[mkeespiet]

Can come CO from candles as well? We do not have any flames inside as we cook with induction, we only use candles as indoor flames. Nice fact: Here in the Netherlands a fire alarm will be obligated starting from this summer

[mindcrime]

Yes, any combustion will produce CO as a byproduct. Candles, fireplace, kerosene heater, the aforementioned water-heater, furnace, etc. And Carbon Monoxide is particularly deadly because it's a "chemical asphyxiant" meaning it binds more readily with the hemoglobin in your blood than Oxygen does.