Carbon monoxide is a by-product of combustion and can come from defective, improperly installed or worn out appliances or flues. Wood burning devices can be a source of CO; however the most common suspects are oil, propane and natural gas appliances: water heaters, gas dryers, gas and oil furnaces, gas fireplaces. One time I had a fellow call me, panicked that he might have a CO problem. Ends up that he had no wood burning devices and he had an electric hot water heater, an electric range and dryer, electric baseboard heaters. That was one individual who did not have to worry much about CO levels -- unless he was sneaking a barbecue in the house during cold weather.
Personally, and it is beyond the standards of a regular home inspection, I run a carbon monoxide test with a sophisticated meter if I have a concern about an appliance -- usually an older furnace, sometimes a water heater. By the way, most gas ranges, as the burners are being lit, put out CO -- hence the strong recommendation for a range hood over gas ranges. I will hold the CO detector in different locations but I was taught by an HVAC professional that one of the smartest things to do is to put the meter on a heat supply register that is near the furnace. That best simulates CO exposure into the home. For your information here are a few key CO levels, and what to expect from them, listed in parts per million, the measurement that is used:
9ppm..... maximum allowable concentration for short term exposure in a living area
35ppm....maximum allowable concentration for continuous exposure, over 8 hours, in industry
200ppm... maximum concentration allowable in a 15 minute period. Likely to cause headaches, nausea after a couple hours.
400ppm....headaches in a couple hours, life threatening after 3 hours. This is the maximum allowable CO in flue gas, so you can see why you do not want a leaky flue at the furnace or water heater.
12,800ppm...okay, there are lots of numbers from 400ppm up to this point, but they are all really bad for you after about 150 ppm. At 12,800 levels approach instant death -- in 1 to 3 minutes.
Different home inspectors have different ways of looking at this issue. Some do not have a CO detector and some do not want one. Personally, if I have a fear of a furnace or installation, I feel better knowing I put the CO meter on it. Obviously, if readings are high -- such as the furnace in the photo below -- you better believe I warn that an HVAC pro should be called in prior to running the furnace at all. Actually, if I do not like the condition or age of the furnace, even if CO levels are normal, I still recommend that an HVAC technician evaluate and service the furnace and the heat exchanger. I guess that I run the CO test for my own peace of mind. At least I know that, even if the client puts off the HVAC service, he or she will not be exposed to dangerous levels of CO. I do not want a client going into a permanent sleep, the result of logistic issues. Other inspectors, who choose not to use a CO meter,will normally recommend HVAC service, as I described above. They just do not do the test.
Safety: I think that, unless a person lives in a home with all electric heat and the appropriate electric appliances, the new, affordable CO alarms should be installed. These emergency "dummy alarm" devices for consumers are not at all the same as the meter in the photo. It takes higher levels of CO to alarm them. They really are an emergency device. When they go off, take it seriously, the device has either detected significant levels of CO, or it has gone on the fritz. As an example, a basic CO alarm, unlike an expensive CO meter, has a time function tied to it as it reads the level of the concentration. Under say 100 ppm the alarm might not alert for several minutes, yet if it detects 400 ppm it will sound more quickly. This gauging of the level of CO vs time is intended to simulate the absorption of CO into the body, without causing false alarms due to smokers, gas burners and that kind of thing. For more detailed information on CO, please visit this site.
SOS --The lights and alarms are sounding. 145ppm, way too high
Thanks for dropping by.
Bellingham WA home inspector
Steven L. Smith