I have a fairly new gas range and a Rheem tankless water heater. Both are under 6 years old. I have a CO sensor (Sensorcon) in my kitchen and would rarely see over 10 ppm. Tankless water heater worked fine.

About a year ago the tankless hot water unit would shut off after ignition with error 11. Cleaning the flame sensor helped a bit, but same issue. Replacing both flame sensor and ignition coil made no appreciable difference. To be clear, the flame does ignite correctly but then errors out, but only about half the time. The other half of the time it runs fine, so it’s not broken.

Around the same time, I started to see much higher levels of CO in the kitchen. Up around 60 ppm with a single burner. I bumped up ventilation to keep levels low, but nothing changed in the range or kitchen.

These problems started around the same time, and so I think they are related. Could it be from the gas burning too rich? What tests can I do?

  • your exhaust vent (chimney) is partially blocked.
    – Traveler
    Commented Sep 4, 2023 at 21:04
  • 2
    What colour is the flame? It should be blue with maybe only a tiny tip of yellow. Anything else and get it repaired before using.
    – crip659
    Commented Sep 4, 2023 at 21:04
  • @crip659 The flame was blue but is now more yellow.
    – Erick T
    Commented Sep 6, 2023 at 19:52
  • @Ruskes the exhaust is clear - it's about 12 inches for the water heater
    – Erick T
    Commented Sep 6, 2023 at 19:52
  • Gas problems should not be taken lightly. Getting a gas appliance repair person and your local gas service people in to check it out, will be a good idea. Yellow flame might mean not waking up.
    – crip659
    Commented Sep 6, 2023 at 20:16

1 Answer 1


When natural gas burns normally, it produces CO2, water, and a little bit of other stuff.

When natural gas does not burn normally, it will often produce CO instead of some of the CO2. As noted in a comment, when natural gas is burning normally it will have a blue flame. But color alone doesn't tell you much, so CO detectors are important in any home with natural gas (or propane) appliances.

There are five things that I can think of that might cause gas to not burn normally:

  • The gas itself. If you are on a propane tank then this seems plausible. But if you are on utility natural gas then it is unlikely.
  • The gas pressure. There are ways to test this, but I would recommend leaving this to the utility or a licensed plumber/gasfitter. (Many plumbers do gas and water, many only do water.)
  • A physical problem with the appliance - e.g., a clogged nozzle. Generally if this happens, some burners will be more affected than others, though some problems could affect all burners.
  • Not enough oxygen for full combustion. Do the high CO readings happen even if you have doors and windows open? Have you cut down on fresh air - e.g., by replacing old leaky windows with new well-sealed windows?
  • Not removing combustion products - e.g., exhaust fan not working properly. I actually don't think this is the problem, or at least not the primary problem, because you should have very low CO readings even if the fan is off, unless there is some other problem (gas, appliance or oxygen).

And finally, make sure the problem is not with your sensor. Do you get 0 (or very close) readings when all gas appliances have been off for a while? Can you borrow another sensor to verify the readings?

  • 1
    In full agreement with everything here but natural gas utilities sometimes don't pump 100% methane but a mixture of methane, propane and AIR. You read that right. When done right, no one notices. It burns just like pure methane without any need to adjust jets or air ports. When they mess up and don't add enough air, hundreds of customers' CO alarms are set off. Happened in MA/NH a few years ago. Not suggesting this is necessarily OP's issue, just that we can't assume utility natural gas is a constant any more. See PDF: standby.com/sites/default/images/PDF/propane_peak_overview.pdf
    – MTA
    Commented Sep 4, 2023 at 22:05
  • That (and other possibilities) is why I said But if you are on utility natural gas then it is unlikely. - unlikely != impossible. Commented Sep 4, 2023 at 22:06
  • How would I ask the utility to look at the gas? The two appliances (range and water heater) share no venting and are on opposite sides of the house. That they both started working poorly makes me think it's a problem with the source, but I don't know the term I'm looking to use when I call PSE :)
    – Erick T
    Commented Sep 6, 2023 at 19:54

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