Hi Meelis,
glad we inspired your project!
You should avoid of course frosting. You have few options for this, either you stop the ventilation when exhausting air riches limit temperature, that is around 2-3 °C, or you reverse the flows so to heat up the frozen side of the exchanger. Alternatively you should preheat the incoming external air so to avoid freezing.
Both motors have to be at the warm side. In my case one motor pushes the air whilst the other pulls it.
As for the humidifier, I'm still working on it. It sure helps, as RH drops down to 25% when really cold outside, so humidification becomes mandatory. It is, though, a bit complicated to integrate it to the exchanger and the pipes, you have to be very careful not to have water drops around and water remains as the could form harmful mold.
Here's a couple of photos of the finished version. It is a completely stand-alone machine with it's own arduino board.
I'll post some data as soon as I can.