[jeff5may]

Ok, let's think about this for a minute.... It all depends on how much the unit runs and how hard and fast. If the unit is running pretty hard, the water can only help. If the unit is not struggling, it may not help too much. During cooling season, it's a no-brainer.

During heating season, the method you employ will make a big difference. If it's not too cold, say above 40 degF outside, you can simply spray water onto the coil to improve performance. But when you dip into "defrost territory", spraying water onto the coil will only help a little. The reason is not related to defrost or freezing so much, but is related to the saturated suction temperature (SST).

When it gets cold outside, the evap coil pressure and SST follows the air temperature change. This limits the capacity of the heat pump "from below" at the decreased suction pressure entering the compressor. With an air heat exchanger, there is no way around this limit, since the refrigerant gas will always be colder than the outdoor air as it leaves the hx. Adding water to the air coil only makes it unstable.

What you have to do is reduce or eliminate the airflow to the heat exchanger, while making sure you have an ample supply of warm enough water. The easiest way to do this is to change heat exchangers, thus removing air from the outdoor side of the system. The not-so-easy way is to slow or stop the fan blower and substitute airflow for water flow. As long as the refrigerant leaving the evap coil is warmer than outdoor air, the unit will provide better heating than "stock" operation.