How does it work? The simplest explanation is that water lines are buried within a tightly packed mound of decaying organic products, the water in them heats up as they travel through the mound, and they re-enter the house at a much higher initial temperature, thereby reducing costs for water heating.
Is it realistic? For Vermont, perhaps. A rural setting is preferred given that the space required for such a massive Pain mound would be the size of many city backyards. Also, in a rural setting there are no neighbors to complain about an unsightly mound whereas in the city, you can be sure to expect some inquiries from neighbors, not to mention inspectors.
Cost-benefit? At this point, the cost outweighs the benefit in my opinion. Though I have no evidence for it, it would seem that the cost of wood mulch, plumbing fixtures and piping and the labor involved in building the pile would be greater than the money saved on simply heating water using fossil fuels. If, however, the benefit of not being reliant on fossil fuels is more important than any costs associated with being off the grid, then perhaps it is worth it for you. For me? Not so much.
If you are interested in learning more about this concept, how it was built, the specifics of costs associated, the BTUs generated from the pile…I’d be more than happy to share documents…get in touch with me by leaving comments or by emailing me…