two-speed with solar panels

[mas985]

Is there really a difference between "negative head" and "vacuum"? As I see it, unless there is still positive head at the top of the panels, the water on its way down is pulled by gravity and produces a "suction" at the top. By suction I mean a pressure lower than atmospheric. If the system is sealed, that negative pressure "sucks" the water up from the inlet. But if there is a valve, the suction will make the valve open. As I said, it's possible that the valve will stay closed with a small amount of suction, but eventually it will open, and I would imagine the margin to be too small to be of use.

Thanks!
Luigi

Yes there is a difference between negative head and a vacuum. The primary difference is the reference. Vacuum usually refers to pressure below atmospheric or negative PSI. Your solar valve on the roof will not release until the pressure in the pipes is below atmospheric which requires the pump to be off and water to be draining out of the pipes. This causes suction.

However, with the pump on, there is a net positive pressure within the pipes usually between 10-20 PSI above atmospheric pressure (i.e. your filter PSI reading). This keeps the valve closed. So while the water going down the pipe offsets the water going up the pipe, this is called static head, there is still a substantial positive pressure to keep that valve closed. There just isn't any head added to the system because of elevation. The only pressure difference is the dynamic head loss experienced in pipe and solar panels. The water falling simply makes it such that less pressure is needed to push the water up the pipes.

The easiest way to see this is when the solar is first turned on. You will see the PSI rise substantially and then after the pipes are filled, the PSI will drop slightly but still higher than without solar.

So in formula terms:

Filter PSI Solar on = PSI Solar off + PSI elevation Rise - PSI elevation fall + PSI Dynamic Head Loss Panels and Pipe

PSI elevation Rise - PSI elevation fall nets to 0 so there is no PSI impact due to elevation.