The following sequence is for the Rinnai tankless water heater. If you can learn this sequence, you will have a good understanding across the board for all brands out there as they all follow a similar sequence of operation.
Step 1. Upon a call for hot water, the water flow sensor will sense water flow and send a pulsing signal to the PCB (control board). The gallons per minute minimum to start the unit start trial for ignition will differ across models and brands, but 0.4 gpm is common