Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 68

Staged Gas Heat Board (SCB)
When optional modulating gas heat is used, the SCB board is
installed and controls additional stages of gas heat. The SCB
also provides additional sensors for monitoring of the supply-
air and limit switch temperatures. For units equipped with
modulating gas heat, the SCB provides the 4 to 20 mA signal
to the SC30 board that sets the modulating gas section capacity.
This board is located in the main unit control box.
Timer Relay Control Board (TR1)
The TR1 is used on modulating gas-heat-equipped units only.
It is located in the gas heat section and is used in combination
with the SC30 to provide control of the modulating gas heat
section. The TR1 receives an input from the IGC, initiates a
start-up sequence, powers the SC30, sets the induced-draft mo-
tor speed, and provides the main gas valve high fire input.
When the start-up sequence is complete, the TR1 checks the
input from the SC30 to determine which state to command the
induced-draft motor and main gas valve. See Table 45.
Signal Conditioner Control Board (SC30)
The SC30 is used on modulating gas-heat-equipped units only.
It is located in the gas heat section and is used in combination
with the TR1 to provide control of the modulating gas heat sec-
tion. The SC30 is powered by an output from the TR1. It re-
ceives a capacity input from the SCB, provides a capacity out-
put to the modulating gas valve, and provides an output to the
TR1 to determine which state to command the induced-draft
motor and main gas valve. See Table 45.
Table 45 — TR1 Board LED Indicators
LED
DESIGNATION
ON 24 VAC Supplied to TR1
SR Input received from IGC2, starts timer no. 1
Modulating Gas Valve modulated except during fixed
MR
output delay time
FR IDM2 operates at high speed
CR Modulating Gas Valve operates in high pressure stage
Modulating Gas Control Boards (SC30 and TR1) Logic
All gas modulating units are equipped with one timer relay
board (TR1) and one signal conditioner board (SC30), regard-
less of unit size. The boards provide control for variable heat-
ing output for the gas heat section.
Similar with staged gas heat option, each IGC board is con-
trolled separately. The IGC functions are not affected by the
modulating gas control logic. When a call for gas heat is initi-
ated, W on the IGC board and the timer relay board (TR1) are
energized. The LED on TR1 board will be turned on. See
Table 45 for LED explanation.
When TR1 received an input from the IGC board, the relay
board starts Timer no. 1 or start-up sequence: sets the gas valve
stage and the inducer motor speed, and enables the signal con-
ditioner board SC30. During Timer no. 1, the SC30 board
keeps a fixed heating output. When Timer no. 1 expires, the
modulating gas control boards start Timer no. 2. Throughout
the duration of Timer no. 2, the boards determine which state
to adjust the capacity output to satisfy the heat demand. When
Timer no. 2 expires, the boards receive a capacity input from
the SCB board and continuously modulate the heat output until
the mode selection sensor is satisfied.
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. 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.
RESULT/ACTION
RELOCATE SAT FOR HEATING-LINKAGE APPLICATIONS
If Configuration  HEAT  LAT.M is set to YES, the supply
air temperature thermistor (Temperatures  AIR.T  SAT)
must be relocated downstream of the installed heating device.
This only applies to 2-stage gas or electric heating types (Con-
figuration  HEAT  HT.CF=1 or 2).
Determine a location in the supply duct that will provide a fair-
ly uniform airflow. Typically this would be a minimum of 5
equivalent duct diameters downstream of the unit. Also, care
should be taken to avoid placing the thermistor within a direct
line-of-sight of the heating element to avoid radiant effects.
Run a new 2-wire conductor cable from the control box
through the low voltage conduit into the space inside the build-
ing and route the cable to the new sensor location.
Installing a New Sensor
Procure a duct-mount temperature sensor (Carrier P/N
33ZCSENPAT or equivalent 10,000 ohm at 25°C NTC [nega-
tive temperature coefficient] sensor). Install the sensor through
the side wall of the duct and secure.
Re-Using the Factory SAT Sensor
The factory sensor is attached to the left-hand side of the sup-
ply fan housing. Disconnect the sensor from the factory har-
ness. Fabricate a mounting method to insert the sensor through
the duct wall and secure in place.
Attach the new conductor cable to the sensor leads and termi-
nate in an appropriate junction box. Connect the opposite end
inside the unit control box at the factory leads from MBB J8
terminals 11 and 12 (PNK) leads. Secure the unattached PNK
leads from the factory harness to ensure no accidental contact
with other terminals inside the control box.
TEMPERING MODE
In a vent or cooling mode, the economizer at minimum posi-
tion may send extremely cold outside air down the ductwork of
the building. Therefore it may be necessary to bring heat on to
counter-effect this low supply-air temperature. This is referred
to as the tempering mode.
Setting up the System
The relevant set points for tempering are located at the local
display under Setpoints:
ITEM EXPANSION RANGE UNITS
Tempering
T.PRG
Purge
SASP
Tempering
T.CL in Cool
Offset
Tempering
T.V.OC
Vent
Occ SASP
Tempering
Vent
T.V.UN
Unocc.
SASP
Operation
First, the unit must be in a vent mode, a low cool, or a high
cool HVAC mode to be considered for a tempering mode. Sec-
ondly, the tempering mode is only allowed when the rooftop is
configured for modulating gas, SCR electric heat, or hydronic
heating (Configuration  HEAT   HT.CF=3 or 4). Also, if
OAT is above the chosen tempering set point, tempering will
not be allowed. Additionally, tempering mode is locked out if
any stages of mechanical cooling are present.
If the control is configured for staged gas, modulating gas,
SCR electric heat, or hydronic heating and the control is in a
vent, low cool, or high cool HVAC mode, and the rooftop con-
trol is in a situation where the economizer must maintain a
68
CCN
POINT
–20-80 dF TEMPPURG
5-75 ^F TEMPCOOL
–20-80 dF TEMPVOCC
–20-80 dF TEMPVUNC
DEFAULT
50
5
65
50