The theory of steady-state heat loss determination from buried heating pipes has been reviewed as well as previous approaches to treat transient heat losses in the case of constant water temperatures. A new method has been developed to find an undisturbed ground temperature by which the transient heat loss can be calculated by the use of the steady-state heat loss equations. By numerical simulations, as well as by experiment, the position of this undisturbed ground temperature has been found. The position of this ground temperature is closer to the ground surface in the case of uninsulated pipes-or pipes with the insulation in poor condition-than in the case of insulated pipes. For pre-insulated pipes the position corresponds approximately to the top of the pipe casing. Copyright (C) 2000 John Wiley & Sons, Ltd.