John - I did this in a different context, to heat a small greenhouse, and avoided the complication of heating water. I also did it using less than ideal materials - mainly clay, as opposed to the vastly superior insulated firebrick, ceramic fiberboard and ceramic fiber batting. I think that if those materials were employed, it would perform quite well. Mine worked well enough, and I was able to heat a masonry bell on the coldest nights of the year. It wasn't very practical in the sense that I wasn't willing to make a fire every day. Take a look: http://donkey32.proboards.com/thread/2321/indoor-outdoor-greenhouse-heater
It has certainly been an idea that has been rolling around in my head for a while - to do it in a residential setting with optimal materials.