[franciscop]

What people definitely don't give enough importance is CO2 buildup as well. The more isolated a place is, and smaller, the more environment friendly it is BUT the more likely it is to build up CO2, which leads to headaches.

I installed a CO2 meter both at my current home, and at my parent's home for when I go back for Christmas. In my home, it's a fairly big open place so it takes ~2 days of closed windows (0-3C outside) to reach 2000PPM (recommended under 1000, above 2000 starts affecting you, 5000 is the legal limit[1]). However, at my family home where I grew up it's a tiny room and it reaches 3000-4000 just by sleeping there with the window and door closed. So the headaches of "visiting family" might in big part be explained by this.

PS, incidentally in Spain!

[1] https://www.kane.co.uk/knowledge-centre/what-are-safe-levels...

[Arbalest]

These are starting to catch on, as someone else said Heat Exchanger, but that's a more general concept and this is the thing you'd actually want https://en.wikipedia.org/wiki/Energy_recovery_ventilation

[shostack]

I suspect I'm experiencing some of this in a small home with poor circulation. Any suggestions for inexpensive room monitoring sensors?

[franciscop]

I just bought one on my local Amazon version by searching "CO2 meter". While true numerical accuracy is "who knows", on rest/full ventilation it shows 400PPM and blowing at it (even gently) shows the CO2 increases drastically, as expected, so I'm happy with it.

[hazza_n_dazza]

that is why you need a Heat Exchanger