What is Air-Free CO

If you have used a combustion analyzer or CO detector, you may have come across the term “air-free  CO reading.” A common question is “What is the difference between the regular CO reading and the air-free CO reading?” You may have noticed the air-free reading is always higher than the “regular” reading. Basically, the “air-free” reading is calculated to determine what the CO concentration in flue gas WOULD BE if all the excess air were removed. CO readings tell how many parts per million of the sampled gas are CO. If you add a bunch of gas that  was not part of the combustion process, it dilutes the CO reading because now all those other gas “parts” are being counted, even though they had nothing to do with the combustion processs. All gas appliances are designed to operate with some excess air. Excess air is left over in the flue gas after the combustion process – thus its name; excess air. The purpose of excess air is to insure adequate combustion air by providing more than is needed. Lack of combustion air leads to CO production, so having more than enough air helps reduce CO levels. Older, natural draft furnaces have higher levels of excess air than induced draft burners; but all should have some excess air in the flue gas. Excess air is necessary for safe operation. The problem is that the excess air dilutes the flue gas, lowering the CO reading. The air-free calculation corrects for this dilution effect.

Basically, the CO reading is multiplied by the ratio of the atmosphere’s oxygen percentage (20.9)  to the excess oxygen percentage. The formula is
Air-Free CO  ppm = Measured CO ppm x (20.9 / (20.9 – O2% in flue gas))
 So if the measured CO ppm is 50 and the measured oxygen in the flue gas is 10.5%
Air-Free CO = 50 x (20.9/(20.9 – 10.5)) = 100 ppm (approximately)
If the CO reading is exactly the same, but the O2 reading is 14%
Air-Free CO = 50 x (20.9/(20.9-14))= 150 ppm  (approximately)
The second furnace is producing much more CO than the first, but the CO meter reads the same because the extra excess air in the second furnace has diluted the flue gas. This is why you should use air-free readings whenever checking flue gas CO levels. Most digital flue gas analyzers will do this for you. If you are using the old hour glass bubblers, you will need to do the math yourself.  

For a more detailed look at air-free CO and carbon monoxide in combustion gas, take a look at this article by Richard Karg http://www.karg.com/pdf/coairfree_article.pdf

It is actually about ovens, but the processes and science are relevant. 

Airflow in Heating

Technicians who know to check airflow in cooling sometimes neglect to check airflow in heating. But poor airflow can cause heating issues too. If you have a heat pump that is tripping out on high pressure – check for problems that can affect airflow. Dirty air filters, dirty indoor coils, or duct problems can all cause airflow problems. Since the indoor coil is the condenser in the heating cycle, poor indoor airflow will cause high discharge pressures and hot compressors. These can cause tripped high pressure switches or open compressor internal overloads. You would not want to condemn a compressor because the air filter was dirty!

Electric strips may cycle on the their thermal overloads, or the fusible links may open. Anytime you replace a fusible link in a strip, check the airflow and all problems that can cause poor airflow. A stopped up evaporator coil can cause a problem in the heating cycle, even if you do not have a heat pump because it is creating an airflow restriction. Another problem that can be easy to miss is an overly restrictive CLEAN air filer. Most 1" pleated air filters have a very high static pressure drop even when brand new. Homeowners often replace the lightweight filters with these and inadvertently cause airflow problems.

Gas and oil furnaces can cycle on the high limit due to poor airflow. This can be more difficult to catch because most high limits automatically reset. When a furnace cycles on the high limit, the blower keeps running and the burners cycle on and off. Furnaces often cycle on the high limit when they have an airflow restriction. This will shorten the life of the furnace, and is a dangerous situation because you are depending on the safety control to prevent overheating. If the limit would fail to open, the furnace could dangerously overheat. Remember, good airflow in the heating season is really just as important as in the cooling season.

HVACR Language

New students can easily be confused by HVACR techno-speak We have our fair share of acronyms in HVACR. Used properly; acronyms speed up communication by reducing long polysyllabic phrases to a few letters. For example, speakers would get a bit winded if they used the phrase “heating, ventilating, air conditioning, and refrigeration” repeatedly in a conversation. The abundance of industry specific acronyms and the use of more than one acronym for the same item can be truly bewildering to students. For example, one major valve manufacturer uses the acronym TEV to represent “Thermostatic Expansion Valve,” while another uses the acronym TXV for the same thing. They do not represent two different types of components, just two different ways to abbreviate the same words.

Try to avoid using acronyms when introducing new material. Teachers who have had years of experience with HVACR techno-speak often forget that the acronyms we casually throw around often have no meaning for our students. Take the following sentence for example: “The evaporator delta T is controlled by the TEV adjustment, the return air wb, and the CFM.” Now I believe that most any air conditioning instructor understands this sentence, but it is essentially unintelligible to many air conditioning students. It might as well be in Russian! Replacing the HVACR specific jargon with the wingdings font it looks like
“The evaporator deltaT is controlled by the TEV adjustment, the return air wb, and the CFM.”
THAT is what this sentence looks like to a new student!

In general it is a good idea to resist using an acronym for something until that item or process has been discussed. Otherwise, the acronym will appear to students to be a mysterious grouping of letters used by air conditioning shaman to communicate with each other. The acronyms and jargon become a secret language which they are not familiar with. I believe that acronym free language promotes better understanding.

An acronym should first be explained and defined before it is used. It is much easier to remember an acronym if you understand what the letters stand for. When using an acronym for the first time make sure and explicitly spell out what the letters represent, this will increase student’s understanding and retention of the term. It also helps for the students to have a good mental picture of the object or process being described. They are far more likely to remember what Delta T stands for if they have actually measured a temperature difference and you have discussed it in class. That way the abbreviation is not an esoteric piece of jargon attached to something they don’t understand, but a name for something they have done. In Fundamentals of HVACR, we always use the complete word or phrase before introducing an acronym. It helps to explain concepts plainly. After introducing the concept, we give the technical terminology that is used to refer to the concept. The students are more likely to remember the terminology if it is logically connected to something they understand.