Our heater had to be emptied of some air that prevented the warm water to flow inside. I did not have the right key at hand so I quickly designed one in SketchUp and printed it. It's a tight fit, but works. I suppose that most european heaters should be compatible.Printed it using the following settings on my PrintrBot Simple Metal: Layer height: 0.2 Shell thickness: 1.2 (0.4mm nozzle) Bottom/Top: 0.4mm Fill density: 100% (it's so tiny, it does not make any difference) Adhesion: Used a raft in Cura You might need to rotate the key by 180 degrees. The handle should be on top. You don't need any supports as the inside has a slight angle that should easily print.