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.
Add Comment
4 Answers

I like all the other answers, the only thing I would add is that while you are correct in most cases about the CO in the flue being less than 50ppm.  I’ve found on some units (specifically boilers and tankless heaters), they tend to be higher sometimes closer to 100ppm.  I think one must understand all that goes into the combustion process.  It’s hard to advise on your situation without more details, such as combustion air, 80% or 90% furnace, etc… I’m guessing the pan is warped from over-heating, and that the humidity (and probably warping) are due to oversized equipment on undersized duct system.

Junior Tech Answered on April 14, 2018.
Add Comment

Your Answer

By posting your answer, you agree to the privacy policy and terms of service.