Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Stumper- can't get boiler CO ppm down in flue- ideas?

Smith CO. boiler 140,000 BTU natural gas
CO in flue 1400 ppm and not budging after performing everything we can think of;
1. Have cleaned/brushed out burners and HX, and ppm isn't dropping.
2. Took top off boiler for more thorough cleaning.
3. Verified we have correct manifold gas pressure, proper draft, and proper combustion air.
4. Verified CO measurements with a 2nd combustion analyzer.
5. Manufacturer had us clock gas meter to verify boiler is is not over firing and operating @ correct amount of BTU's, and it is well w/in range (~132,000 BTUs).

We've never run into this before where the CO in the flue wouldn't budge. Any more ideas?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    Usually it's mis-aligned burners. I can't think of anything else to check. If it were a furnace, cracked heat exchanger. Everything is sealed back up correctly, no leaks? Any CO detected outside/around the boiler? Any rollout?
    Draft hood?

    There was an error rendering this rich post.

  • captainco
    captainco Member Posts: 796
    steve ask the important question. Does it have a drafthood? What is your O2 and flue temperature and draft? Are you sure the CO stopped at 1400 ppm or did you just stop testing? Burner alignment or bad burners would be the only other reason if your O2 and CO are stable.
  • ChrisJ
    ChrisJ Member Posts: 15,767
    How did you confirm there's enough combustion air?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • HomerJSmith
    HomerJSmith Member Posts: 2,467
    edited September 2022
    Happened to me once and I was getting all these weird numbers. My meter was set to nat gas fuel and the owner pointed out to me that it was a propane sys. Duh! Is the meter set to the proper fuel type and recently calibrated?
  • captainco
    captainco Member Posts: 796
    It doesn't matter at all what fuel the analyzer is set for when it comes to the O2, CO and Flue temperature. The fuel setting is strictly for calculated CO2 and Efficiency and has nothing to do with the actual measurements.
  • ch4man
    ch4man Member Posts: 296
    you stated you verified you had the correct draft. where did you take this reading and what was it? if the captain is here that'll help him help you
    EBEBRATT-Ed Member Posts: 15,607
    try cutting the gas pressure slightly and see if the CO changes. If it doesn't change at all your tester may be bad
  • captainco
    captainco Member Posts: 796
    In 37 years of using and checking the calibration of combustion analyzers I have never found one that was off any where near that amount. Without the actual O2, Flue Temp and draft we all our guessing at life or death.