The unit is designed not to turn the heater on unless it is plugged in to a properly wired 3-hole receptacle. On my unit there is a 4" plastic tube attached to the output port on the pump that helps to push the water around the tube and ensures the heated water is properly circulated so the water temperature is even throughout the tub. If you are doing some electrical work in your darkroom to fix the grounding on your plug, you should take the opportunity to install a ground fault circuit interrupter if you don't already have one.