was doin some snoopin in my utility room, opend up my venmar AVS 1.5 solo, noticed the heat exchanger core was made of polypropolene, wouldnt you get better effiency with a aluminium or copper core? most plastics make good insulators. i'll measure its heat recovery performance one of these days.

Without going to the effort to look up data, I'm going to venture a comment that it probably has to do with specific heat, i.e., how many Btus of heat it takes to raise the temp of 1 lb of material 1°F. The plastic may be better in that regard than aluminum or copper. You want to be able to suck a lot of heat out of the warmer air without having to raise the temp of the medium very much. For instance, on a volume basis water holds a lot more heat than does rock.