As Kirk mentioned, radiators do work well, but, first of all they require pretty good flow, and because of the shape and design, they like to be vertical.
Besides, the rad is usually a big pipe in and out, the A/C condensor is usually a 1/2" or 3/4" inlet & outlet. Also they generally have nice radius turns and the liquid flows really well through them.
I used several inline with each other to heat the operators cab on a firewood processor, it used the hydraulic fluid return line as a heat source. It cooled the hydraulics and warmed the cab at the same time.
It worked too well though, the operator is nearly down to his skivvies making firewood unless its a very cold day.