I have a Stiebel Eltron solar hot water system approx 15 years old for which I've been doing most maintenance the past 7 or so years. The system slowly loses pressure - used to be over a period of a year or longer but now more like 3 months. When it does it eventually blows off steam on the pressure valve at the top of the collectors. I'm not certain whether there is some pressure blow off earlier in the pressure cycle nor whether the system is always sensing temps and pumping to avoid a high temperature blowoff when the pump should be operating. The sensors occasionally read out very high temps when I look at them . Also the programming for max tank temperature and releasing excess heat from tank might be off; I sometimes find the sensor temps at collector as very high well after dark. So I can't say for sure whether the problem might be in the pipes & pressure valve, or in the sensors & electronics. Any suggestions what to do and where the likely failure mode is? Do the pressure valves need to be replaced or reset? The pipes for this installation used quick sealing joints with a tool to install them , not solder; might the seals after 15 years have reached end of life? Any gotchas around the Stiebel electronic controller ? Thanks for any pointers on tracking this down.