Seattle Home Inspector's Blog


So you think you know everything there is to know about CO detectors?


In recent years the push to install CO detectors in homes has resulted in them being required in many jurisdictions and certainly any jurisdiction that has adopted the 2009 or later IRC Building Codes will require them in New Construction.  Washington State currently requires them on each floor level and in the vicinity of each sleeping area when a home is sold.  In most homes this means there will typically be two of the devices but with large sprawling homes with basements and/or multiple stories there could be several more.

CO detectors for residential construction must meet the requirements of UL 2034. 

CO DetectorWhat is not commonly understood about these detectors and UL 2034 is that they are “not intended to alarm when exposed to long-term, low-level carbon monoxide exposures or slightly higher short-term transient carbon monoxide exposures, possibly caused by air pollution and/or properly installed/maintained fuel fired appliances and fireplaces.”

(please reread the previous quote and the information below VERY carefully as it is counter-intuitive)

Following these standards, the alarms are: 1, not “allowed” to alarm when CO is lower than 50 parts per million; 2, they are required to alarm within 50 minutes at levels up to 150 PPM; and, 3, they are required to alarm within 15 minutes at Carbon Monoxide levels up to 400 PPM. 

What the standards do not address is the fact that some individuals are greatly affected by being exposed to lower levels of CO over a longer period of time.

The Kidde user’s guide states: “While anyone is susceptible, experts agree that unborn babies, small children, senior citizens and people with heart or respiratory problems are especially vulnerable to CO and are at the greatest risk for death or serious injury.


They go on to state: CO alarms provide early warning of the presence of carbon monoxide, usually before a healthy adult would experience symptoms.

So does this mean that those most in need of protection are not in fact protected?

Other alarm makers have similar recommendations.

This next caution from Kidde is very important, as I can attest to, from an incident at a recent inspection:  “CAUTION: THIS ALARM WILL ONLY INDICATE THE PRESENCE OF CO GAS AT THE SENSOR. CO MAY BE PRESENT IN OTHER AREAS.”

When I turned on the oven at the inspection and it had been operating for just a couple of minutes, I started to experience some of the symptoms of CO poisoning.  (Once you have experienced these symptoms you can almost become your own CO detector---but unfortunately human beings sleep, and then there is the problem that you might not notice very low levels of CO and thus be no more effective than UL 2034.)  My “real” CO detector found over 600 PPM and yet the CO detector plugged in at the countertop on the other side of the kitchen did not go off and would not (under UL 2034) be required to go off until the unit experienced 400 ppm for as much as 15 minutes.  Depending on air currents in the home, the unit might never see appropriate levels---even after the person using the stove succumbed to the gas.

The biggest concern that I have with all of this information is: “Who reads the instructions?”  Are we creating an awareness of CO detectors where people are assuming they are protected when they in fact are not?  While I still think installation of these alarms is probably a good idea, and that perhaps more appropriate residential CO detectors may be forthcoming, I think it is at least as important to better educate people to recognize the symptoms associated with Co poisoning as well as to be aware of what these detectors do and don't do.

Again from Kidde: “Be aware of the symptoms of carbon monoxide poisoning: – headaches, dizziness, weakness, sleepiness, nausea, vomiting, confusion and disorientation.

Recognize that CO poisoning may be the cause when family members suffer from flu-like symptoms that don’t disappear but improve when they leave home for extended periods of time.”

Carbon Monoxide in homes is a serious issue, and legislation/codes do not adequately address all the concerns associated with it.  It is in all likelihood dangerous to assume that they do---if we and our families are to be safe in our homes.  This is especially true as homes become tighter in relation to becoming more energy efficient.


Charles Buell, Real Estate Sales in Seattle




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Comment balloon 13 commentsCharles Buell • January 07 2013 06:16AM


Great info Charles...I know I don't necessarily read the instructions for these...but , I know now!

Posted by Kristin Johnston - REALTOR®, Giving Back With Each Home Sold! (RE/MAX Realty Center ) about 6 years ago

My parents, now in their upper eighty's, always sleep with the bedroom window open, about an inch, even in winter. Fresh air is a great defense against CO and indoor pollutants.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) about 6 years ago

Wow! This information makes my head swim...and not from CO poisoning.  It seems that these monitors can give a false sense of security.

