Yes, any combustion will produce CO as a byproduct. Candles, fireplace, kerosene heater, the aforementioned water-heater, furnace, etc. And Carbon Monoxide is p

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.