There are many common misconceptions about furnaces, water heaters, and carbon monoxide that I hear repeated on a daily basis, and I'd like to clear a few of them up.
False: Carbon Monoxide is also called CO2. Carbon Monoxide is CO. Carbon Dioxide is CO2. (Mono = 1, Di = 2)
False: Cracked heat exchangers create CO. CO is caused by incomplete combustion, period. A cracked heat exchanger does not create CO. A heat exchanger is the part of a furnace that transfers heat from the flames to the household air. A functional heat exchanger keeps the household air and the combustion gases completely separate from each other. If a furnace has a cracked heat exchanger, the combustion gases can mix with the household air. It's usually just a little bit, but this is still unacceptable, and it means the furnace or heat exchanger should be replaced. The photos below show cracks in heat exchangers (click the photos for full-sized images).
False: Cracked heat exchangers can be fixed. They can't be fixed. The heat exchanger or entire furnace needs to be replaced.
False: High CO levels = cracked heat exchanger. See above. We test the CO levels in the flue gas, which has nothing to do with a cracked heat exchanger. Heat exchangers fail when the metal rusts through or when it cracks. CO does not cause this.
False: High CO levels in the flue gas mean the furnace is leaking CO. If there is a high level of CO in the flue gas, there is a potential for the exhaust gases to mix with the household air, or ‘leak'. One way would be for the exhaust gases to backdraft, which means that instead of rising up and out of the house, they come back down the flue. The other way would be because of a cracked heat exchanger. If we find high levels of CO in the flue gas, we recommend immediate repair - it doesn't matter if the gases are mixing with the household air at the time of the inspection or not, because this condition could potentially change at any time. Higher CO levels can often be fixed.
False: Backdrafting at a furnace or water heater means CO is coming in to the home. Backdrafting means that exhaust gases are spilling back in to the home, rather than going up the flue. A properly functioning water heater or furnace will not create CO, so you can't say CO is coming in to the home unless you test the exhaust gases; we do this at every inspection. Click here for a news clip me testing a furnace. While backdrafting doesn't mean CO is coming in to the home, this is still a potentially hazardous situation that requires immediate correction. Backdrafting has the potential to allow CO in to the home, and will always contain CO2 (carbon dioxide), which can cause sickness and headaches in higher concentrations.
Wrong Term: Hot water heater. Just 'water heater'. The heated water that comes out is hot.
To summarize, high levels of CO need to be fixed, cracked heat exchangers need replacement, and backdrafting is never ok. These three things are all independent, but a combination of these conditions is especially dangerous. When using these terms, make sure you have them correct. It makes a difference.
Reuben Saltzman, Structure Tech Home Inspections