Burnham SM-6 Installation, Operating And Service Instructions page 17

SEQUENCE OF OPERATION
1. When the thermostat calls for heat, relay coil (1M)
is energized closing two sets of NO contacts. One
set (1M2) energizes the 24 volt limit circuit and the
other set (1M1) completes the 120 volt circulator
circuit.
2. With the thermal cutout switch (on the burner
access panel), the high limit switch, and the suction
pressure switch in their normally closed positions,
the fan relay coil (2M) is energized closing two sets
of NO contacts. One set (2M1) completes the 120
volt fan circuit and the other set (2M2) energizes the
24V suction pressure "check" circuit.
3. With the fan operating, the suction pressure switch
closes its NO contacts. The "check" circuit contacts
(2M2) keep power on the fan relay (2M) during the
heating cycle.
4. When the suction pressure switch proves fan
operation the ignition module is energized,
beginning a 30-50 second prepurge. During
prepurge the module performs a safe-start check
that tests the internal components of the module for
a fl ame simulating condition. If a fl ame simulating
condition is present in the module, the heating
system will not start.
5. After prepurge, the module energizes the pilot gas
valve operator. The pilot gas valve opens, allowing
gas to fl ow to the pilot burner. At the same time, the
electronic spark generator in the module generates a
spark at the ignitor-sensor to light the pilot.
6. If the pilot does not light within 90 seconds, or the
pilot fl ame current is not at least 1.0 milliamps and
steady, the module will not energize the main gas
valve and the main burner will not light. Then the
module goes into safety lockout, de-energizing the
pilot gas valve operator causing the pilot gas valve
to close. Five to six minutes after shutdown, the
Ignition Module restarts the ignition sequence.
7. When the sensor senses pilot fl ame during the 90
second trial for ignition the module will shut off the
spark generator, reset the safety lockout timer, and
energize the main gas valve operator. The main gas
valve opens allowing gas to fl ow to the main burners
where it is ignited by the pilot burner. Since the
main gas valve is a step-opening valve, a limited
amount of gas will be admitted to main burners
for ignition (low fi re). After a short time interval,
the regulator on the gas valve will permit full fl ow
through the main gas valve (high fi re).
Should a loss of fl ame occur, the main valve closes
and the spark reoccurs within 0.8 second. The
ignition module has an internal 100% lockout
function to completely shutdown the system should
the pilot gas fail to ignite with approx. 90 seconds.
Five to six minutes after shutdown, the Ignition
Module restarts the ignition sequence. The ignition
trial, shutdown, and wait sequence continues until
either the pilot lights or the Thermostat is set below
room temperature (to end the call for heat). The
ignition sequence can be reset by setting down the
Thermostat for one minute.
8. Burners, circulator and fan will continue to operate
until the thermostat is satisfi ed.
9. If the high limit setting is reached before the
thermostat is satisfi ed, the switch in the high limit
will open to de-energize the ignition module and
fan relay coil (2M), causing the gas valves to close
and the fan to stop. (The circulator will continue
to operate as long as the thermostat is calling for
heat). When the boiler water temperature drops to
a point where the high limit switch closes, the fan
will restart (via 2M) and the ignition module will be
energized to repeat the sequence in Steps 4 through
7 above.
10. If for any reason (such as fl ame roll out into the
vestibule) the thermal cutoff switch located on the
burner access panel is subjected to temperatures
above its setting the TCO switch will open to
de-energize the ignition module and fan relay coil
(2M), causing the gas valves to close and the fan to
stop. (The circulator will continue to operate as long
as the thermostat is calling for heat). Since the TCO
switch is a one-time fusible link, the reason for over-
heating must be determined and the switch replaced
in order for the boiler to function again.
11. In the event the fan is inoperative or fails to provide
suffi cient air fl ow, the suction pressure switch
connected at the inlet of the fan will not activate, the
NO contacts will not close, the ignition module will
not be energized, and hence the gas valves cannot be
opened.
12. If the contacts in the suction pressure switch were
to weld together during the heating cycle (COM to
NO), the ignition module could not be energized
on the next call for heat due to the fact the fan relay
coil (2M) could not be energized through either the
suction pressure switch or the now open "check
circuit" contacts.
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