Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 67

The electric heat outputs are located on the MBB. The analog
output that sets the SCR electric heat section capacity is locat-
ed on the SCB.
Limit Switch Temperature Monitoring (LIM.M)
Variable air volume applications in the low heat or tempering
mode can experience low airflow and as a result it is possible for
nuisance trips of the gas heat limit switch, thereby shutting off
all gas stages. In order to achieve consistent heating in a temper-
ing mode, a thermistor (Temperatures  AIR.T  S.G.LS) is
placed next to the limit switch and monitored for overheating. In
order to control a tempering application where the limit switch
temperature has risen above either the upper or lower configura-
tion parameters (SW.L.T, SW.H.T), the staged gas control will
respond by clamping or dropping gas stages. See Table 43.
Table 43 — SCR Electric Heat Control Steps
RELAY OUTPUT
STAGE
Heat1
0 OFF
1 ON
If the Limit Switch Monitoring configuration parameter
(LIM.M) is set to YES, all the modes will be monitored. If set
to NO, then only LAT Cutoff mode and Capacity Clamp mode
for RISE will be monitored.
If S.G.LS rises above SW.L.T or if (LAT – LAT last time
through the capacity calculation) is greater than (RISE) de-
grees F per second, the control will not allow the capacity rou-
tine to add stages and will turn on the Capacity Clamp mode.
If S.G.LS rises above SW.H.T, the control will run the capacity
routine immediately and drop all heat stages and will turn on
the Limiting mode.
If S.G.LS falls below SW.L.T, the control will turn off both Ca-
pacity Clamp mode and Limiting mode with one exception. If
(LAT – LAT last time through the capacity calculation) is
greater than "RISE" degrees F per second, the control will stay
in the Capacity Clamp mode.
If control is in the Limiting mode and then S.G.LS falls below
SW.L.T, and LAT is not rising quickly, the control will run the
capacity calculation routine immediately and allow a full stage
to come back on if desired this first time through upon recovery.
This will effectively override the "max capacity stage" clamp.
In addition to the above checks, it is also possible at low cfm for
the supply-air temperature to rise and fall radically between ca-
pacity calculations, thereby impacting the limit switch tempera-
ture. In the case where supply-air temperature (LAT) rises above
the control point (HT.C.P) + the cutoff point (LAT.L), the con-
trol will run the capacity calculation routine immediately and
drop a stage of heat. Thereafter, every time the capacity calcula-
tion routine runs, provided the control is still in the LAT cutoff
mode condition, a stage will drop each time through. Falling
back below the cutoff point will turn off the LAT cutoff mode.
CONTROL BOARD INFORMATION
Integrated Gas Control (IGC)
One IGC is provided with each bank of gas heat exchangers; 2,
3, 4, or 5 IGCs are used depending on unit size and heat capac-
ity. The IGC controls the direct spark ignition system and mon-
itors the rollout switch, limit switches, and induced-draft motor
Hall Effect switch. For units equipped with Modulating Gas
heat, the IGC in the Modulating Gas section uses a Pressure
Switch in place of the Hall Effect sensor. The IGC is equipped
with an LED (light-emitting diode) for diagnostics. See
Table 44.
CAPACITY (%)
Heat2 Min. Max.
OFF 0 0
ON 0 100
Table 44 — IGC LED Indicators
ERROR CODE
Normal Operation
Hardware Failure
Fan Off/On Delay Modified
Limit Switch Fault
Fame Sense Fault
Five Consecutive Limit Switch Faults
Ignition Lockout Fault
Ignition Switch Fault
Rollout Switch Fault
Internal Control Fault
Software Lockout
NOTES:
1. There is a 3-second pause between error code displays.
2. If more than one error code exists, all applicable error codes will be
displayed in numerical sequence.
3. Error codes on the IGC will be lost if power to the unit is interrupted.
Integrated Gas Control Board Logic
This board provides control for the ignition system for the gas
heat sections.
When a call for gas heat is initiated, power is sent to W on the
IGC boards. For standard 2-stage heat, all boards are wired in
parallel. For modulating gas heat, each board is controlled sep-
arately. When energized, an LED on the IGC board will be
turned on. See Table 44 for LED explanations.
Each board will ensure that the rollout switch and limit switch
are closed. The induced-draft motor is then energized. For
units equipped with 2-stage gas heat the speed of the motor is
proven with a Hall Effect sensor on the motor. For units
equipped with modulating gas heat the motor function is prov-
en with a pressure switch. When the motor speed or function is
proven, the ignition activation period begins. The burners ig-
nite within 5 seconds. If the burners do not light, there is a 22-
second delay before another 5-second attempt is made. If the
burners still do not light, this sequence is repeated for 15 min-
utes. After 15 minutes have elapsed and the burners have not
ignited, then heating is locked out. The control will reset when
the request for W (heat) is temporarily removed.
When ignition occurs, the IGC board will continue to monitor
the condition of the rollout switch, limit switches, Hall Effect
sensor or pressure switch, and the flame sensor. Forty-five sec-
onds after ignition has occurred, the IGC will request that the
indoor fan be turned on.
The IGC fan output (IFO) is connected to the indoor fan input
on the MBB, which will indicate to the controls that the indoor
fan should be turned on (if not already on). If, for some reason,
the overtemperature limit switch trips prior to the start of the
indoor fan blower, on the next attempt the 45-second delay will
be shortened by 5 seconds. Gas will not be interrupted to the
burners and heating will continue. Once modified, the fan de-
lay will not change back to 45 seconds unless power is reset to
the control.
The IGC boards only control the first stage of gas heat on each
gas valve. The second stages are controlled directly from the
MBB board for staged gas. For units equipped with modulating
gas heat, the second stage is controlled from the timer relay
board (TR1). The IGC board has a minimum on-time of
1 minute.
In modes such as Service Test where long minimum on times
are not enforced, the 1-minute timer on the IGC will still be
followed and the gas will remain on for a minimum of
1 minute.
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LED INDICATION
On
Off
1 Flash
2 Flashes
3 Flashes
4 Flashes
5 Flashes
6 Flashes
7 Flashes
8 Flashes
9 Flashes