"Ellie." "Ellie who?"  "The Elephant in the living room."


The elephant in our room is NOx, and elephants eat a lot.  Then they poop.  A lot. We need to watch where we step. 


As technicians following the BPI Standards, we test for CO in the flue as well as in the ambient air as part of any audit where there are combustion appliances.  Imagine my surprise when my new Testo 327 combustion analyzer gave me flue gas content results that seemed impossibly low . . . in some cases zero. This was something I very rarely – if ever – saw with my trusty Bacharach equipment in the past.


Many of us test our natural gas leak detectors by exhaling out of our mouths directly onto the sensor.  The detector should go off, but that doesn't mean that we produce natural gas  (at least not by breathing out!).  The sensors in our gas leak detectors react to combustible gas or hydrocarbons generally, but they are also reactive to other gases, including VOCs the humidity in our breath and VOCs.  VOCs are the reason why pipe dope sometimes sets them off.  Still, the hydrocarbon most likely to be present at a joint in a gas line is the one we’re looking for—methane—and there is no good reason to use more sophisticated (and therefore more expensive) gas leak detectors to simple find a leak unless you need to quantify it. 


Accuracy is more important when we’re testing flue gas.  But many of the CO detectors and combustion analyzers that we use are similarly indiscriminate: They react to Nitric Oxide  (NO gas, often called NOx) and other cross-interfering gases, and they include some proportion of the reaction in the displayed CO concentration. The issue is this: Where there is smoke there is fire, and where there is fire there is NOx.  This seems like an important consideration to me.


We don’t like NOx because it’s toxic and it’s a greenhouse gas (as is its sulfurous sibling, SOx, another cross-interferrant gas). What’s more, NO converts to NO2 (another toxic gas) shortly after leaving the stack.  If the test instrument does not discriminate effectively between CO and NOx, the number we read for CO in the flue is inaccurate by a factor that can be significant.     


While this issue came up twice in the past on the RESNET-BPI group on LinkedIn, it did not generate much buzz. I believe that Auditors and Raters should be aware that their measurements might not accurately represent the level of CO in the flue gas being sampled. It is unlikely that the original BPI standards take into account this complicating factor.


The issue is the CO reading in the flue only.  NO converts to NO2 about 35 minutes after leaving the stack. NO2 does not interfere with a CO sensor. And its presence is not an issue when sampling ambient CO.



On the good news side, BPI is gathering information on the issue.  On the bad news side, BPI (like any standard-setting organization) moves at a pace that would embarrass your average tectonic plate.  Since we routinely evaluate the safety of combustion appliances based on action levels for CO in flue gases, and since different people are measuring them with different degrees of accuracy, it only makes sense to get the matter of CO testing more into the public forum.


Many thanks to Bill Spohn of Trutech Tools for making sure that this old English major has the chemistry right.








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Comment by David Meiland on March 27, 2012 at 10:13am

The Testo 327 has built-in NOx filtering, the Fyrite Insight has an accessory filter available that you can add. Without, you may get high CO readings that are not accurate. If you are combustion testing and need to determine whether action should be taken, seems you would need an accurate tool.

Ed, maybe you should cover air-free readings in your next installment, just to make things even more puzzling.

Comment by Joseph Lamy on March 26, 2012 at 12:34pm

When i discovered these GIZMO-related issues and shared them with the Social Workers and ER docs in a Portland, OR, hospital (Providence St. Vincent) i also learned that even when a CO-filled house seemed to be the culprit, they had NO plan for dealing with the problem. They sent them home with NO plan for follow-up on the cause of the poisoning, be it a cracked heat-exchanger, leaky supply ducts blowing conditioned air outside the envelope, leaky returns sucking on the CAZ or disconnected flue. No contractor with CO detector showed up to 'doctor' the house, to diagnose the source, the pressure, hole or flow. Nothing. Again, we have a problem not just with the dang GIZMO-related lies, but also the integration of house and health issues. Looks like an-tapped market falling all over itself to hook-up with Home Energy Pros to make house-calls, huh?

Comment by Joseph Lamy on March 26, 2012 at 11:45am

Here's a reference for anybody who wants to refer their ER doc to the facts. The back-up of a blood draw is seldom performed if there is no OTHER reason to suspect the CO may be the culprit.


Comment by Joseph Lamy on March 26, 2012 at 11:39am

Another related issue comes to the fore when I see how we RELY on gizmos. In the ER, the finger oxymeters that are supposed to tell the professional health folks like Doctors, nurses, etc what your oxygen level is inside your bloodstream actually CAN NOT tell the difference between carboxyhemoglobin and oxygen!

In other words, a gassing victim can show 98% oxygen and still be puking his guts out, have a disabling headache and be carrying enough CO to kill a horse but the dang oxymeter LIES! If no other indication of carbon monoxide presents, well, they'll give you pills for the flu-like symptoms and send you HOME to die in the CO-filled house that started all the trouble to begin with. Houston, we have a problem.

Comment by Ed Voytovich on March 26, 2012 at 11:29am

David B . . . I'm concerned about the consequences of having two people conduct tests with different instruments and getting significantly different results.  That being said, the more accurate reading with the NOx filter is probably more importance to a heating tech than to an energy auditor, but the anyone testing for CO should be informed that sometimes their apples aren't apples.

My intention in writing this particular blog was to try to make people aware of what could be an issue in the course a quality assurance check or some other review of an auditor's work.

Comment by Ed Voytovich on March 26, 2012 at 11:11am

That's great input, Joseph, and another nail in the coffin of a product (unvented gas heaters inside the building enclosure) whether the building is loose, tight, or tied in a knot.  Thanks!

Comment by David Berg on March 26, 2012 at 11:05am

In my Testo 327-2 analyzer, there is a NO filter that removes the NO before the CO sensor.  It is one of the maintenance items that has to be replaced when the absorbing chemical is consumed.  Are you concerned about the CO readings or the NO content of combustion gases?

Comment by Joseph Lamy on March 26, 2012 at 10:26am

When those pesky little 'Ventless' gas space heaters landed on the Home Depot floor, i found out about the NOx/CO relationship. The ventless units make very little CO but gobs of NOx by putting pullenty of air into the fire. So they get what they asked for - almost none of the silent killer whose name resonates within the public consciousness. They also got gooolobs of the NOx because the air has ~80% nitrogen and this little item can EAT unpainted sheetrock! but does NOT resonate within the awareness of most us Joe Q. Publics. Where our gas ranges (cooktops) allow us to set our skillet right on the flame, Canadian stoves has a ceramic cover to catch that NOx and dump it outside because their homes are tighter, their air quality more critical and their group consciousness includes the nasty gas which we disregard. ventless units lost much of their appeal through the grapevine mostly due to the moisture content of the methane and propane wreaking havoc. running down the window glass and feeding mold in carpet, in corners, behind couches, etc. But  truth be told, in a tighter home, the NOx may be even more of an issue for those concerned about alveolar and longevity, air quality and health.

Comment by Don Ames on March 26, 2012 at 1:17am

Ed,  great article , thanks.  It's hard enough for us energy auditor types to simply run a combustion gas analyzer, let alone understand how the reading may not be accurate. We do not run into a true dangerous CO situation very often, but when we do, we had better be right and we better get the people out of the house. Thanks, again.

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