Which decentralized ventilation system is sensible?

Hmm, I thought I had read that they change their airflow direction every 70 seconds. It actually makes sense, since the ceramic element then has to be warmed again by the warm room air. Hence the constant switching.

Whether 2 fans working in push-pull principle (so one always in the opposite direction of the other) are mandatory is not really clear to me yet. That you move more air volume with 2 is clear.

If you want a ventilation system with heat recovery, one is not enough. One can only exhaust air.

They alternate between sucking out and blowing in. You don't use just one alone because in airtight houses this leads to overpressure and underpressure, and the devices no longer perform well.

Otherwise, heat recovery is not possible if each unit only operates in one direction. In that case, you might as well save the devices and just open and close windows directly.