Carbon Monoxide and Home Inspections
by Martin Newmark
Copyright 12/2006

Carbon monoxide (CO) as you may know is an odorless, colorless, tasteless gas. In low concentrations its effect can cause no symptoms at all to headaches, fatigue, flu like symptoms, and others. In high concentrations it can cause confusion, severe headaches, brain damage, death, and again, others. Over two hundred people die each year in the US from CO poisoning from fuel burning appliances.

CO is created in the home by any appliance that burns a fuel. Examples are gas furnaces, gas water heaters, gas stoves, fireplaces -- both wood and gas, running cars in a garage, and gas dryers. The fact that these appliances and systems create CO is part of the reason that they are usually vented to the exterior. I say usually because there are some cases where they don’t have to be vented to the exterior such as gas stoves and some other “ventless” appliances.

During an inspection neither NAHI nor ASHI require the use of a CO detector. However, many inspectors like myself do use them. You don’t need a CO detector to spot potential CO problems, though. Backdrafting of water heaters or older furnaces can be detected with an inspection mirror. Backdrafting is when combustion products (exhaust from the appliance) don’t go up the flue but instead escape into the indoor air. Other signs of CO are yellow in the flames of the appliance and a strong odor. The strong odor won’t be from the CO but from other byproducts of incomplete combustion. If any of these symptoms are observed, a qualified service professional should be called to investigate and correct any problems.

For those of us inspectors who do use CO detectors, what do we look for? Industry standards guide us in this area. The EPA states that a person should not breath 9 ppm (Parts Per Million) or more of CO over an 8 hour period or 35 ppm or more over an hour. The EPA also states that CO levels near properly adjusted gas stoves are often 5 to 15 ppm and levels near poorly adjusted stoves may be 30 ppm or higher. The AGA (American Gas Association) specifies the maximum allowable concentration of CO in the flue of a gas burning appliance is 400 ppm.

One other benchmark that should be used is comparison to  the levels of CO we normally see in other homes we inspect. Putting the standards mentioned above together with what we see in practice, gives us guidelines for when to recommend the services of a qualified service technician. Here’s an example. Of the furnaces I am able to check the CO levels in the flue (many flues these days are not readily accessible), most have levels around 12-20 ppm. This indicates a pretty clean burning furnace in the Denver area. While the AGA limit for CO levels in a flue is 400 ppm, if I see a level of 50 ppm I conclude that it is out of the norm. This suggests to me that the unit should be serviced by a qualified professional.

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