When an air source heat pump (ASHP) unit operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil which normally has multiple parallel circuits on its refrigerant side for minimized refrigerant pressure loss and enhanced heat transfer efficiency. On its airside, however, there is usually no segmentation corresponding to the number of refrigerant circuit. Frosting deteriorates the operation and energy efficiency of the ASHP unit and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. This paper, the first part of a two-part series, reports on the experimental part of a study of the reverse cycle defrosting performance on a multi-circuit outdoor coil unit in an experimental 6.5 kW heating capacity residential ASHP unit. Firstly the experimental ASHP unit is described and experimental procedures detailed. Secondly, the experimental results are reported. This is followed by the discussion on the effects of downwards flowing of the melted frost along a multi-circuit outdoor coil surface on defrosting performance. Finally, the evaluation of the defrosting efficiency for the experimental ASHP unit is provided. In the second part of the series, a modeling analysis on the effects of downwards flowing of the melted frost along the multi-circuit outdoor coil surface on defrosting performance of the experimental ASHP unit will be presented. (C) 2011 Elsevier Ltd. All rights reserved.