Experimental measurements of fouling resistance were carried out in a double-pipe heat exchanger (HE), with geothermal water flowing in the inner tube and tap water in the annular space. Tests were performed using waters from a geothermal well in central Serbia whose production temperature remained practically constant (52 degrees C) throughout the 90-day experiment. Analysis of the data showed that the change in fouling resistance was a linear function of time. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.