Solar heating question

[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.