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Building Science

A UL-Listed Carbon Monoxide Alarm May Not Protect You

Why you probably need a low-level CO monitor

A low-level carbon monoxide monitor from CO Experts. The new models have a different case, but work on the same principles.
Image Credit: Energy Vanguard

Don’t judge a book by its cover? That certainly applies to what may be the best protection against carbon monoxide poisoning you can buy. The two best carbon monoxide monitors, the CO Experts monitor and the NSI 3000 from the National Comfort Institute, don’t have the approval from Underwriters Laboratories (UL) that so many manufacturers crave. There’s a good reason for that.

The CO Experts and NSI 3000 monitors are low-level monitors that tell you what’s going on with the carbon monoxide levels in your home in real time. They cannot get listed by UL because UL has decided that only high levels matter, apparently to reduce the risk of ‘nuisance’ alarms. Here’s a quote from UL standard 2034, as given on the CO Experts website:

Carbon monoxide alarms covered by this standard 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.

Are low levels of carbon monoxide safe?

Because UL-2034 mandates that CO alarms remain silent during “long term, low level carbon monoxide exposures,” you may logically conclude that low levels of carbon monoxide are safe. Is that true? Not according to George Kerr, the brains—and the passion—behind CO Experts. He cites numerous studies showing that low levels of CO are harmful. In fact, problems show up even at really low levels. A UCLA study found that CO levels above 5 parts per million (ppm) was associated with pregnant women having underweight babies with smaller heads.

David Richardson, a combustion safety trainer for the National Comfort Institute, compares the protection you get from a UL-labeled CO alarm to a home security system. “If you had a home security system that alerted at comparable levels the intruder could already be in your home, standing in your bedroom door while you slept before the alarm went off,” he wrote in an article on CO alarms last year.

The existence of the studies quoted by Kerr probably helps to explain the disclaimers that companies making UL listed CO alarms put on their products. Here’s the warning from the Kidde-Nighthawk Model #KN-COPP-3, as given in the user manual I downloaded from the Kidde website:

You should take extra precautions to protect high risk persons from CO exposure because they may experience ill effects from carbon monoxide at levels that would not ordinarily affect a healthy adult. Are there any infants or small children in the home? Be sure to check them for signs of possible CO poisoning because they might have trouble explaining their symptoms. Infants and children are more susceptible to CO poisoning than a healthy adult.

Pregnant women should be aware that their unborn fetus could be harmed by exposure to carbon monoxide, even when the mother suffers no ill effect herself. Any pregnant woman who suspects she may have been exposed to carbon monoxide should immediately contact her physician.

Is there anyone in the household who is elderly, or who has anemia, heart disease or respiratory problems, emphysema or chronic bronchitis? These individuals are at higher risk for CO poisoning and for health problems from exposure to low levels of carbon monoxide.

Basically, the problem with UL listed alarms is that they’re meant to offer protection to healthy adults during very high levels of CO in a home’s air. So how much CO does a UL listed alarm allow you to breathe?

  • 30 ppm for up to 30 days
  • 70 ppm for up to 4 hours
  • 150 ppm for up to 50 minutes
  • 400 ppm for up to 15 minutes

If you think about what those numbers mean, it’s actually worse than it looks. For example, as Kerr points out on the CO Experts site, a UL listed CO alarm would allow you to breathe air with 358 ppm for 45 minutes—with NO alarm at all. I don’t know about you, but I want a lot more protection than that.

The CO Experts low level CO monitor starts giving you warnings when the CO levels it detects are above 6 ppm. As the CO levels rise, the warnings go from visual to auditory and increase in frequency. The highest level of alarm occurs when it reaches 70 ppm, when it gives you a series of beeps every 6 seconds. The NSI 3000 is similar, giving you a visual readout for levels as low as 5 ppm and beginning auditory alarms at 15 ppm.

Don’t judge a book by its cover

Back to that old aphorism I opened the article with, there are two reasons you might rush to judgment about the CO Experts monitor. I’ve already mentioned one: No UL listing. I hope I’ve convinced you already that that’s not a valid reason. The problem isn’t that this low level monitor isn’t UL listed. The problem is with the requirements for the UL listing.

The second may already be apparent to you if you clicked any of the links above: The CO Experts website is a complete mess. George Kerr is knowledgeable and passionate, and if you spend any time talking with him about this issue, you’ll be convinced that you and everyone you know needs a low level CO monitor. I met him in 2005 at the Affordable Comfort conference. The problem is that Kerr is also an engineer, and his website shows that. It’s full of great information presented poorly. It’s one of those ’90s websites that somehow has survived a dozen years into the new millenium.

Don’t be put off by the website appearance, though. This CO monitor is the real deal.

Where can you buy one?

