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Heat pipes that operate in the medium temperature range (550-700 K) are very rarely used in industry despite the potential demand of use. There is no consensus about suitable working fluids in this temperature range as research on possible working fluids is limited. One proposed working fluid is naphthalene. In this paper, a number of tests have been undertaken on both an individual naphthalene heat pipe and a naphthalene heat pipe heat exchanger. Unlike room temperature working fluids, medium temperature working fluids are solid at ambient temperature therefore they have unusual transient start up behaviour. Testing has indicated that these heat pipes start to operate when the temperature of the adiabatic section reaches approximately 200 °C. The tested heat pipes were 8 mm in diameter and 278 mm long. The container and mesh material were stainless steel. They were found to have a thermal resistance of approximately 1 °C/W and a maximum rate of heat transfer of 40 W. The orientation was found to have a large effect on the performance of the heat pipes. Compared to the bottom heat mode orientation, when in a horizontal orientation the heat exchanger effectiveness more than halved and when in the top heat mode orientation heat exchanger effectiveness was significantly further reduced.