Page A5 Schematic Analysis for Sequence of Operation (AUTOMATIC Spark Ignition System)

Sequence of Operation

1.At the bottom of the schematic you will notice there is 24v coming to the circuit from a transformer.

2.Voltage comes into the 861 pilot switch, but it is a dead end.

The ‘Pilot Switch’ is a heat responsive switch. It is a bimetal that is placed in such a location that it is in the actual pilot flame. It has two sets of contacts, one NO and the other one NC. When it is ‘cold’ the contacts are as shown. But when the pilot is lit, the contacts will change their position based on this ‘hot’ condition.

3.So the voltage keeps on moving to the ‘Warp switch’.

The ‘Warp Switch’ is a heat responsive switch, a bimetal. Unlike the pilot switch, which needs an external source of heat, it has its own internal source, the heater that is shown inside the device. It also has two sets of contacts which will have a ‘hot’ position and a ‘cold’ position.

4.The voltage comes to a contact which is ‘closed.’ So it continues on its way but can only head south and go two places.

4a.Through the warp switch heater and back to 24v.

4b.Through the 861 NC contacts and through the igniter transformer primary and back to 24v.

Remember, this system has a ‘standing pilot.’ So gas is flowing to the pilot assembly. The ignition transformer primary is energized, so the secondary will now produce its voltage of several thousand volts. This will ignite the gas at the pilot assembly. After a period of time the 861 pilot switch will get ‘hot’ and its contacts will change position.

5.Pilot heats 861 pilot switch and its contacts change position. This one closes. This one opens.

6.Circuit is broken to the ignition transformer, so spark ignitor ‘sparking’ stops. BUT circuit is maintained to Warp Switch.

7.The Warp switch’s heater has been energized long enough now for the bimetal warp switch contacts to change position. This one closes. This one opens.

8.Voltage can now go through the ‘closed’ contacts to the MV (main gas valve) and up to the thermostat.

9.When the thermostat calls for heat and its contacts close, the circuit will complete and the main valve will open allowing gas to the main burners.

This completes the sequence of operation for the automatic spark ignition system.