Mat... the line you see could be the return line ... putting the "warm" water to the bottom to be reheated... remember the heater draws from the top when there is no recirc pump there ... did you see a pump anyplace...( they are usually 4" square or smaller) under .... any sink ... there should be an electric line from the switch to the pump ... Good luck on that ... be easy on that back !!!
Crank it up... you can always back it off later !!!