RE: Combustion testing and CO range


 I use a TPI for checking combustion. What are some possible causes or sources of CO ppm measurement. Most furnaces I wanna see less than 20 ppm. I’m told 50> is acceptable range and 50<  “we can do better”. I know that high gas, not enough O2 and bad heat exchanger (delamination) and so forth can cause high co readings. 

I recently found myself working on a Goodman furnace. It was a warranty call, New construction, single pipe, two years old 92% single stage. I believe 100k BTU w/ 4 ton drive. Customer complain was awful smell that hasn’t been fixed since closing. Only happens in heating season! My co reading outside was 30ppm and indoor at flu natural steady state 30 ppm. I had 9.2% oxygen, 30ppm co, gas was at 3.3″wc. I forgot to write down the Co2 reading. Furnace doesn’t trip limit. 

The smell was a plastic type gas smell. Utility gas company came out to check for gas leak and detected “incomplete” burning of fuel. Never pinpointed a joint leaking gas.

Also add that the client complained about high humidity in home during summer (65% to 70%). 

By the time I tested everything I condemn the gas valve as I thought it was over feeding after being cut off from heat cycle. I also ordered a new evap pan, after inspecting heat exchanger I noticed the pan was warped. I thought maybe the pan being warped may have contributed to high humidity in home during summer. Maybe the condensation flipping out into air stream? 


Sorry was so long, hoping for an educational discussion. Hope I included enough info, and testing out this cool forum. 




Mikayo88 Rookie Asked on April 12, 2018 in Residential HVAC.
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4 Answers

I’m sort of going through this in my mind…..Most manufacturers that I have known vary their CO percentages.


Most that I’ve seen ask for less than 100PPM on low fire/stage. Bacarach published industry established guidelines for all types of equipment. I keep a copy in my combustion analyzer kit for reference and to chow my clients what the numbers and thresholds mean.


Excess CO is a symptom of incomplete combustion. There are a lot of possibilities but at the end of the day, there is an issue with the air/gas ratio.


I’d rule out impingement since it doesn’t seem like the CO numbers you are getting would indicate it.


The first thing that I’d do is verify that your analyzer is  correct….a bad sensor can really wreck your day. I use a calibration kit that I purchased whenever I suspect a potential sensor issue.


I’d actually start over and totally recommission the equipment to get all of my specifications exactly where the manufacturer wants them. That way, there is a baseline that can be used to further diagnose.





Rookie Answered on April 13, 2018.
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