Water absorbed dose calorimeters will be used as PTB standards for the dosimetry in radiation therapy. In this investigation, heat conduction effects in the calorimeter due to the irradiation of the non-water materials of the plane-parallel detector are investigated experimentally and by model calculations. It is shown that the effects during Co-60-gamma-irradiation can be adequately described by finite-element simulations and that corresponding corrections in the order of 0.2% result for the typical operating mode of the calorimeter. Correction factors k(C) are determined for a variety of different irradiation conditions.