Anything that has combustion will produce carbon monoxide. That includes gasoline or diesel motors, appliances that run on natural gas, propane, and fuel oil, fireplaces, woodburning, pellet and coal burning stoves, kerosene heaters, candles, and even smoking. People and animals exhale carbon monoxide. Granted, the last few are a minimal threat, but when houses are closed up tight for extended periods in cold weather, they have to be considered. My utility compay control center was primarily electric, but we also dispatched for the gas department, which meant I had to file reports to the Public Service Commissions for carbon monoxide situations, regardless of the cause. Over the years I can recall at least one due to a fireplace and one where a closed uo for cold weather house had several heavy smokers. The serviceman was required to test the entire premise in every case and the heavy smoking was, indeed, determined to be the only source of carbon monoxide. The residents had symtoms, but they weren't serious... yet. I know we like to be efficient and keep our houses draft-free when it's cold out, but a little fresh air is a good thing.
Unless your house is entirely electric and free of other sources, do have a CO detector. When it goes off, leave immediately and call for help.
CO poisoning is one of those things that I'm just terrified of. 
