An experimental study on defrosting performance for an air source heat pump unit with a horizontally installed multi-circuit outdoor coil

•Negative effects due to gravity eliminated when outdoor coil horizontally installed.•Defrosting efficiency increased 9.8% after outdoor coil horizontally installed.•Defrosting efficiency decreased 6.6% when air fan reversed to blow the melted frost.•Total mass of the retained water collected decreased 222g less after wind blowing.•Higher DEV respected better defrosting performance for multi-circuit outdoor coils. When frost forms and accumulates over the outdoor coil’s surface in an air source heat pump (ASHP) unit, system operating performance will be dramatically deteriorated. Reverse cycle defrosting is the most widely used standard defrosting method. A previous related study reported that downwards flowing of melted frost due to gravity over a vertical multi-circuit outdoor coil would decrease the reverse cycle defrosting performance. If the outdoor coil can be changed to horizontally installed, the flow path of melted frost over coil surface can be shortened, and the flow directions of refrigerant and melted frost changed from opposite to orthogonal. Consequently, a better defrosting performance is expected. In this paper, therefore, an experimental study on defrosting performance for an ASHP unit with a horizontally installed multi-circuit outdoor coil was conducted. Experimental results show that, when a vertical outdoor coil was changed to horizontally installed, the defrosting efficiency increased 9.8%, however, with the same defrosting duration at 186s. Furthermore, when the outdoor air fan was reversed to blowing the melted frost during defrosting, the total mass of the retained water collected decreased 222g. However, the defrosting efficiency was not increased, but decreased 6.6% because of the heat transfer enhanced between hot coil and cold ambient air.