Solar Pool Heater

[Poconos]

The efficiency and amount of heating (BTU / Hr) is dependant primarily on square footage, how you plumb them in and water flow. Ideally you want the surface to be cool to the touch, that is, no discernable heating. This indicates maximum heat transfer to the water. In a practical sense you don't need to ram full pump output through the heaters....the flow is a compromise. I only divert part of the pump flow to the heaters, something around 15 gpm or a little less on a 4x20' mat. However many heaters you get it would be best to plumb them in parallel for two reasons. 1)--less restriction on overall flow. 2)--you don't want warm water from one feeding the next. In any case you want to feed from the low point of the heater and discharge from the high point. This will assure a good solid charge of water and a good trapped air purge. As for 160 sq ft not being much, hey, every bit helps and you'll be surprised how much heat you do get. And...it's free.
Hope this helps.
Al

[blk133]

Thanks Poconos. So if I understand it right then you would suggest me to go with 5 of 4x8 vertical than with 2 of 4x20 horizontal.
Hmm $$$$ about to fly away

[Poconos]

Not necessarily recommending one way or the other. Whatever you can fit in the available space the best way and the cheapest $$$ is what I'd go with. Plumbing the things is usually pretty easy.
Al