Service Manual
Sequence of Operation
1. Thermostat Calls for Heat.
Prior to energizing the blower, the gas control checks the safetycircuittoinsure the circuit isopen. Normal switch
positions in the safetycircuit are as follows:
a) Pressure switch normally open.
b) Blower temperature switch normallyclosed.
If the safetycircuit is closed, the gas control waits 4 seconds, then the gas control LED flashes 2 times followedby
a3second pause. The gas control waits 2 minutes then, the blower runs for 30 seconds. This cycle repeats until
the safetycircuitopens.
Power Up Sequence
1. Start Up.
Uponpower up, the control runs a safe-start check withatypical start-up delay of 1-5 seconds.
2. Flammable Vapor Check.
To assure no outputs are energized if the “Flammable Vapor Sensor” is out of range, the control will test the
“Flammable Vapor Sensor” for proper operating range. If the “Flammable Vapor Sensor” is within range the
control resumes normal operationwith no perceptible delay. If the “Flammable Vapor Sensor” is out of range, the
control LED immediately flashes 7 times followedbya3second pause.
Normal Heating Sequence
2. Blower Energizes.
4. Differential Pressure Switch Proves Blower/Vent System Operation.
a) If the pressure switch does not close within 30 seconds, the control LED Flashes 3 times followedbya3second
pause.
b) The blower runs for 30 seconds every2minutes trying to get the pressure switchorblower temperature
switchtoclose. This cycle repeats as long as there isacall for heat.
3. Blower Pre-purge Period(15 seconds).
5. Trial for Pilot Ignition(90 seconds).
a) The gas control lights the pilot by activating the spark igniter and gas flow to the pilot burner.
b) If flame is not sensedwithin 90 seconds, the igniter and gas flow are deactivated. The blower will post purgeand
the gas control LED flashes 6 times followedbya3second pause.
6.Main Burner Ignition
After pilot flame issensed, the gas control activates the main valve for main burner ignition. The gas control will
ignore flame and pressure switchsignals for 3 seconds allowing for the main burner to stabilize.
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UPDX Series
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