What Does “Air Free CO” (Carbon Monoxide) Mean?

 

Almost everyone knows that CO (carbon monoxide) is really nasty stuff and nothing to play around with. Humans will often begin exhibiting symptoms of CO poisoning at 200 ppm (parts per million) of CO in a space. Studies have also shown that the effects of CO exposure can accumulate over time, resulting in health problems that don't show up all at once in extreme symptoms.

All of this is to say that testing for and monitoring CO levels in occupied spaces using stationary and personal protective CO monitors and alarms is a must. It is also a best practice to inspect fuel-burning appliances for combustion safety and efficiency using a combustion analyzer.

One of the readings taken in the flue of a fuel-burning appliance is the CO in PPM, but in many cases, we refer to it as CO “air free.” Many combustion analyzers calculate it automatically.

In essence, CO air free is a calculated PPM of CO that takes the “excess air” or air that was not used in combustion and removes it from the equation. This means that the CO Air Free number will always be higher than the uncorrected CO number.

The way air free CO is calculated is by first calculating the difference in the flue O2 (oxygen) percentage compared to the percentage of oxygen in regular air at sea level, which is 20.9%.

So, the formula is:

Air Free CO (in ppm) = Measured CO ppm x (20.9 ÷ (20.9 – O2% in the flue gas)

Or, set by step:

20.9 minus the measured percentage of oxygen in the flue; let's say we measured 5%, so that would equal 15.9%.

20.9 ÷ 15.9% = 1.31

We then multiply the measured ppm of CO (let's say it was 87) times the 1.31 we calculated, and we get an air free CO ppm of 87 x 1.31 = 114 ppm of CO adjusted to air free.

Using air free CO helps create a more level playing ground for comparing one appliance to another by preventing high levels of excess air from diluting the CO and giving an improperly low reading.

While it is widely recognized that 400 ppm CO is the allowable limit for flue CO levels, it is generally possible and recommended for flue CO levels to be below 100 ppm air free.

—Bryan

Related Tech Tips

Cleaning Condenser Coils Still Matters
There was a story that came out recently based on an ASHRAE study performed by David Yuill from the University of Nebraska. The study appeared to indicate that condenser coil cleanliness makes no difference on system performance and efficiency. Those of us who have worked in the field know that coil cleaning matters because most […]
Read more
Introduction to Pool Heat Pumps
Air-to-water pool heat pumps are seeing more and more popularity in the climates that can support them over the more traditional gas and electric pool heaters we usually see. While they definitely contain some familiar operating principles to an air-to-air heat pump, there are some striking differences in how the heat is transferred and how […]
Read more
Fan (Blower) Efficacy
For those of you who use the MeasureQuick app for system diagnosis and performance testing, you may have noticed the “fan efficacy” results and wondered what it is. It is simply the CFM output of the system divided by the wattage used by the blower. It is only for the blower motor and has nothing […]
Read more

2 responses to “What Does “Air Free CO” (Carbon Monoxide) Mean?”

  1. I recently passed and am now an A Gas Fitter in Alberta, Canada. We take very little combustion analyses at trade school. I am an HVACR tech up here as well. So will be doing combustion analyses using the Testo analyser on boilers, direct and indirect( mostly) up in the Oilsands of northern Alberta, on units up to 5,ooo,ooo btu. Mostly with GP power burners. Looking for a manual and/or any info you can provide. Look forward to your response

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from