Most household water pipes really aren't insulated, or aren't insulated well enough. Plus, moving water under the frostline means that the ground it's moving through might be upwards of 55 degrees F.
My house came with an outdoor wood boiler (it's more a rural cabin than a house, really). It moves 180*F water from the boiler 50 feet underground into the house, where it runs through an air exchanger in the central air system and pipes it back out to be reheated.
Newer models of these things can get upwards of 98-99% burn efficiency, and ~85% heat transfer efficiency- as good as or better than you can get with standard efficiency furnaces.
Most household water pipes really aren't insulated, or aren't insulated well enough. Plus, moving water under the frostline means that the ground it's moving through might be upwards of 55 degrees F.
My house came with an outdoor wood boiler (it's more a rural cabin than a house, really). It moves 180*F water from the boiler 50 feet underground into the house, where it runs through an air exchanger in the central air system and pipes it back out to be reheated.
Newer models of these things can get upwards of 98-99% burn efficiency, and ~85% heat transfer efficiency- as good as or better than you can get with standard efficiency furnaces.