As we do fall maintenance and head into the heating season, combustion analysis becomes a big deal. (It’s always kind of a big deal, but now it’s a really big deal.) First and foremost, it tells us how much carbon monoxide (CO) is produced by burning. CO can make people ill, contribute to cognitive deterioration over long periods of low exposure, and kill people in large concentrations. But CO isn’t the only vapor that causes trouble; it shares the podium with NO_x.

What is NO_x?

NO_x refers to gases made up of nitrogen and oxygen, specifically nitric oxide (NO) and nitrogen dioxide (NO₂). Even though the combustion equation doesn’t contain nitrogen, it’s in the air around us. Nitrogen doesn’t react with oxygen at typical ambient temperatures, but combustion adds a lot of heat and energy, and nitrogen and oxygen molecules will react to form NO_x gases.

These gases are pollutants. NO_x gases can create smog when they interact with VOCs, especially in cities where lots of cars idle in traffic, burn gasoline, and release NO_x through their exhaust in a concentrated area. NO_x gases can also cause acid rain, along with sulphur oxides (SO_x).

Nitrous oxide (N₂O), which makes you giggly in the dentist’s chair, is NOT a NO_x gas; the formula is different. It’s also not a huge air quality pollutant, but it does have a small ozone depletion potential (0.017, just under 2% of R-11 and R-12) and has global warming potential (273), just like refrigerants. 

Why is NO_x a Problem for HVAC Professionals? 

Yes, NO_x gases are pollutants, but your typical home doesn’t have a seven-lane highway with standstill traffic running straight through the center. The amount of NO_x gases a furnace can give off is a drop in a bucket and probably won’t have a huge impact, right?

Well…

This same group of gases that’s partially responsible for acid rain is in contact with metal surfaces as the blower pushes air through the appliance. You have a heat exchanger and probably have a coil in there as well for air conditioning (or a heat pump in dual-fuel setups). When you have moisture, such as in 90%+ high-efficiency furnaces, NO_x gases react with it to form nitric acid, which will eat away at those metal parts.

In the human body, NO_x gases can contribute to or worsen respiratory conditions like asthma. Exposure at low levels can irritate the eyes, nose, and throat, and fluid buildup in the lungs can start within two days of prolonged exposure to low levels. High levels of NO_x gases can cause respiratory tissues to swell and build up fluid, and death is a possibility.

NO_x in Combustion Analysis

Another problematic feature of NO_x gases is that they interfere with our combustion analysis readings, particularly CO readings. 

In short, some combustion analyzers have sensors that don’t differentiate between CO and NO_x gases. The electrochemical sensors react the same way to NO_x gases as they do to CO, so you get readings that add the NO_x to the CO, thus inflating your CO readings. If you want the full picture from someone who really knows tools, Bill Spohn from TruTech Tools wrote a paper about NOx gases and combustion analysis.

When you’re looking for a combustion analyzer, look for one that has a NO_x filter. As you can see in the chart below, NO_x elevates CO readings when it’s taken by a combustion analyzer without a NO_x filter. (Bill Spohn also interpreted this chart in a livestream we did back in 2023, which you can watch HERE.)

Credit: TruTech Tools

Luckily, there are several combustion analyzers that do indeed filter out NO_x. Among those are UEI’s C161, C163, and C165+, and their C165+N also measures nitric oxide (NO) separately. 

If you’re looking for a new combustion analyzer or want to see if your current one has a NO_x filter, you can check the manual of your combustion analyzer. PDFs of the manuals are usually available on the manufacturers’ websites in a section for downloads, resources, or user manuals. In the case of UEi combustion analyzers, that information is in the “Features” section of their manuals. For Fieldpiece, that information is in the specs of the actual sensors.