Solar heating question

[cleancloths]

1 PSI is equivalent to roughly 2.3 feet of water column.

[Poconos]

Normally with the panels close to pool water level you can put a bypass valve in the line to allow most of the water to bypass the solar. You get more overall flow and there is still enough water going through the panels to operate efficiently. However, 20' in the air and with a relief valve at the high spot to allow them to drain, you will probably have to run the entire flow through the panels which will cut the overall flow rate. If you bypassed any of the flow there would not be enough pressure in the solar lines to prime them. Just be aware of this.
Al

[cleancloths]

How does that relief valve work? I ask because the elevation should only come into play to start the flow, once you have it established you are taking water from grade and returning it to grade, thus the only pressure you need is to overcome friction losses due to the length of pipe and equivalent length of the fittings.

[mas985]

That's correct. Elevation only comes into play during priming. The valve remains open until water reaches it then closes off. At that point, the pump still needs to push out the remaining air through the returns so it still needs to overcome elevation head plus the dynamic head where the water is in the plumbing. Once the solar return pipe is full of water, the elevation head is then canceled.

But because of both the solar pipe and panels, there is quite a bit more dynamic head loss with solar than without. If there is not enough pressure at the top of the panels to keep the VRV closed, it will open and suck in air thereby removing the canceling effect. The pressure at the top of the panels is always dependent on the elevation rise so it will always be lower than the pad pressure by at least height / 2.31. So for a 25' roof, the pressure will be at least 11 PSI lower than the pad and that doesn't take into account the extra pressure loss due to dynamic head.