Posted by Kathryn Maguire, Serving Chesapeake, Norfolk, VA Beach ( (757) 560-0881) about 6 years ago

Charles -- the other thing that most people do not realize about CO monitors is that they have to be REPLACED every 6-8 years.  Original one we had in house accepted battery (sensor) replacement during that period, but then they had changed design and battery(sensor) unit was no longer supported.  When I checked with the company, they told me ALL CO monitors require replacement at regular intervals.

Posted by Steven Cook (No Longer Processing Mortgages.) about 6 years ago

Kristin, so many of these very technical safety devices come with cryptic manuals loaded with fine print that nobody reads---let alone all the kids in the house---or how about rentals?

Robert, it really is---and I wish I was as smart and as hardy as your parents :)

Kathryn, that is my big concern with them.

Steven, and that is a little bit sooner than with smoke alarms---they need to be replaced every 10 years.  They are now required to be "date stamped"---but who is going to look?

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) about 6 years ago

I actually am learning more and more each day about carbon monoxide detectors. I had a faulty one installed in my house by Comcast with my Security System... 

Posted by Paul S. Henderson, REALTOR®, CRS, Tacoma Washington Agent/Broker & Market Authority! (RE/MAX Northwest.) about 6 years ago

When I was about 12 or so, we had a propane water heater that wasn't properly vented.  When the wind was right, it wouldn't vent all the exhaust gases (it didn't have a blower).  We had a CO detector, but it never went off, at least until one night when my younger sister and I were home alone.  My grandparents came over (their house was just across the yard) and opened the windows and we went to their house.  Afterwords, my mom (who stayed in the house most of the winter) said she complained of headaches all the time and that was probably the cause.  My dad, sister and I, who were in the house much less, didn't have any symptoms.  It wasn't until it got high enough that it went off.  I always get CO detectors that have a digital indicator now.

Posted by Wyatt about 6 years ago
Charles, this is really important....and sobering information! I share your concern that lots of people may be feeling WAY more protected than is warranted. Thanks for giving me some valuable safety info to pass along to clients (and friends).
Posted by Nancy Conner, Olympia/Thurston County WA about 6 years ago

I've seen them work very well and I've seen them fail. I replace mine every 4 me paranoid. Smoke detectors are worse, I just had to replace 3 in my house that were only 3 years old. Not sure what you can and can't trust any more when it come to the gadgets. 

Posted by Suesan Jenifer Therriault, "Inspecting every purchase as if it were my own". (JTHIS-Professional Home Inspection Team) about 6 years ago

Paul, I think there are many that are unaware of what these devices do and cannot do.

Wyatt, this is a familiar story I am sure.

Nancy, thanks---just trying to get the word out

Suesan, of course you are correct about these devices.  I think they can create a false sense of security.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) about 6 years ago

Near our furnace and water heater I have the exact detector that you have pictured.  I have a plug-in Kidde upstairs near the bedrooms.  And I have experienced personal poisoning, as a Boy Scout, riding to an overnight trip in a car with a hole in the floor!  By the time I got there I was sick, very sick, and it lasted all night and into the next day.  It took two days to recover.  Everyone assumed it was flu, but I now realize what it was.

You are so right about the long-term, low-dose effects.  But for now, what can we do?

Obviously the manufacturers are aware of this.  Perhaps they will work on the technology and make improvements.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) about 6 years ago

Excellent post, Charles.  I just posted a blog here on AR with the exact same message, looking at it from a slightly different angle.  

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 6 years ago

Jay, you can certainly get "sensitized" to it so that you can develop an ability to dectect it---even if you can't smell it.  Once you have been made really sick from it it can certainly take a while to clear the sytstem.  So did you get your CO badge? :)

Rueben, thanks---I saw it when it came out on your website---they make good companion pieces. :)  I think we will be hearing a lot more about this issue as public awareness increases.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) about 6 years ago