Frost problem with Lennox HRV

I have a Lennox Healthy Climate HRV3-150-TPD installed. I live in a cold climate and have a constant problem with frost build-up in the HRV in winter months. The HRV ices-up both internally and even on the housing, then a few days later the ice melts creating a big puddle, and then freezes up again blocking the airflow and depressurizing the house. I know it because I use the HRV to control radon level in the basement and when the heat exchanger is iced up, the radon level jumps through the roof. The HRV has a motorized damper on the FA intake and is supposed to automatically defrost, although I've never seen the damper in a closed position. I checked DIP switches and they seem in the right position according the installation manual to enable automatic defrost. The contractor who installed the HRV was good at selling it to me, but couldn't troubleshoot if you put a gun to his head. He says he needs to do some research and he's been doing that research for a couple of years. I went to the Lennox site, but they don't want to talk to me unless I have a gas fitter license (I'll never allow another Lennox product in my house again). I have a feel the solution should a simple adjustment to HRV controls, but I haven't been able to find how. Any suggestions on how to resolve my HRV frost problem (other than rip the darn thing out and throw it away) will be appreciated.

source https://hvac-talk.com/vbb/threads/2236254-Frost-problem-with-Lennox-HRV?goto=newpost