Hi Mirar,
Sorry for my very late reply 
unfortunately I don’t have my test setup running anymore, so I can’t easily do that test.
The MH-Z14a was indeed the only “real” co2 sensor. Since it gave me the most consistent results, I eventually wend with that sensor and created a board for it:
!
On top of the board the mh-z14a can be placed:
and together they can fit in this housing:
Unfortunately the thing still has a few flaws:
- The co2 sensor stops reporting values after a while. Sometimes after a hour, sometimes after several days
- the si7021 sensor I combined it with to report humidity and temperature is giving much too high temperature values, possibly because of the proximity to the ESP8266