I'm not sure whether you'll get an informed answer or not, and I don't have the experience to know whether your salesman is correct or not.

But you can find out for yourself, easily enough:

1. Install accurate inline temperature gauges on BOTH your pool piping, upstream of the solar return, and also IN the solar return.

2. Check and compare temps under full sun, under clouds, in rain, and at sunrise, after clear nights. If you the solar return temp is often lower than the pool temp, you may need a controller of some sort, to prevent the panels from cooling the pool.

Something like this should work:
PIC Liquid Filled Thermometer Bottom Connection, Brass Well
Be careful about accuracy; a lot of commercial gauges have a 2 - 3 degree error, and in your application, that could cause problems. BUT . . . you don't actually have to have an accurate pair of gauges; just a pair that read alike. So compare them in water baths with both cool, warm, & hot water: if they read the SAME, even though they are both off by a few degrees, you're good to go, since you need difference readings, rather than absolute accuracy.

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