Water absorbed dose calorimeters have been developed for the application as PTB standards for the dosimetry in radiation therapy with photon and electron radiation. Different detector types for various applications are available. In this paper, heat conduction effects of "sealed" detectors are investigated, caused by the material-dependent temperature rise during the absorption of Co-60-gamma-radiation in the detector walls and in the temperature probes embedded in water. The corresponding corrections can be minimized by a proper design, allowing the resulting relative standard uncertainty to be kept well below 0.1%.