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Navien NCB-240E boiler makes vibrating noise on startup and 5-10 minutes after hot water shuts off

MichaelZMichaelZ Member Posts: 2
Good Afternoon,
We have a new Navien NCB-240 in our new construction home. We use it for hot water and for hydronic heating. It worked perfectly fine for several months, but now started making loud vibrating sounds on start up, and - much more bothering - some 5-10 minutes after I turn off hot water. Basically, I turn on hot water (even form some 30 seconds), then turn it off and the boiler starts making this vibrating noise... It does not happen when the boiler is used for heating. I.e. as soon as the thermostat switches heating to 'system idle' - the boiler gets absolutely quiet almost immediately. But again, after using hot water - it makes this vibrating noise.
I noticed that - when it makes this noise - there is some kind of whistling sound around intake / exhaust pipes. So, I guess it moves air through this pipes, even though it is not actively heating at this moment.

Another symptom that might or might not be related is that hot water temperature setting is 125F, but I noticed that when we run shower, the current temperature indicator consistently shows 131F. Basically, it kind of "ignores" 125F settings and heats water to 131F. But it is not an issue by itself. This strange and new noise - is what really annoying...

Thank you in advance for your help!


  • mikeg2015mikeg2015 Member Posts: 332
    Those units I think have a little buffer tank internally, so the system will keep heating that tank on low fire for a while. They also have a long post purge that might partially be intended to cool off the heat exchanger to prevent overheating the vent pipe from residual heat.

    However, 5-10 minutes seems long. Do you have a recirculation loop to circulate hot water? Could be the re-circulation pump?

    I've installed a couple of these, and the vairable speed inducer is not quiet. Not loud, but not quiet. I could see it make a bit of a rumble in some cases.

    Might make sure the screen is in place and clean.
  • mikeg2015mikeg2015 Member Posts: 332
    The whistling makes me wonder if they didn;t seal the pipes.

    Also, make sure the metal enclosure cover is installed correctly and tight. Gasket is in good condition. The unit free draws the combustion air from the enclosure, the pipe to the outside just feeds the enclosure.

    If loose, it might cause it to resonate and rumble... and whistle. I think there' 4 screws that secure it. The top of the front lid hooks over the cabinet.

    While your in there, you might shut down the unit and isolate it and check your strainer screens (check the manual). I'd check them every 6-12 months unless you never see any debris.
  • MichaelZMichaelZ Member Posts: 2
    Thank you very much for your response. I removed the front cover to inspect the gasket, etc. It looks like what makes noise is the part at the left upper corner (attaching an image). A strange thing about it is that it does not make this (presumably post-purge) sound after heating cycles. It shuts down and turns to be completely quiet immediately (along with air handler and circulation pump), as soon as thermostat says 'System idle'. However, after turning on and then turning off hot water - it starts making this sound... And there is no gas burning during this time (I was watching the meter and the flame in the combustion box. So, again, no burning, no hot water, nothing but the noise...
    Even more strange: if - while it makes this sound - I turn on hot water, I do see it starts burning gas, I see flame, etc. and it turns to be reasonably quiet (while water is running). Turn off the tap - noise is back and is there for some 5-10 minutes...

    If it was about cooling down - can't figure out why it does not happen after heating cycles? Also, why it decides it has to cool down even if I was running hot water only some 10 seconds... Completely puzzled...

    Thanks again for your response and - if the described above new details somehow help to understand what is going on - please let me know.
    Thank you. I really appreciate your help.

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