Heater savings: up to 95% heat recovery reliably calculable?

What do you mean then? What else should be compensated for besides the ventilation losses?

It was just a mistake ...

If the question is: "Can the efficiency (rather heat recovery factor) of a heat exchanger be calculated?" then I would say: yes, that is possible. However, it will never reach 90%, but at most 80%, and that probably only at low volume flow rates and only with suitable designs.

Oh, I think it works quite well. The picture is a few years old, but still: