two-speed with solar panels

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OK. I will indulge myself by being doggedly stubborn and incapable of understanding this.

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet


OK. But can this work if the high point in the loop is higher than 13 feet over the pool water level, and there is a valve at that point that opens at negative pressure? (Taking atmospheric pressure as the origin.)

If you think this works, then it means that the pump at low speed generates enough head to get the water all the way to the top, and there is no need for the high speed.

More specifically: you say the low-speed dynamic head is 13ft, or 5psi (by the way, 13ft is closer to 6psi). So on the way up to the panels, the pressure drops, both because of friction and height. What's the pressure in the pipe at the top? Is it 2psi? 1psi? Or -1 psi? (The reference point is atmospheric pressure, about 15psi at sea level, so -15psi is the lowest value.) If you give me a positive number (or even 0 as a limit case), I'll tell you that in this case 5psi is enough to get the flow started, not just maintained. If you don't agree, please explain why. If you give me a negative number, I'll tell you that that's not possible, because the valve will open. If you don't agree, please explain why.

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.

I am sure it doesn't hurt to have the higher speed available. I still don't see why it's necessary. I agree completely that the two-speed pump makes sense if there is a siphon effect. But the valve prevents that effect. What's wrong with this reasoning?

By the way, I just took readings of the pressure at my filter. It's 16 psi without solar. When solar kicks in, the reading rises to 24 psi, then drops down to 23 psi. That's well over the static head for the panels, so I am positive I could run at much less power and flow.