When the hot water tap is turned on, the water enters the heater. A flow sensor detects the water flow and the computer automatically ignites the burner. The water circulates through the heat exchanger and the heat exchanger instantly heats the water at the designated temperature (this takes approximately six seconds). When the hot water tap is turned off, the unit shuts down automatically. This process may vary by different brands of tankless water heaters. Call 1st Choice Plumbing at (770)596-5064