Evacuated Tube Solar Collectors - Maintenance?

Solar water heaters, the ones with a bank of 12-30 parallel tubes, are very common in this area.  Some are gravity fed and others are pressurized.  Research reveals they may require routine maintenance every couple of years or so.  Does everyone perform routine maintenance on their system?  If so, who do you employ for such a service and at what cost?

When mine was installed about 3 years the hot water it generated was blisteringly hot and if the water was not consistently drawn off it would boil off excess as steam.  The system no longer gets quite as hot and I can't remember the last time it evacuated steam.  Looking for advice/suggestions...

If the incoming water supply is treated, i.e. soft water, then I do not think there is any maintenance required other than wiping down the outside of the vacuum tubes to remove dust/dirt. Our pressurized solar system does not blow off steam during the months of cloud cover like we are experiencing now. Test the Temperature Pressure relief valve to ensure it can open. If it is stuck closed, the tank may build up pressure and leak at a seam due to expansion.

When my solar hot water system was installed the guy suggested it should be serviced every 12 months.  I though he was just trying generate a little extra business for the future.

When old system was removed (not sure how old it was; 7-8 years?) a huge amount of scale came out of the drain hole on the bottom of the collector tank along with some nasty jelly-like substance.  There was no water softener in the house at that time so I added one.

The temperature/pressure release valve works okay.  The system has 30 tubes and a large tank so maybe it's just the shorter days, sun lower in the sky and my wife taking overly long showers!

I might shut off the water supply and loosen the drain plug a little and see if anything unusual comes out...