I read about this problem and brought it to my HVAC-R class and instructor to find a solution yesterday.
We didn't have time to discuss the problem but I felt I should try to help as my late wife also had breast cancer and died from it.
I can appreciate your wanting to make your wife as comfortable as possible.
So I did some research based on your problem and symptoms. I may have a solution too.
Yes, when you freeze up or get a block of ice at the evaporator, it usually means your superheat is near zero or below it.
This confirms you have either a low refrigerant charge or the pressure-reducing valve isn't functioning correctly (blockage, mechanical problem, feedback mechanism).
From what I read about the technical specs of your water furnace it uses an EEV as a metering device or two of them in the case of its configuration as a heat pump. And the superheat ought to be 5-7 degrees.
And yes, because your EEV's aren't operating in their full range, then you are not operating efficiently and worse, may not be cooling your compressor adequately thus reducing its life expectancy.
Superheat is measured as a result of two temperature readings. One measures the temperature of the metal pipe right before it enters the compressor (suction line). The second temperature reading corresponds to the refrigerant's P/T chart from the pressure reading at the suction line's pressure tap. Superheat = temperature at the suction pipe - temperature corresponding to the suction line pressure reading.
Hence, your system had a circuit design to accomplish the superheat calculation and then use it to drive the EEV to open more or close it to maintain this range of superheat based on your heating or cooling load which is based on the indoor and outdoor temperature conditions.
Here is a solution you can implement with the help of an HVAC-R tech.
Find an EEV controller that will read the suction line temperature and pressure sensors that ought to be mounted on your system.
Use this EEV controller to drive the existing EEV metering device to maintain the 5-7 degrees of superheat.
Yes, this may mean your tech has to disconnect the existing connections from these three devices from that circuit board that doesn't work or continue to use the pressure and temperature connections in parallel with the new controller and old circuit board. I am not sure if your old circuit board needs the temp and pressure signals to operate or for safety (i.e. low-pressure sensor).
The above isn't pretty but ought to work.
Good luck if you wish to implement the above.
Good health and success to you, wife, and family too
