Hi,
Frosting is a big issue with ASHP. Unfortunately dehumidifiers are not a good candidates for conversion if one keeps the original evaporators.
In oposition to an ASHP, in a dehumidifier is equipped with an undersized evaporator by purpose. The leading design idea is to have as low evaporation pressure (temperature) as possible. This ensures lot's of water condensing out from ambient air with the smallest possible compressor. The temperature gap between the air and HX is arround 20Kelvin.
If you ran such unit as ASHP your evaporator reach wery early the freezing temperatures and start to develop ice.
On an efficient ASHP you have to have as small temperature difference between evaporating temperature and ambient air as possible. To achieve this you need huge size of evaporator (comparing to dehumidifier), good airflow and low superheat.
Nowdays minisplit systems starting to develop ice at arround 45F as the designed dT is much smaller comparing to dehumidifier. It is done by increased surface and airflow.
If we want to make a good, efficient ASHP, we need to strech the evaporator surface area even furter than a standard minisplit. My heatpump is running on a compressor, half the power the evaporator is designed for and with good airflow. This way there is no frosting down to 2C/35F ambient and maybe 10% of the area is frosted after two hours running at -2C/28F without any defrosting.
Another important, but not obvious thing is to keep superheat low. to achieve this you have to set refigerant charge level spot on (in case of capillary metering) or better, switch to TXV or EEV. Every degree of extra superheat rise the corner temperature (where the frosting starts) roughly by the same ammount.
I run my hp with an EEV and superheat set point is at 0.5K.
To make a working HP is easy, not requires much effort. To make a good efficiet HP is much more work!
T.
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