We take filtered water, (but not chlorinated water) and feed it up to the roof using a solar pump. Water then runs at approx 2m per second down the line and returns.
The ideal length is 10m. This means the water is in the line for 10 seconds total i.e. (Up the line for 10m + back down the line for 10m = total 20m / 2m per sec) = 10 seconds approx.
Water is heated on the roof and returned back to the pool.
Roof spaces will hold some serious heat. The shoulder months of the swimming season, really take advantage of this and a roof can be as hot as 35 degrees in Sept and May.
The ideal combination is to use a 4mm thermal blanket, to retain the heat and reduce evaporation and running your solar to pull as much heat into the pool as possible.