Damn $100 a month?? That's impressive. My design is just a theory and really just a side thought when I was bored. I also never understood why the water heater and the furnace don't share the same heat exchanger, I suppose because one goes bad before the other and people huck the whole thing out in favor for another. I thought it was so cool long ago when I first saw an atv starter that was also the alternator. We could all save space, weight, moving parts if things had dual purpose.