What is the best defrost scheme for a packaged water heating heat pump. I have heard about using reversing valves to defrost. Can anybody explain the hot gas defrost? I am specifically interested in components needed to have hot gas defrost (i.e., for reverse cycle need reversing valve, may be an accumulator?).

I am trying to assess the cost, reliability of defrost cycle method, and effectiveness of defrost (obviously that comes with logics). By effectiveness, I mean having an efficient defrost cycle with minimum energy usage and energy losses. Thanks.