text: what is better to use two heat echangers in parallel or series? we have two primary circuits(hot water from boilers) that supply two heat  exchangers separetly, also we have one secondary circuit (cold water from swimming pool that we need to rise its tempreture),what is better to put the heat exchangers in parallel or series to get the best effect. series for sure.   swimming pool: sp heat exchangers: he1, he2      in parallel, both he heat the water to a temperature t1 higher than t0, the initial temperature in the sp.   so 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.  in 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. the water from sp is heated more than in the 1st case, but 2 times more slowly.