450 ppm or less (I think). Window always open on tilt : better put - rarely if ever shut. The wind direction and open-window/door configuration is very important. The chimney effect is very noticeable
Sensor positioning appears to be important - I have found 'dead spots' in our house. As I write this at my desk (a dead spot) its 775 - with a 3m window wide open 4 meters away. Bedroom seems to be 'OK' to the extent that I can tell. The CO2 meter needs an a/c socket nearby, so the test hasn't been as extensive as I would like (flex length).
The only time I've noticed a 'red' alert ( on the meter) is when over Christmas the house was packed with friends and family, cooking on full blast and some people were vaping like it was going out of fashion.
I think that MVHR might well lessen the number and extent of dead spots. It'll be interesting to do a comparison.
The question is how to do a fair comparison?
Because for six or seven months of the year we have at least two windows open. Maybe @SteamyTea or @TerryE could help my thinking process with a 'fair' design ? Please?
The main point is I realise that our approach is not for everyone: most want to switch MVHR on - forget it - and pay the bill. But until recently we couldn't afford MVHR, and so have been forced to experiment and 'feel' what the house will do for itself.
To me, it seems to do better than I expected : slow temperature change up or down, and to an extent sauerkraut resistant. It is very clear though we have to open a combination of doors and windows every day. Whereas with MVHR we wouldn't need to think about it.