The downhole heat exchanger (DHE), used extensively in Klamath Falls, Oregon, in over 500 installations, and in Turkey and New Zealand, provides heating for one or more homes, schools, and apartment buildings from a single geothermal well. The DHE eliminates the problem of disposal of geothermal fluid, since only heat is extracted from the well. The heat exchangers consist of a loop of pipes or tubes suspended in the geothermal well, through which "clean" secondary water is pumped or allowed to circulate by natural convection. The maximum output of large installations is typically less than 3 GJ/h or 0.8 MWt, with well depths up to about 150 m, and may be economical under certain conditions at a well depth of 500 m. However, the typical output for an individual home in Klamath Falls tends to be less than 265 MJ/h (0.07 MWt). In order to obtain maximum output, the well must be designed to have an open annulus between the wellbore and casing, with perforations near the top and bottom of the submerged heat exchanger, just below the water surface and at the hot aquifer at the bottom of the well. Natural convection circulates the well water down inside the casing, through the lower perforations, up through the annulus and back into the casing through the upper perforations, with the new geothermal water mixing with the old. This vertical convection cell exposes the DHE to the near-maximum temperature of the well water and thus increases the heat output of the DHE. The heat output from a DHE system is dependent on the bore diameter, casing diameter, DHE length, tube diameter, number of loops in the well, flow rate and temperature of the geothermal fluid. Based on local experience in Klamath Falls, the "rule-of-thumb" is that contractors estimate approximately "1 foot of DHE pipe per 1500 Btu/h" (5200 kJ/h/m or 1.44 kW/m) as an average output. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.