mas985, the 1/3 is the figure they calculated and derived after putting the flow-reversal system into practice many years ago... It assumes a pool cover and an in-ground pool installation with a deep end...

You do bring up a good point, is a single return sufficient on a 27ft above ground pool or larger?

I would imagine on a 21ft or smaller pool a single return with good flow would be just fine, but larger pools of 27-33ft I would have to imagine a single return would be a stretch to cause proper mixing...

So, would two returns work better for the larger pools? Maybe 3 or even 4?

I just don't get the HUGE difference in pool flow designs between in-ground and above ground pools. I understand that in-ground pools have varied layouts and designs and some of that requires multiple returns/skimmers/etc to effectively clean and mix the water, but it just seems that in-ground pools are wayyy overbuilt and complicated compared to above ground pools and if the in-ground are clearer with better, more effective turnover, why not apply a lot of that to an above ground pool for the same benefits... And yes, I know, a lot of complex plumbing is for spas and integrated water features and such, but take those out of the equation and a lot of in-ground pools of simplistic designs have at least double the returns/skimmers of their above-ground brethren for the same or even less volume at times...