The Rinnai 104000042 Water Heater Heat Exchanger Kit transfers heat to incoming water to reach the required temperature for domestic hot water. This assembly uses heat generated by the water heater's burner and transfers it through internal passages as water flows through the exchanger. Installed within the water heater, it connects to the heating and plumbing systems to support the w... https://www.hnkparts.com/104000042-rennai-kit-heat-exchangerr98elsev/4-104000042.html