My grandparents in Louisiana heated their home with natural gas space heaters in all the rooms of their home. I loved them because fires are just so fun! (I'm not going to tell you about that time my friend Bubba and I caught a field on fire, and the fire department had to come and put it out. Nor will I tell you about the fire that the two of us set in my mom's clothes dryer. That's really not relevant to this story at all.)
These atmospheric combustion appliances do create problems in homes from a building science perspective, but they don't have the same type of problems that I wrote about with atmospheric combustion furnaces and water heaters a couple of weeks ago. The major problem is one that most people may not even recognize.
Yes, carbon monoxide inside the home is certainly a danger, and that's the best reason to get rid of unvented gas space heaters. It's just never a good idea to burn a fuel inside your home without having a vent for the combustion gases.
Carbon monoxide results from incomplete combustion of natural gas. During complete combustion, natural gas, which is mostly methane (CH4), combines with oxygen (O2) from the air and produces carbon dioxide (CO2) and water vapor (H2O). When the combustion process isn't complete, you get more carbon monoxide (CO) and less carbon dioxide.
The often unrecognized problem with these heaters, though, is the huge amount of water vapor that they put in your home's air. In my grandparents' home, I think the gas space heaters worked well most of the time and didn't produce much CO. They did produce a lot of water vapor, however.
About 5 or 6 years ago, I got a call from someone who wanted me to come look at his grandmother's home. I knew as soon as I walked up to the front porch that they had a big problem. They had the front door open, and the storm door had so much condensation on the inside of the glass that it was dripping down. Inside the house, microbial infestations were growing on the walls and ceiling in the back bedrooms. (I wish I'd taken a photo of that door but unfortunately didn't think to do that.)
The problem was that she was heating the whole house, which was probably only 1000 square feet or so, with two unvented natural gas space heaters. Because the house was poorly insulated and air sealed, there were plenty of condensing surfaces for that water vapor, the front door being the most obvious. When you put water in materials that act as food (the paper on drywall, for example), the spores, which are everywhere, will activate.
If you - or your grandmother - have unvented natural gas space heaters, get rid of them right away. They're not good for homes, even when they're operating perfectly.
Photo by warrenski from flickr.com.