Tom,
Now I know which one you mean. Mine was the metal color and was made in Germany. It had tiny mechanical relays inside!
Mine worked perfectly for a couple years. Someone, maybe at the factory, put it in the clips upside down. One day a fitting in the back started leaking and the compartment filled with a water mist. I didn't catch it for a couple days. With the box upside down, water got in it and the relays got "sticky".
At the time, the German price was $900 for a replacement. Maybe the "black" ones are cheaper. That was the day I decided that I was not sure I liked having to maintain a HydroHot. It was a love/hate relationship for about another 10 years!
Before it was over, it was that brain box, two burner tubes, three stir pumps, a burner bearing, one electric element, one electric element relay, one over temp switch, one mixing valve, two rebuilds of the plastic plumbing in the back, eight or ten nozzles, as many filters, a couple boiler caps and a fluid level switch. The zone pumps, fuel pump and circuit board never failed. A few of the screws in the connectors holding the wires worked loose a couple times.
We stayed warm and most of the showers were hot!