A tiny amount of CO is very bad, period. An increase of CO2 and decrease of O2 is bad, but not deadly or quick, without you feeling it. Our bodies have no clue how much oxygen they get, which is why you die almost instantly in an inert gas atmosphere, without feeling it. We detect CO2, so if it gets high, you will feel like you're suffocating in your own house. Unvented indoor heat will deplete oxygen eventually without some exchange to the outside.
I have no idea what the practical way to do this is, just adding the biology of it.