Why should a heat pump receive low return temperature?

[TSN79]

One is always told that a heat pump (heating water) should receive 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?

 

[Andrew Mason]

A heat pump works by transferring heat from the cold reservoir to the hot reservoir. In order for heat flow to occur from the cold reservoir to the system, the part of the system that is in thermal contact with the cold reservoir must be colder than the cold reservoir. The colder that part of the system is, the greater the rate of heat flow to the system and then delivered to the hot reservoir (eg. the room to be heated). But the colder it is, the worse the co-efficient of performance becomes. So with heat pumps one tries to reach a balance between efficiency and rate of heat transfer.

AM

 

[Averagesupernova]

A person can keep this sort of thing straight in their head if they think about it in the manner in which it is named after. Absorption refrigeration. Keep in mind that heat only travels from hot to cold so in order to make it go the other way we manipulate the state of the refrigerant to make part of the loop colder than the reservoir we wish to cool and allow heat to absorb into the refrigerant which is then carried away and made hotter than the reservoir we wish to dump the heat into. For a long time I thought that evaporative refrigeration was what a plain old refrigerator or heat pump was assuming it was called that since we evaporate the refrigerant in part of the cycle but of course I was wrong and this is not the case.