290699-YIM-B-0607
Unitary Products Group
43
a greater temperature swing in the conditioned space.
Reducing the value below the correct setpoint will give
shorter “ON” cycles and may result in the lowering of
the temperature within the conditioned space. Refer to
Table 28 for the required heat anticipator setting.
GAS HEATING SEQUENCE OF OPERATION
When there is a W1 call for heat, the heat relay (RW1)
is energized by the unit control board (UCB). The
RW1-1 contacts immediately close energizing the igni-
tion control board (ICB). The ICB checks the state of
the flame sense circuit, the roll out switch, the centrifu-
gal switch and the primary / auxiliary temperature limit
switch circuit. If they are in the expected state, then the
ICB energizes the draft motor and verifies that the cen-
trifugal switch located on the end of the draft motor
closes. After the centrifugal switch closes, a 15 second
heat exchanger purging period is completed. After this
purging period, the ICB will simultaneously energize
the pilot gas valve and the ignition coil. Once the flame
sensor senses a pilot flame is present, the ignition coil
is de-energized. The ICB checks for pilot flame stability
and once the ICB is satisfied that the pilot flame is sta-
ble, the main gas valve is energized by the ICB. The
UCB will energize the indoor blower after a 45 second
delay from the call for heat. The ICB and UCB both
monitor the furnace safety devices during the furnace
operation. When the call for heat is satisfied, the ICB
closes the pilot and main gas valves and performs a 30
second purging of the heat exchanger by continuing
the operation of the draft motor. The UCB continues
the operation of the indoor blower for a configurable
amount of time after the call for heat is satisfied.
GAS HEAT OPERATION ERRORS
During furnace operation, the ICB monitors the flame
sense circuit, the centrifugal switch, the primary limit
switch and the roll out switch. If a signal from any of the
inputs moves to a fault state, then the ICB immediately
closes the pilot and main gas valves. The ICB will
determine the device that is signaling a fault and flash
a code for that device. A primary limit trip, centrifugal
switch trip or flame sense fault triggers a temporary
lock out. An auxiliary limit or a roll out switch trip
requires intervention to reset the ICB. The UCB also
monitors the primary limit and gas valve.
TEMPERATURE LIMITS
The primary limit is located such that the tempera-
ture sensitive switch can sense the temperature of
the heat exchanger tubes. On single stage models
the limit is mounted to the condenser partition panel.
On two stage models the limit is mounted just above
the inlet of the heat exchanger tubes on the right
side. If a primary limit (LS) fault occurs (the primary
limit opens due to excessive heat exchanger temper-
ature), then the ICB will flash the appropriate code
(Table 33) and monitor the primary limit. The UCB
will energize the indoor blower and the ICB will ener-
gize the draft motor while the primary limit is open.
When the primary limit closes and the call for heat
still exists, the ICB will start the ignition sequence
over and the UCB will de-energize the blower for 45
seconds. However, the auxiliary limit is in series with
the primary limit and it takes first control. If the
excessive heat has been high enough to cause the
auxiliary limit (AUX) to open, then the ICB will flash
the primary limit code but the furnace will not retry
TABLE 28: ELECTRIC HEAT ANTICIPATOR
SETPOINTS
HEATER
KW
VOLTAGE
SETTING, AMPS
TH1
TH2
5
230-3-60
0.35
-
7
0.35
-
10
0.35
-
15
0.35
0.19
20
0.35
0.38
30
0.35
0.38
7
460-3-60
0.35
-
10
0.35
-
15
0.35
-
20
0.37
0.29
30
0.37
0.29
10
575-3-60
0.35
-
15
0.35
-
20
0.37
0.29
30
0.37
0.29
FIGURE 18 - GAS VALVE PIPING
GAS
VALVE
GAS MAIN
MAIN VALVE
TO MAIN
BURNER
REDUNDANT
VALVE
TO PILOT BURNER
