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Natural gas in exhaust

JEFF5255 Member Posts: 9
Hello all,
I'm experiencing a situation on a job we just installed 2 new Navien NCB-240 Combi boilers (natural gas)
The customer was in the yard gardening and smelled gas so he called the gas co. They came out and confirmed there is a high level of natural gas being expelled out the exhaust pipe outside and he proceeded to red tag the units. I thought it was isolated to just 1 of the 2 units but on the 2nd time out he check both and they are both having the same issue. I was present the 2nd time out and witnessed his meter and it showed a 20% combustible gas reading.
I took my digital manometer and checked the gas pressure at the burner and my reading was 19"WC which I knew was extremely high. But it was possible as the area has high pressure gas mains and a step down regulator in the basement. I thought we had discovered a bad regulator and surely the cause of the problem until the gas co came back out to check the pressure just after the regulator and the reading was 8" WC. The customer sent a picture of the manometer showing 8"WC.
Has anyone experienced this before? Can you give any suggestions on what might be going on here with all this natural gas coming out the flue??

I did have the local Navien rep out to see if he could help but he only came to check the settings, dip switches and visually inspected the flame. He explained to me that some raw gas will be present in the exhaust especially during pre and post purging. He also went on to say that there also is some unburned gas always in the exhaust because the units are not 100% efficient. He said but the correct way to check it is during "high fire". He said that you never really have this call because all the old exhaust with the gas in it went up the chimney now it's going out thru the side wall where it can be smelled. I'm no gas expert but this just sounds a bit absurd, what he telling me is that if you have an 81% efficient burner the 19% of the gas is not being burned goes out the chimney?? Or in our case the 93% eff burner will expel 7% raw gas out the flue pipe? I don't believe what I'm hearing, this would be an explosion hazard if someone was out smoking in the garden... Can anyone elaborate on this so I can understand more about the combustion process.
Back to the issue we have a 20% natural gas or "combustible gas" reading coming out of our exhaust and rightfully so a customer that is quite upset.

Any feedback would be helpful

Thank you,



  • Zman
    Zman Member Posts: 7,569
    I am thinking that Navien should provide the rep with a combustion analyzer.
    No offense intended, but you should really have one also..
    20% is a ridiculous amount of gas. The unit should be red tagged until someone qualified sorts this out.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Steamhead
    Steamhead Member Posts: 16,832
    That Navien rep must be on a different planet. There should not be ANY unburned gas in the exhaust- It should all have been burned. It's scary to think someone calls this normal. Something to keep in mind when considering Navien equipment.

    The building regulator might be sticking intermittently. You will have to catch it in the act. If the gas supply checks out, those Naviens will have to go.

    Where does the exhaust come out in relation to the garden?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jamie Hall
    Jamie Hall Member Posts: 23,280
    The only way you could have natural gas in the exhaust when the burner is running -- on any firing level -- is for the burner to be way way out of adjustment and running far too rich -- or a serious gas leak in the unit, outside of the combustion zone. it's worth noting that you are well in the combustible range -- in the exhaust, yet -- so if that exhaust manages to get enough oxygen mixed into it, which could easily happen, it's going to make very exciting loud noises.

    I agree with ZMan. Red tag until someone can figure out what's wrong.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • don_9
    don_9 Member Posts: 395
    I have experience that before with boiler n force air equipment when a spider get in there and make a home in the burner tubes.I have also seen it with crack hx on a force air system when the blower come on and is to strong for the inducer motor.I agree find someone with the right tool n knowledge to fix the problem.
  • Sam81
    Sam81 Member Posts: 37
    If the pressure was to high then u might change the gas valve orfice had that once because of high pressure
  • kcopp
    kcopp Member Posts: 4,432
    I would go to the next person @Navien to see what is going on. Call Tech support and share w/ them the reps thoughts. Not Cool.
  • j a_2
    j a_2 Member Posts: 1,801
    My opinion you did all the right things, in your investigating. Do get yourself a combustion analyzer asap…Truth be told I have seen this quite often….the gas smell…
  • Tim McElwain
    Tim McElwain Member Posts: 4,621
    Most of the comments are correct. It looks like the gas air mix is too rich on the gas side. You should never smell gas when a gas fired piece of equipment fires. It always indicates something is wrong if you do smell gas.

    Someone needs to go through the setup on the burner and adjust the gas air mix typically on these type of units it is a small adjustment to the throttling screw with a combustion analysis being conducted.

    There could also be problems with the venturi on the CAB, especially if you are getting delayed ignition.

    What is your location?
  • captainco
    captainco Member Posts: 793
    I do not believe there is a standard gas leak detector that can measure in flue gas. Every one I have ever tested is sensitive to moisture, temperature and airflow so I am pretty sure that it is a false reading. The smell may just be aldehydes of poor combustion. If there is raw gas in the flue gases there is carbon monoxide beyond measurements.

    The Navien rep is absolutely clueless
  • j a_2
    j a_2 Member Posts: 1,801
    What were the combustion readings ? What was the static and dynamic gas readings? I hope by now you have a analyzer…Todays testers are very user friendly…
  • kcopp
    kcopp Member Posts: 4,432
    What ever became of this? kcopp
  • j a_2
    j a_2 Member Posts: 1,801
    I was wondering the same thing….Those who ask should answer...
  • icesailor
    icesailor Member Posts: 7,265
    Probably resolved by the use of that Egyptian River Disease. Denial. The only disease that says that there is nothing wrong.