Retrofitting bathroom ventilation with heat recovery

Because you want to get rid of moist air. With inverters, that doesn't happen half of the time. With cross-flow heat exchangers, I would be concerned that it might freeze for you, as it already condenses heavily inside the device (but I don't know).

I am dealing with the same topic, but so far I haven't felt like doing a core drilling in a young house.

Exhaust air is perhaps actually the most efficient and also powerful. but where does the supply air come from?

From holes in the building envelope.. if in doubt, through the gap under the front door (even if you don’t usually notice it). Showering is very punctual - a lot of throughput in a short time; the cubic meter amount is therefore very low overall.

Or smoke from the fireplace into the living room...

:D fair enough. But you would have that with an inverter too.

Our front door also has a seal at the bottom, no air gap. With enough negative pressure, air will eventually come in, but according to the BlowerDoor test, it has to be quite a bit.

from on the go