A heat flux sensor (HFS) can be used to measure the heat loss from buried district heating (DH) pipes if the HFS is calibrated in conditions resembling the actual condition of use, i.e. not in one-dimensional conditions. Owing to the fact that the thermal conductivity of the HFS often differs from the thermal conductivity of the surrounding media, the heat flux through the HFS will differ from the true heat flux. Consequently the development of compensated HFSs is discussed. The influence of how the HFS is fixed to the pipe casing is discussed as well as the influence of the casing diameter and the soil thermal conductivity on the calibration factor. The long-term stability of HFSs is discussed with reference to measurements on a DH transmission line, which were carried out over a period of more than one year. Copyright (C) 2000 John Wiley & Sons, Ltd.