Excellent... so some experimentation with the valve and the pressure readings at the filter will give me some indication.

My filter is currently running about 12PSI (sorry I am quite new to this). If I gate back the main line a bit, forcing more water through the solar loop in the concrete, then it will go up. If I gate it back more, forcing more water through, it will go up more. What are safe limits to this? How high can (should?) this pressure go? I know I don't want to force all the water from a 1.5" line through a .5" line - it will put a HUGE amount of work on the pump I would expect.

I am also told I should backwash when the PSI climbs about 5PSI higher then normal... is this also correct?

Cheers
Josh