[benjamir]

* I wrote about CO2.

* The boiler's flame is visible (through a slit).

* The system doesn't have a separate supply stream -- it takes oxygen from the flat.

An CO sensor seems obligatory to me: after the chemney sweeper (obligatory in Germany) measured 10,000ppm CO in the exhaust stream (defect, not normal operation), I didn't want to depend on a 25y+ boiler's electronics alone.

CO kills relatively fast. Even if rare, I don't want to die caused by a rare event that could have been avoided by spending ~50€ (here)