This system was plumed based on this diagram from Pentair, with modifications found on this forum.
There are two differences in my system. It does not include the check valve between the solar panels and the return, and does include manual bypass and shutoff valves as shown in this diagram and photo.
MY SOLAR DIAGRAM.jpg
Here are some photographs of the installation:
First let me describe the system operation at startup. The two speed pump is run by the electronic controller. I set the unit to run the pump on high speed for five minutes, and then switch to low speed for the rest of the day. During the high speed interval the solar system which has drained down over night is primed and all air is forced out.
Unfortunately during the one minute interval it takes for the timer to switch as programmed from high to low speed, the solar system drains down enough that the pump at low speed cannot force the air from the system and so water does not move through it. If I make the switch from high to low speed manually, with just a few seconds interval the system runs just fine. In other words, the solar system works fine on low speed if the panels have not been drained down.
A fix that just addresses the drain down during the start-up switch from high to low speed alone will not resolve the problem entirely. The same issue arises during normal daytime operations while the pump is in low speed mode and the solar valve closes, causing a drain down and then later opens automatically.
In short, they system runs great with the pump in the desired low speed mode …IF… the solar panels are primed. The pump in low speed mode simply does not have enough oomph to overcome the resistance of the air in the solar panels if they have drained down.
I look forward to any advice you can offer, and wish to acknowledge and say thank you for all the information and inspiration this forum and its members have provided.