Water heater 3D wiring model

Suburban gas, direct spark

The water heater is a short chain: your switch, the thermostat, a safety cut-off, then the board that sparks the burner and opens the gas valve.

Parts in this model

On/off switch: Supplies 12 VDC to the water heater. The light on it shows no ignition.
Thermostat: Surface mount against the tank. Opens when the water is hot, closes when it cools.
Eco / hi-limit: Resettable cut-off. Opens if the water gets too hot. It has a manual reset button.
Switch light
Spark
Gas valve: Power off, wires off: the coil should read about 40 ± 4 Ω.

Off. The switch is off, so nothing is powered.
Switch on. 12 V runs through the switch, the thermostat and the ECO to the red wire at the module board. The red light on the switch comes on while the board tries to light the burner.
Trial for ignition. The board sparks the electrode and opens the gas valve. Suburban's guide says the red light should show for 5 to 7 seconds on a normal start.
Burner lit. The board senses flame through the electrode, so the spark stops and the light goes out. The valve stays open.
Water is hot. Around 130°F the thermostat opens and shuts the burner off. When the water cools, it closes and the cycle starts again.