In the 20th Century many buildings became totally dependent on fossil fuel energy to make them habitable. Recent legislation and building regulations, however, ensure that designers are contracted to aim for zero carbon sustainability or buildings which function with much lower levels of fossil fuel dependency. Heat pipe technology has found increasing applications in the building service industry in recent years: particularly in enhancing the thermal performance of heat exchangers and increasing energy savings in commercial HVAC systems. In this paper, an experimental investigation of the thermal performance of an angled air-air heat pipe heat exchanger, which utilizes thermosyphon technology, will be presented. A thermosyphon is a particular type of heat pipe which relies on gravity assistance for its operation. In this paper, the investigated heat exchanger utilizes nine thermosyphons in a modified inline configuration filled with water as the working fluid and having finned evaporator and condenser sections. Different sets of experimental tests were carried out by varying the heat load as well as the inclination angle of the heat exchanger. The values of the overall effectiveness and thermal resistance of the heat pipe heat exchanger are shown to vary with the air flow rate, as expected, but the results also allow the prediction of effectiveness variation with the variation of the operating temperature. The paper also includes a performance model for predicting the effectiveness of the thermosyphon based heat exchanger under test, developed using empirical mathematical correlations. (c) 2011 Elsevier Ltd. All rights reserved.