Isn't it a carbon monoxide (CO, not CO2) detector that you need? CO is more dangerous than CO2, though I do have a pretty table in front of me listing the effects (e.g. coma and death) of increased CO2 concentrations ...
All this talk of furnaces, boilers and heat pumps is music to my ears xD I'm a chemical engineer, you see, so these sort of thermodynamic cycles are fun!
What exactly is a household 'furnace', though? Is it just the same thing as a boiler, or what? When I think of furnaces, I think of a) blacksmiths, metal-working and forges or b) power stations.
If you believe everything you read, better not read.