A simple system can do much better than the average. Picture the water flowing in a "U", with the hottest water at the bend. The coldest incoming water is warme

A simple system can do much better than the average. Picture the water flowing in a "U", with the hottest water at the bend. The coldest incoming water is warmed just a bit by the coldest outgoing water, then the slightly warmer incoming water is warmed by water a bit further upstream from the output. Carry that logic through and you recapture the heat with a fairly simple system (give or take longer pipes to let enough heat exchange happen).