[Greenbuilding] Solar thermal hot water messed up

[Ktot g]

Being a holiday weekend, I cannot reach my installer for a few days, thus my question here.

I have a solar thermal hot water system with (of course) panels, a large tank, various pumps, and a Rinnai for backup. 

The Rinnai is not coming on as the meter I adjust to cause it to turn on is set at 100 degrees and the temp is staying above that even though it’s pretty cloudy out. So I’m thinking the Rinnai is not the problem.

I can get very hot water; when the upstairs meter is at 140, for example, the water is very hot, as it should be. But it lasts only two or so minutes then gets cold. Obviously this makes for very short or very cold showers, makes it hard to effectively wash dishes and so on.

The pressure gauges by the tanks look fine to me (two tank ones around 140, the pressure tank at 32—should be about 30. And the solar thermal panel pumps are working (get hot, vibrate).

There’s a Grundfoss pump just past the Rinnai that seems to always be flashing between 6 watts and 0 gym, even when I turn on a faucet and run hot water. So I’m wondering if that’s the problem.

Or if there’s anything I can try to at least diagnose, if not fix, this myself. I live very far from the installer so if I have to have him out it’s going to cost me a fortune. Plus I prefer understanding how things function, fixing them myself, if at all possible.

Any suggestions?