8 Maintenance (continued)
9.24VAC is supplied to the intermittent alarm field safety contacts, B1 and B2.
10.24VAC is supplied to the water flow switch.
11.24VAC is supplied to the TH terminals on the ignition module.
12.As power is applied to the TH terminals on the ignition module, 120VAC is switched from the F2 terminal to start the combustion air fan at full speed on Models 497 - 747 and at low speed on Models 987 - 2067.
13.24VAC is supplied to the C terminal of the air pressure switch. As the low air switch makes, power is supplied to the ignition module.
14.24VAC is then supplied to the PS terminals on the ignition module as the air pressure switch makes to prove fan operation.
15.The combustion air fan operates for the
16.At the end of the
17.The hot surface igniter proves 1800°F (982°C) ignition temperature by current draw through the ignition module.
18.The ignition module supplies voltage to the MV terminals on to the gas valve(s).
19.The first stage gas valve(s) opens and supplies gas to the orifice and burner inlet to
20.The gas/air mixture is forced into the burner and out of the burner ports under pressure.
21.The hot surface igniter lights the gas/air mixture and then serves as a flame sensor to prove main burner flame by rectification.
22.Burners are now firing for approximately 50% of the heater’s input.
23.24VAC is supplied from the second stage of the electronic temperature control across the 2C and 2NO terminals on the terminal strip on to a control to a relay.
24.The control relay switches the fan on Models 987 - 2067 to high speed and energizes the gas valves for the remaining 50% of burner input.
25.The remaining gas valves open and supply gas to the orifice inlet to
26.The gas/air mixture is forced out of the burner ports and light by carryover from the burners currently firing.
27.The heater is now firing at full rated input.
Installation & Service Manual
Heat transfer process
28.Heated products of combustion pass over the heat exchanger transferring heat to the water.
29. The rate of flue product movement is controlled by “V” baffles on the heat exchanger to maximize heat transfer.
30.Flue products pass into the flue collector and are exhausted from the unit.
End of sequence
31.Set point for the high fire on the electronic temperature control is satisfied.
32.Power to all valves is turned OFF on F9 systems and power to the high fire gas valves is turned OFF on M9 systems.
33.The water temperature continues to rise and the set point for low fire is satisfied (M9).
34.Power to the low fire gas valves is turned OFF (M9).
35.The combustion air fan runs for a 30 second post purge timing and turns OFF.
36.The appliance is now in a Standby Mode waiting for the next “call for heat”.
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