To be clear I did not mean HVAC, which requires (some) energy in itself.
At home I do not have HVAC. However, the outbound air passes through a rotating metal structure that sucks up the heat from outbound air and releases it into the colder inbound air. Just using the temperature gradient that is there naturally. Passive energy transfer.
Same principle can be used with hot water (e.g. from showers).
Then one can put HVAC on top of this for active warming.