[Mromson]

Personally I've addressed that concern with a CO2 alarm in every room (which you should have regardless, so I don't count that against passive houses). I suspect that there exist ventilation fallback solutions that can address a power failure scenario, but I've yet to find any viable ones.

It should be possible to have a system that generates automatic airflow should power fail.

Best ones I've found are solar panels, or geothermal power generator (geothermal is generally really amazing for everything), but that still leaves mechanical failure.

That said, it takes quite a bit of time before ventilation failure leads to a dangerous CO2 buildup. And the addition of having a single entry point for air means that it's easy to filter my air through a HEPA filter, ensuring top quality air all year round.