A brand new furnace...

Your body.. The New York Times has an amazing article on super sealed homes in Germany that are built without furnaces because they keep warm air in and keep cold air out. Relying on tightly sealed entry ways and windows, plus enhanced ventilation system. The house keeps itself warm by using the ambient heat from electronics (my laptop would be a good candidate) and the occupants body heat.

This is quite amazing and it will be interesting to see if this can take hold in the U.S., the cost of the materials is only 7% more than traditional housing materials. The downside of these designs is they are optimized for 500 square feet per person which while not spacious, is less than the average American household. Of course there does seem to be a movement toward smaller homes. Is this what we call convergence.