Depends on how old the furnace/boiler is.
Originally Posted by Worker 11811
I don't know about the regulations in Canada and the US but here, a boiler will either be a balanced flue or a fan assisted flue. In both cases, the flue is two co-axial pipes. Fresh air is drawn in around the outside and hot flue gases escape via the inside pipe.
The whole of the combustion area is sealed from the room so if the gas is burning incorrectly and putting out CO or CO2, it will all be vented and will not enter the room.
Also, any combustable gases within the room will not come into contact with the burner so will not ignite.
However, if you have an old burner/furnace then this might not be the case.