[GistNoesis]

Here OpenERV use a push-pull ventilation design where air direction is reversed every 30s. This allows energy recuperation and dispense connecting the inlet and the outlet to each other, as each ventilation port alternate role simultaneously.

The alternative design is a counter-flow heat exchanger. Using 3d printing and gyroids it seems possible to build quite compact ones. (metal 3d printed heat exchanger for helicopter https://www.youtube.com/watch?v=1qifd3yn9S0 )

3d-printing a counter-flowing heat-exchanger seems interesting but maybe there are some molding issues that need to be taken care of (maybe HEPA filters on the inside in/outlet are sufficient).

The main advantage of the heat-exchanger solution is that you won't need specific electronic control and can reuse the standard fans for controlled ventilation, but there is more thermally isolated piping required (and the pipes are quite big (~10cm diameter) because they need to move a lot of air even if the fans are weak).

The push-pull system is harder to DIY because most of the off-the shelf fans can't be reversed easily (and 3d printed fans are noisy and inefficient).