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...
Doctor Drivel wrote:

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... Once the panel circuit fills with water, the pump is only pumping
against the running friction, and not the head.

It is pumping against the head (pressure) always...

Wrong.

Not entirely. To prevent forming steam in the top most portion, the
pressure there must be kept above the saturation pressure for the
temperature. So if the collector outlet temperature is 180F, the pressure
must be kept above 7.57 psia. With the return line open to atmosphere (14.7
psia), then the vertical fall from the collector to tank is limited to
somewhere in the range of 16 ft.

If you arrange so the major friction loss is on the return line, say a
throttle valve right before the tank return, then the pressure drop across
it will add to the pressure 'seen' at the panel (i.e. the backpressure
raises the pressure in the panel). This would support more vertical height
panels.

The *worst* thing would be if the throttle valve/restriction were on the
riser side.

daestrom