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Typical Gas Furnace Control Circuit
(Modulating Burner, Mixed Air Intake)
Sequence of Operation: When the rooftop unit is energized, 120 volt power is supplied through the system on-off switch S1 to OBA3 contacts. Upon a call for
heat, the control system will close OBA3, thus energizing relay R20. 120 Volt power is furnished through the system on-off switch S1, through relay R20 closed
contacts, through the burner on-off switch S3, through the high limit control FLC and through the optional automatic reset low pressure switch LP5 and the
optional manual reset high gas pressure switch HP5, to power terminal 6 on the flame safeguard control FSG. The flame safeguard then energizes its terminal
4, which powers the burner combustion air blower motor BM. Blower operation is sensed by the air switch AS which makes terminal 6 to 7. After a 90-second
prepurge period, terminal 8 (pilot gas valve GV1) and terminal 10 (ignition transformer IT) will be energized. The pilot flame will ignite and be detected by the
flame safeguard through the flame rod FD. Upon detection of pilot flame, terminal 10 (ignition transformer IT) will be de-energized and terminal 9 (main gas
valves GV2 and GV3) will be energized and the main flame will come on. Also, the flame safeguard contains LEDs (lower left corner) that will glow to indicate
operation.
Low fire start is provided by relay R23. The relay drives the gas valve actuator VM1 to the minimum firing rate position whenever the flame is not on and holds
it there until the flame has lit and been proven.
Whenever the burner is in operation its firing rate will be determined by the “floating” gas valve actuator VM1. This actuator positions the butterfly gas valve
and combustion air damper and can set the firing rate between 33% and 100% of normal rate. When the main control system closes OBA5, the gas valve
actuator will reposition toward a higher firing rate until either OBA5 opens or the actuator reaches its maximum position. When the main control system closes
OBA4, the actuator will reposition toward a lower firing rate. If neither OBA4 or OBA5 are closed, the actuator will remain at its present position.
In the event the pilot fails to ignite or the flame safeguard fails to detect its flame within 10 seconds, terminal 4, 8, 9 and 10 will be de-energized, thus de-
energizing the burner. The flame safeguard would then be on safety lockout and would require manual resetting. The heat alarm relay R24 would then be
energized and would then energize the remote “heat fail” indicator light and send a fail signal to the MicroTech input board ADI.
If the unit overheats, the high limit control FLC will cycle the burner limiting furnace temperature to the limit control set point.
Piping Diagram
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