One is always told that a heat pump (heating water) should recieve return water with a temperature as low as possible. I've never really understood why. My initial thought is that if the return is higher, then it's "easier" for the heat pump to heat the water up to whatever supply temperature you want. Or is it because a low return indicates that your system makes good use of the heat the heat pump provided in the first place? Is there such a thing as too low return? Or does that just mean that your system's heat emitters are too small?