Gas Heat System Sequence of Operation

Heat Demand Initiated

• Triggered by user command to Preheat or Start Cook Cycle.

• GMU (Gas Management Unit) begins heat sequence.

Safety Circuit Check

• GMU verifies the following are closed:

  • Electronic Fuse (U9) – Must be closed to proceed.

  • High Limit Circuit – Must be below trip point (430°F).

  • Combustion Fan Pressure Switch – Must be open before fan starts.

Tech Tip: If the fan doesn’t engage after step 3, check the high-limit switch.

Combustion Fan Activation

• GMU sends 24V DC signal to Blower Control Board (BCB).

• BCB closes and sends line voltage to combustion fan.

Expected Behavior: You should hear the fan motor engage.

Pressure Switch Verification

• Fan airflow closes the normally open pressure switch.

• GMU confirms switch closure to proceed.

Troubleshooting Cue:

• E-20A: Pressure switch failed closed.

• E-20B: Pressure switch failed open.

Gas Supply Enabled

• Non-CE Units: Gas flows directly to combination valve.

• CE Units: GMU sends 24V AC to Safety Shutoff Valve, allowing gas to reach pilot valve.

Pilot Ignition

• GMU sends 24V AC to Ignition Module (TH terminal).

• Ignition Module:

  • Sends PV signal to GMU.

  • Sends high-voltage spark to pilot electrode.

• GMU sends PV signal to pilot solenoid, opening gas flow to pilot.

• Pilot flame ignites and is verified by flame sense rod.

Expected Behavior: You may hear a clicking spark followed by a soft flame ignition.

Main Burner Ignition

• With pilot flame confirmed, Ignition Module sends MV signal to GMU.

• GMU sends MV signal to main solenoid, opening gas flow to burners.

• Burners ignite and heat exchanger tubes transfer heat to oil.

Expected Behavior: You should hear a steady burner flame.

Flame Rollout Monitoring

• J-type thermocouple above burner entrance monitors flame behavior.

• If flame rollout is detected:

  • GMU triggers E-20R error.

  • Gas heat system shuts down until error is acknowledged.

Key Spec and Notes

• Full Vat Output: 75,000 BTU/hr (gross), 19.8 kW (net)

• Split Vat Output: 37,500 BTU/hr (gross), 9.9 kW (net)

• High Limit Trip Point: 430°F (221°C)

• High Limit Reset Point: 374°F (190°C)

• Pressure Switch: Normally open, closes with fan airflow

• Flame Sense: Requires ≥1 micro-amp DC current to confirm pilot flame