[stevehull]

NiHao is correct, but the efficiency difference may be small. Try it both ways and see. The greatest heat flow (Q) is proportional to dt (delta t - temperature difference). So a counter flow system maximizes this with the coldest return air being exchanged for the warmest outflow water. There are many equations that model this, which is why I put in those terms above. You can find them in books on thermodynamics and also on the internet.

But you may have physical constraints that limit which pipe goes where. I suggest hooking it up as easily as possible (counter flow is best) and see how it works.

Let us know.

Steve