below 50
_
F (10
_
C) dampers will be modulated (open or
close) to bring the mixed air temperature back within
control. If mechanical cooling is utilized with free
cooling, the outdoor air damper will maintain its current
position at the time the compressor is started. If the
increase in cooling capacity causes the mixed air
temperature to drop below 45
_
F (7
_
C), then the outdoor
air damper position will be decreased to the minimum
position. If the mixed air temperature continues to fall, the
outdoor air damper will close. Control returns to normal
once the mixed air temperature rises above 48
_
F (9
_
C).
The power exhaust fans will be energized and
de--energized, if installed, as the outdoor air damper opens
and closes.
If field installed accessory CO
2
sensors are connected to
the EconoMi$er IV and X control, a demand controlled
ventilation strategy will begin to operate. As the CO
2
level in the zone increases above the CO
2
setpoint, the
minimum position of the damper will be increased
proportionally. As the CO
2
level decreases because of the
increase in fresh air, the outdoor air damper will be
proportionally closed. For EconoMi$er IV and X
operation, there must be a thermostat call for the fan (G).