The indirect unit has a temperature sensor or aquastat that goes into the side of the tank to sense the temperature. This usually only needs to touch the bottom of the hole or (well) for it to sense the temperature of the tank. If the light is solid green, that is an indication that the tank is at the set temperature. If the light is flashing green, that means it is calling for more heat. If it doesn't satisfy (shut off at the correct temperature), please have a qualified person check the system out.