Loss of pressure when adding basic solar heater

[BigDave]

Agreed, but, in this case the OP has a low volume, low pressure pump that is unlikely to overwhelm the capacity of the panel. The observation of warmer water returning from the panel demonstrates this. Flow rates approaching the panel's capacity would be less noticeably warmer.

[mas985]

It depends on the size of the panels but typical AG panels are 2'x20' or 40 sq-ft and I got the impression there is only one which would require only 4 GPM. So most of the flow rate can probably bypass the panels and still provide pretty good heat transfer. But once the OP posts back, perhaps he can tell us the size of the panel.

[CarlD]

sometimes it's simpler. I re-routed soft panels when I had them so the water went in one corner and out the other, and ALL the return water passed through them. The efficiency of heating was amazing that way.
It's fundamental to thermodynamics that the faster the water flows through the panels without cavitating, the more heat is transferred. Usually, the limiting factors are a loss in pressure, or more pressure than the panels can tolerate before they start to fail and leak.

I successfully ran a 4'x10' (same as a 2x20) with a little tiny Intex cartridge pump this way, and was able to heat my parents' 13k AG with only 6 hours of direct sun a day. For both, I did it by re-routing the panels so water went in one corner and out the furthest.

[Air_Cooled_Nut]

There are many factors that can affect pressure drops: http://www.pipeflow.com/pipe-pressure-drop-calculations

Also, though the pressure may have dropped that doesn't necessarily mean the flow has. Time how long it takes to fill a gallon bucket from the solar output and the normal output (basically the end of the hose that connects to the pool to keep things consistent).