Gas geysers that operate on LPG fuel are replaced with Biogas. In a gas geyser, cold water from a tank flows when a hot water tap is opened, then water flow is detected by a sensor and biogas flow starts through the gas burner. The ignition is needed to start manually by operating a gas lighter. The water typically follows a serpentine pattern through the heat exchanger, absorbing as much heat as possible. The heat exchanger is a device which transfers heat from one medium to another.