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.

Carbon Monoxide Detector

Carbon Monoxide Detector

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

What 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.
By Charles Buell, Real Estate Inspections in Seattle

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  1. For information pertaining to low level CO detectors and where to obtain them see:

  2. What timing! I just wrote a post on this same subject yesterday afternoon, which I’m planning to post on Tuesday. I’ll be sure to link to this post for another great angle on the same subject.

  3. Robert Frank says

    In the “old” days, carbon monoxide detectors would go off if the level was over 30ppp. Then fire departments made a fuss that there were too many false alarms. Well they weren’t “false ” alarms, they were transient conditions. So it was changed so that there had to be 8 continuous hours over 30ppm at lowish levels of CO for the alarm to go off. If 7 hours and 58 minutes at 45ppm, and went under 30ppm for 1 minute, it would take a further continuous 8 hours above 30ppmThe digital detectors would still show the level of CO (for example, 25ppm). Now, no alarm unless over 50 PPM for over 8 continuous hours and no digital reading unless over 50ppm for shorter term. It frustrates me when they dumb down a serious safety concern like this.
    Charles, feel free to post this at another site if you want to.

  4. CO FROM AN OLD OVEN???? I am intrigued by this discussion. Some friends have tenants who bake and cook a lot and sit in the kitchen with their 2 year old daughter while doing so. They were experiencing headaches. They brought the CO detector from another room into the kitchen. It went off. They called the fire department which detected CO coming from the oven and disconnected the gas to the stove/oven.

    The landlord called an appliance repairman to fix the leak/emissions and reconnect the stove/oven. The repairman said in fact, there were no CO emissions from incomplete combustion, but that the metal panel at the bottom of the oven in an old stove can give off CO when it is hot. He also said old pots and pans in the drawer below the oven can give off CO when the oven is hot.

    He replaced this metal panel and retested the oven to find no dangerous levels of CO emissions. He said this is a common problem that most people do not know about.

    This sounds plausible to me, that old metal or enamel might give off CO when heated. However, I would be interested to hear opinions from this obviously august discusion group about whether this makes sense chemically or otherwise.

    The repairman did reconect the gas after replacing the metal panel and as far as I know, the CO detectors have not gone off since. Comments????

    • Charles Buell says

      Sounds pretty strange to me

    • Greg Redmond says

      That repairman is not giving out good information. Incomplete combustion IS the source of CO. You can certainly get higher levels of CO than normal when cooking under certain conditions, eg on initial start up if flame is hitting relatively cool metal, several large pots on the stovetop with burners turned up, burners and/or jets that need to be cleaned, etc

      or basically what Robert Frank said.

  5. Robert Frank says

    Carbon Monoxide is typically increased when there is impingement on the flame, such as a pan can do on the burner or in the oven. That is why gas logs cannot be installed willy-nilly, but need to be placed in exactly the configuration the manufacturer recommends. Misplace logs can cause a huge jump in CO due to excessive impingement.

  6. Joel Potrykus says

    I am the maintenance manager of an HVAC company in SouthEastern Michigan and I’ve taken a couple classes specifically speaking on these same topics, and it is amazing how many people are unaware of the differences and the proper use of their CO detectors. Thank you for spreading the word and I am definitely subscribing for future information.


  1. "If there's a CO problem in my home, why didn't the CO alarms go off?" | Structure Tech Home Inspections says:

    […] off – long enough for you to start experiencing symptoms of carbon monoxide poisoning.  Seattle home inspector Charles Buell just wrote a post on this topic, where he tells the story of how a kitchen stove was producing CO […]

  2. […] off – long enough for you to start experiencing symptoms of carbon monoxide poisoning.  Seattle home inspector Charles Buell just wrote a post on this topic, where he tells the story of how a kitchen stove was producing CO […]

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