Provided you actually have water available to the unit (ie any inline valves in your water pipework are open) then most likely problem is the flow control solenoid. This is at the bas of the unit left of the inlet spigot. When you turn your shower on for flow the solenoid should open. If it doesn't then no water can get to the heater. Check connections are ok after isolating the power. If connections ok then solenoid is faulty. I am just passing through this website so may not be back for some time. Good luck
Quantumfoam, August 2008
