Burnham SM-6 manual Sequence of Operation

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 flame simulating condition. If a flame 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 flow 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 flame 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 flame 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 flow 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 fire). After a short time interval, the regulator on the gas valve will permit full flow through the main gas valve (high fire).

Should a loss of flame 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 satisfied.

9.If the high limit setting is reached before the thermostat is satisfied, 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 flame 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 sufficient air flow, 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.

17