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 186 s. Furthermore, when the outdoor air fan was reversed to
blowing the melted frost during defrosting, the total mass of the retained water collected
decreased 222 g. However, the defrosting efficiency was not increased, but decreased 6.6%
because of the heat transfer enhanced between hot coil and cold ambient air.