Very high solar roof heating Splapool 1.5HP 2 speed not strong enough, need help

Also, have you checked the head curve of your current pump to make sure it has enough head to prime the panels?
 
As Mark said, 1.5" is likely fine, 2" would just have a lower flow resistance.

Mine just ran to the roof and then had about 10 PVC Tees all plumbed together. Out of these the irrigation lines were fed (often leaked around these too). Then there was another manifold of Tees plumbed together to join back up to the solar return line.
 
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