It's winter and it's cold outside. To keep us warm inside, we use heaters. The furnaces our Barths came with waste half the propane they burn.

An unvented heater of some sort is used by many. These can consume oxygen and produce carbon monoxide. They come with instructions telling you to have so much ventilation open. Several square inches per 1000 BTUs, depending on manufacturer. Quite often, that might be way too much, especially with the original ceiling vents, which leak a lot when closed.

If you use a CO detector with a digital display, like an Atwood, you can monitor it and adjust the venting accordingly. Not too much and not too little.

Another advantage of monitoring the CO is that concentrations below the alarm point can be harmful. Here is a site to that effect. More on request.

http://www.bacharach-training.com/CO%20Zone1/What%20is%20Carbon%20Monoxide.htm

Your CO detector might be too old to be reliable. Here is a site with information:

http://www.safe-t-alert.com/press_releases.asp?news_id=59&news_class=press

Some heaters have oxygen depletion sensors that will shut them off before you die.

I am such a safety nut that I will not have any heater or furnace running whil I sleep.