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
Furnace" is NOT a conventional Outdoor Wood Boiler - it is a completely
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)