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Service Reference Manual SEQUENCE OF OPERATIONS
2-23
SRM-HW/HWC 2/99
Simplified Sequence - HW (w/Fenwal 2461D Board)
Refer to Figure 2-4
CALL FOR HEAT
1. A call for heat closes the circuit between R and W
on the thermostat. This sends a 24-volt signal to the
white wire on the unit.
2. The 24-volt signal energizes terminal TH on the
ignition control and the W terminal on the blower
control board.
3. With the TH terminal energized, the ignition control
closes an internal relay. This sends line voltage
from terminal IND to the induced draft blower,
starting the inducer.
4. After a pre-purge cycle of approximately 30 sec-
onds, the ignition control makes a trial for ignition.
Terminal V1 energizes at this time, opening the gas
valve. The ignition control initiates a spark at the
burner to light the main burners.
5. The trial for ignition lasts seven seconds, at which
time the ignition control stops sparking. The ignition
control looks at the flame sense to determine if the
main burners have ignited. If flame sense is not
proven, the ignition control goes through another
pre-purge cycle (approximately 30 seconds) and
makes another trial for ignition. It does this three
times. If the three trials for ignition fail, the ignition
control locks out for approximately 60 minutes.
After 60 minutes, the ignition control again makes
three trials for ignition. This continues as long as
there is a call for heat from the thermostat.
6. When W energizes, the blower control board starts
a countdown to fan “on”. Approximately 60 seconds
after the thermostat initiates a call for heat, the
circulating air blower starts.
7. The unit continues to operate as long as there is a
call for heat from the thermostat.
8. When the heat call is satisfied, the circuit between R
and W is interrupted. With the white wire de-
energized, the ignition control interrupts the 24-volt
signal to the gas valve. This closes the gas valve
immediately and opens the internal relay that de-
energizes terminal IND. The induced draft blower
shut downs several seconds later.
9. The W terminal on the blower control board de-
energizes. This causes the blower control board to
start a countdown to blower “off”. Approximately 90
seconds later, the circulating air blower shuts down.
This completes the heating cycle.
Detailed Sequence follows