Greenheck VSU Installation, Operation And Maintenance Manual page 34

Reference
Typical Gas Train Layout with Direct Spark Ignition
This is a typical gas train. The gas train in your unit may be different.
Burner Differential
Pressure Sensor
(Airflow Switch)
to Burner
Modulating Combination Shut-Off Low Gas Pressure
Valve Valve/Regulator Switch (optional)
Burner Manual
Shut-Off Valve
Low Gas Pressure
Test Port
High Gas Pressure
Test Port
Gas Connection
34 Model VSU Make-Up Air
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