Furnace Efficiency using Combustion Analyzers?


A lot of utility companies or municipalities are asking for  furnace efficiencies based on combustion analysis. Combustion analyzers are okay to use for the purpose of determining if there is excessive CO  or CO² in the flue products and to see if the furnace is burning correctly, but they are not acceptable for trying to verify efficiency on 90%+ units.  You need to let them know this.

The true efficiency of a condensing furnace takes into account the heat being recovered from the condensation process, which cannot be measured by a flue gas analyzer. So a combustion analyzer can never get an efficiency reading above about 89% even though the furnace has a “tested” AFUE of 96 to 98%.t  

You also need to know that even on an 80% furnace, the efficiency readings obtained from a combustion analyzer will be only an approximation of the efficiency of the furnace.

The combustion analyzer can only measure flue loss, it can’t measure AFUE, which is what is publish in the  any manufacturer’s literature.

Combustion analyzers are a great tool when servicing a furnace to see how it is working. This can be done as part of a clean and check of the unit.

As for what the CO and CO²  readings  you should be getting, the answer is: it depends. The readings depend on the model of furnace, the firing rate of the furnace, the length of vent and air intake pipe, the altitude, the type of gas as well as other lesser factors. So, in general, the CO² for furnaces should be between 5% and 9%. The CO should always be less than 400 ppm.  This shows that the furnace is operating within the design parameters and that it has proper combustion.

So if you have a utility or municipality asking for combustion analysis efficiency, you will never show what has been design certified by AGA/GAMA for true AFUE by your manufacturer.

Combustion analysis is a good tool to use because it can show if there are problems with the combustion side of the furnace but the analyzer should not be use to determine a furnaces efficiency.

If you want to do combustion analysis of a furnace, there are any number of excellent meter for doing this.

Even though they claim they can calculate combustion efficiency, they can only get close since they cannot look at the total combustion  heat exchange process.

So,  if you have a utility or municipality pushing for efficiency of a furnace based on combustion analysis, refer them to the manufacturer’s literature or to AGA/GAMA for that information. 

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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4 Responses to Furnace Efficiency using Combustion Analyzers?

  1. Whit Perry says:

    Please keep this up. This is fantastic information. I have been in the business for 20+ years and now teach HVACR at a community college. This is some of the best blogging information I have ever come across. Is this available for anyone? Do we just log in to this website. Please share this knowledge as it is so valuable to our industry. The only way to solve the problems we have in this industry is to educate the technician. Some many of your blogs reasserts the information I tell my students on a daily basis.

    Thank you for doing this…………and please don’t stop.

    • Thank you for the kind words. This is my way of giving back to the industry. Most of the post ideas actually come from questions I get as a service manager. I figure as long as there are questions, I’ll be writing this blog. Feel free to share this with your students or anyone else.

      • Whit Perry says:

        I am also Secretary of CARE, Council of Air Conditioning & Refrigeration Educators. I write, or should I say, put together a monthly newsletter for our members and would like to share this link with the other educators. For the hungry student, this should give them a path to follow. And thanks for allowing me to share.

      • feel free to share this with anyone and feel free to print or use any of my posts if you like. I am also on the Advisory Board of the HVACR program at the College of Dupage (COD.edu) in Illinois. Anything to help students is always in all of our industry’s best interest.

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