The "Hill's Furnace" wood boiler concept.

Very briefly, the "Hill's Furnace" is the result of a DOE grant in the late 1970's to Professor Hill at the University of Maine, Orono. The documents are scanned from the 1979 grant report. The furnace was commercialized for a brief time as the Hampton "Jetstream", but the concept seems to have been largely forgotten with the decrease of woodburning in the mid-1980's and beyond. With increasing fuel costs people are moving back to wood heat, and the "Outdoor Wood Boiler" (OWB) is a very ugly incarnation of wood heat that cannot possibly provide long-lasting clean heat; the firebox of most of these units is a metal water-jacket that keeps the firebox at too low a temperature for efficient combustion. To provide hot water over the day, these OWBs are loaded with wood and smolder for hours at a very inefficient burn.  The "Hill's Furnace" is NOT a conventional Outdoor Wood Boiler - it is a completely different concept....

The furnace is a boiler unit comprised of a water-jacketed firebox and a heat-exchanger with minimal water in the boiler. The water circulates to a large tank that can be remotely located in a cellar and insulated to retain the heat. The boiler is fired full-on hot so it burns cleanly; the heat is captured into the water. The firing is done intermittently with the large volume of stored heated water serving as the source of domestic heat. Ordinarily one would use circulating pumps to circulate the heated water through the house, and zone heating easily accomplishes even heat distribution. Because of the large volume of water (~1000gal) the temperature drops slowly as heat is removed, and the furnace is re-fired as needed to maintain a suitable water temperature. The water storage tank is sized so that it can last ~24hr in the coldest weather, and the furnace needs firing less frequently at other times. The firebox accepts logs vertically so the burn is focussed at the lower end of the wood by the induced draft that is jetted there and the rate is thus controlled.

The furnace could be easily installed in a small outbuilding (Please do not mistake this concept for the current style of "Outdoor Wood Boiler"!!!) and the tank located in a cellar. This will keep all mess associated with handling wood outside of the house, and any potential fire hazard is likewise isolated from the house.

Note that the surface to volume ratio of a large tank is such that it will lose heat slowly, and modest insulation is all that is needed, and the footprint of a 1000gal cylindrical tank is not even as large as the furnace it would replace.

Hill's Furnace document as a Zip of the individual scanned pages (as images)

Hill's Furnace document "Viewer" application for Windows

Wikipedia has an article on the Jetstream Furnace

Disclaimer: I have no association with the Univ of Maine or Prof. Hills and was not involved in the research. I came by a copy of the grant report and knew people who were talking about this system in ~1979 and think it is a concept that should not be forgotten.  (Dale A. Callaham - 6/3/2008)