When the house is closed up for a long time the inside air gets 'stale' .. link after months in the winter.
But just opening a window to get 'fresh air' might work .. it also let out allot of heat.
When all that is needed is the fresh air .. it is ideal to keep as much of the heat as possible.
OEM made heat recovery ventilation (HRV) units do this .. you get fresh air in .. because heat naturally flows from to cold the colder outside air will get warmed up some by the hotter inside air going out.
Never 100% efficient .. but something is better than nothing.
Of course off the shelf OEM units are expensive .. so this is a DIY version I made out of a piece of fluted (corrugated) polycarbonate sheet.
When ~67F inside and ~42F outside ~96% of heat retained.
When ~69F inside and ~17F outside ~79% of heat retained.
Fabrication Video
https://youtu.be/_kJB9LPhZDs
Results / Performance:
https://youtu.be/fCgf1lCK4qQ