text: Series for sure. \n\nswimming pool: SP\nheat exchangers: HE1, HE2  \n  \nIn parallel, both HE heat the water to a temperature t1 higher than t0, the initial temperature in the SP.  \nSo in both parallel pipes we have the same temperature, the temperature obtained with just one HE. The other is useless. Or it gives you the advantage of heating two times faster.\n\nIn series we have the temperature increased from t0 to t1 after passing HE1, and from t1 to t2, a higher temperature, after passing trough HE2.\nThe water from SP is heated more than in the 1st case, but 2 times more slowly.