Sorry...but don't understand the desire not to use metal? By definition, a heat recovery system is an energy saving device...ergo it is green. Engineered for efficiency...I mean...they take the heat out of the air going out and add it to the heat coming in...freakin' practically magic! Computer driven and pretty complex. Regarding metal duct work...the alternatives are not so green. Plastic tubes with fiberglass insulation and a metal spring inside is about the only real alternative left. Metal at least is recyclable. It is also, by far, the most efficient means of pushing air we currently have. I suppose you could get 6" or 8" HDPE (high density polyurethane) and by lots of fittings....but HDPE pipe is for plumbing...so it won't have all the fittings necessary for a heating and cooling system. Heck, the heat recovery unit cabinet is metal and so is the heat exchange mat...

BTW, we use these: http://residential.fantech.net/residential-products/indoor-air-quality/hrvs-heat-recovery-ventilators/

I use a metal heat reclaimer as well and it is unreal!! Its called Magic Heat and it is mounted into your flue pipe of your wood, coal, oil or gas heating stove/furnace. I have used it for two years now and the results are amazing. Went from 4+ cords per year to around 3. From being unable to warm certain far away rooms in my house to having to open the window because it got too hot. The Magic Heat Reclaimer paid for itself during the first winter of use, no doubt! I cannot recommend this unit enough - I know it produces more heat than my wood stove would do on its own. I now heat my house at least 3 times faster than I could before because it has an electrical fan which helps to circulate the hot air. To learn more about these amazing reclaimers visit http://www.magicheatreclaimer.com/magic-heat-reclaimer-specifications/ or check out this youtube video review You will not be disappointed if you get one of these. But if you do consider it make sure to read over the info in that website I posted there are some things to be aware of. Good luck.