Solar system water draw: skimmer or main drain?

[SJSwimmer]

I've never really thought about this before, but is it preferred to draw water to the solar heater through the main drain or the skimmer? It seems that the warmer water would be near the surface, so heat transfer would be better to the cooler water from the pool bottom.

I don't know if this oversimplifies the problem, but I was just curious.

 

[chem geek]

If your pool has poor circulation, then taking water from the pool bottom that would be cooler during the day would be better since it keeps the panels cooler so that heat gained from the sun doesn't get re-radiated back out as much. However, with reasonable pool circulation the water should be at roughly the same temperature at all depths so that it doesn't matter whether you get the water from the skimmer(s) or from the floor drain(s).

As noted in this post, about 1/4th of the sun's energy is absorbed by the first inch of water while about 40% is absorbed in the first foot. In a white plaster pool with a 3 to 6 foot depth, around 60% of the sun's energy is absorbed overall so you can see that without good circulation the surface water will get warmer than water at greater depths. However, it is quite easy for circulation to mix the water to prevent this. I know that in our own pool, I hardly ever notice surface water being any different in temperature than water near the bottom of the pool, but we have pretty good circulation with 3 returns about 1 foot below the surface in a rotating pattern plus two floor drains and one skimmer.

You can also see the effect of differing water temperature on solar panel efficiency in this post where a 9ºF temperature difference can lower panel heating output by around 15% on a sunny day (more on a cloudy day).

 

[mas985]

Also, drawing from just the main drain is likley to increase the suction head loss and reduce flow rates slightly so you might even lose some heat gain not to mention sucking in some air. I would leave both the main drain and skimmer open, the pump should draw from both anyway.

 

[SJSwimmer]

Thanks for the replies. I had seen first of the linked pages, but appreciated the second. It's probably a good thing to have those re-posted, too, for those solar users getting ready for summer.

I had started running water up to the solar system last week, but it looks like (San Jose) we won't really be getting up and running for another week or more: cold temps, rain, and cold nights. At least the water is good to go...