I think that a lot of people are far more concerned about the threat of deadly carbon monoxide coming from natural gas furnaces than they should be, because an 80+ AFUE or a 90+ AFUE furnace that is working properly doesn't really produce any carbon monoxide. In fact, when mom (or dad) is cooking a big dinner on a gas range with all the burners and the oven running, the results (and the danger to the indoor air quality) could be the same as operating a furnace with no outdoor venting at all.
What such furnaces do produce (like gas ranges and ovens) is a lot of carbon dioxide and water vapor. You have likely noted all the moisture on your windows from the cooking process on Thanksgiving Day. Much of this comes from the gas combustion process. And this is why more people aren't killed by all the thousands of furnaces that continue to operate throughout North America with unknown holes in their heat exchangers.
Now, don't get me wrong. I'm not trying to encourage anyone to forego repairs to defective furnace vents or heat exchangers, because furnaces do produce significant amounts of carbon dioxide, which tends to make people drowsy (do you ever get sleepy on Thanksgiving?). This is because CO2 displaces the required amount of oxygen in the dwelling. And if the carbon dioxide levels get too high around a furnace (or any other gas-combustion device), an incomplete burn can result, which creates carbon monoxide and aldehydes that can become deadly. So, any venting problems should be immediately corrected.
However, many people have been sold on the idea today of installing carbon monoxide detectors in their homes, to detect any possible dangers from gas venting problems. Well, as you can see, a carbon dioxide detector would be a much better idea, because too much carbon dioxide would be detected long before the presence of carbon monoxide should become a problem. Yet, there are some situations where carbon monoxide would be created without much CO2, and everyone should be aware of this. It's where a furnace has been tampered with and its design has been altered, or where someone has deliberately cut off its combustion-air supply.
Recognize this: Virtually no carbon monoxide or any soot comes from a blue flame. But when a flame is yellow or has yellow tips (other than from dust particles), soot, carbon monoxide, and aldehydes are likely being produced. Also, you can't always see if a yellow flame is present, once you have opened an air source to a furnace (such as a closet door to the unit). However, the presence of any soot at all in the vent or heat exchanger indicates the creation of carbon monoxide.
Carbon monoxide isn't very likely in the natural-gas combustion process, because there isn't much carbon in natural gas. It is one part carbon and four parts hydrogen, so, complete combustion of each molecule of natural gas creates two molecules of water vapor and one molecule of carbon dioxide. This isn't true of other heating gases (such as propane and butane) and fuel oil, because they all have a higher percentage of carbon molecules. And, the higher this percentage, the more likely the production of CO2 and the danger of creating deadly CO (carbon monoxide) becomes.
Who would be foolish enough to alter a furnace or cut off its combustion air supply? You would be surprised. Right offhand I can think of one job that I looked at in Arkansas several years ago, where someone had installed a furnace in an enclosed closet, then walled it off with a tight covering. Fortunately, the furnace failed due to a soot buildup before it killed anyone, and that's why I was called. I have also seen several jobs where homeowners, in a desire to improve their home's energy efficiency, have completely covered heating closets and cut off the combustion air supply.
On the other side of the coin, there is the problem of where combustion gas is being recirculated to the combustion air supply. As an example, a furnace may be situated in a small space that has provision for an air intake, but the vent pipe fails or a chimney becomes stopped, which causes the vent gases to be recirculated, starving the burner of its combustion oxygen. However, as you can see, most of these problems can be prevented by following basic gas codes. And the pressure switches on most new gas furnaces will detect most of these problems and shut them down before any deadly gases are created. Yet, the real problem may go overlooked by service technicians who are looking for a reason why the unit isn't operating and who may be tempted to override the safety switch. When that happens, the result can be fatal. <<