Pretty much everyone I know in the home performance industry swears by these low-level CO monitors. You probably won’t find them in stores, though. You can buy CO Experts through Kerr’s website if you’re a professional (HVAC, firefighter, energy auditor…), and you can also find them online in various places. The NSI 3000 is only available through pros certified by the National Comfort Institute. Again, an Internet search may be the easiest way to find one, but you can try entering your zip code into their locator.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. bob_kreger_aia | | #1

    Where can you buy one?
    I'd like to buy one of the other low-level CO detector. I've attempted to contact Kerr asking for a discount since I am a licensed architect and licensed builder specialized in high-performance residential. No response. I searched for the NSI units online - all sites want me too call in the purchase - I can do that but why are these so difficult to acquire vs. buying online without human interface. I understand why they are so expensive: probably built in low volumes.

  2. GBA Editor
  3. bob_kreger_aia | | #3

    Where can you buy one?
    Martin: Thanks for these links! The last one is a broken link but I did visit the home link for - very useful.

  4. [email protected] | | #4

    tester vs monitor
    found this unit online, available from amazon, same price as the CO Experts unit, but more of a hand held roving device:


    very important distinction
    In many communities you can call the fire department to come by and check your indoor air for carbon monoxide at no cost. If you have a concern (or a friend suffering from flu-like symptoms that look suspiciously like carbon monoxide poisoning) you might want to call the fire department first and order on-line later.

  6. BobHr | | #6

    The CO experts alarm is


    The CO experts alarm is just that an alarm you plug in and is constantly monitors the area. 24/7/365. The hand held unit does not have an alarm. It does have a few lights and will read low level CO2. If you look at the very bottom of the page you will see that the batteries will last 8 hours. In short it is not an alarm.

    I meet George at ACI in KC. He know about CO2. I do agree that his website is difficult to use.

  7. thehousewright | | #7

    Great information
    We had a building which we suspected to have a CO problem, despite the fact that two different consumer/residential grade detectors did not alert the occupants. We had our HVAC contractor come in with a more sensitive instrument and found the heat exchanger on the furnace was indeed cracked and leaking.

    After that experience I have come to the conclusion that the detectors commonly installed are not nearly sensitive enough to detect all leaks, and questionable in their ability to protect occupants.

    We had tell tale signs of a problem: Noticeable odor when entering the building from outdoors, and trace soot residue in the boots of the floor registers, yet the CO units did not detect a problem.

    Thanks for the information.

  8. lutro | | #8

    Inaccessible solutions
    Thanks for the links to online vendors, Martin. My previous searches had led me to the common conundrum in high-performance building and safety: 1) This problem is important, and everyone should take action; 2) You, as an individual who agrees with statement 1), cannot buy the products that would allow you to act on the information. As an example, the National Comfort Institute has multiple web pages and videos to convince me that CO is a problem, and that I need to solve it now. I'm convinced! When I want to sign up, I get this message: "Sorry We Could Not Find Any National Comfort Institute Members In Your Immediate Area."

    There are reasons, good and bad, why some products are not available to the general public. As a person who embraces many types of innovation long before the general public and the service providers, it is often very frustrating to find access to tools and materials limited or impossible. Almost as frustrating is when a product can only be purchased along with the services of a professional, quadrupling the cost. Professional services are sometimes essential, often valuable, and sometimes irrelevant to the informed homeowner.

    Thanks again for providing links that will allow me to install a low-level CO monitor in my house this month.

  9. lutro | | #9

    What about monitoring CO2?
    Obviously, carbon dioxide (CO2) is vastly less dangerous than carbon monoxide (CO), but it might be valuable to have a warning system for elevated levels of either of those gases. My thinking is this: A well adjusted gas appliance should not produce CO, but will always produce CO2 and water. If there is a problem in the heat exchanger and/or flue, and carbon dioxide is being released into the living space, it would be valuable to be warned about that early on, so that repairs can be made. Rather than getting the first hint of a problem with the HVAC system AFTER the appliance starts producing CO.

    I don't know what it would take to implement this idea, but it would be great if the CO Experts monitoring device (which I ordered yesterday) would indicate both CO and CO2 levels.

  10. GBA Editor
    Allison A. Bailes III, PhD | | #10

    Response to Derek Roff
    If you want to monitor CO2 to ensure proper combustion, you need to do it close to the source of combustion. That's how the manufacturers of ventless gas fireplaces claim their appliances are safe. (Not all agree that ventless gas appliances are safe, however, despite what Bob Vila's website says. I've seen wall-mounted CO2 monitors, but never in residential buildings.

  11. JonathanTE | | #11

    Trying EcoInst link again
    W. Robert and others, I tried tracking down Martin's broken link to Try this:

    When all else fails, enter the following (without the quote marks) into a Google search:
    "CO detector"

    Using "site:" narrows your search to only the relevant website. Comes in handy sometimes.

  12. KeithH | | #12

    Anyone tripping over this in 2017+, CO Experts is no more
    Unfortunately, it appears the sole proprietor, George Kerr, has passed away. The notification on the website seems to indicate the business will close unless a new owner purchase the business.

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