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

MichaelZMichaelZ Posts: 2Member
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 Posts: 851Member
    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 Posts: 851Member
    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 Posts: 2Member
    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.

  • cj5cj5 Posts: 10Member
    Did you ever solve your issue?

    It does not make complete sense based on all the details you provided but I have a mixing valve external to the unit that makes a vibrating noise when the hot water is running. It's set to limit the faucet hot water to 120 F. It stops when the faucet is turned off so a bit different than what you describe. I only mention it because your hot water is coming out at 130F. It may be that the mixing valve needs to be turned to mix in more cold water.
  • mattyrowmattyrow Posts: 1Member
    I believe I experience the same noise as the original poster does. Once you turn it off it sounds like a small engine revving from low to high and then back down to low. On my unit (one of the smaller NPE ones) it only lasts for about 10 seconds. Have been curious what function is occurring to cause that sound.
  • LandoLando Posts: 1Member
    I know this is an older post... but I just started to experience this "low frequency" revving sound from my Navien NPE-210. It was relatively quiet for about 8 months but its starting to get louder. I also hear the sound almost like a faint fog horn (it might be the assembly pump)? Does anyone else have experience with that scenario?

    Has anyone found a fix for either of these problems? It can't be standard operating procedure to have this thing rumbling every time we use the water heater.
  • DZoroDZoro Posts: 771Member
    You should start a new post.
    Rumbling is not standard, something is wrong.

    How old is the unit?
    Natural or LP gas?
    Did your installer leave his combustion numbers with you or in the boiler?
    It could be the water side, but my money is on the combustion side of the equation.
    Maybe post a picture or video of your system.

  • teacherteacher Posts: 1Member
         My NCB-180 began making a loud noise (like a WWII airplane engine) when the hot water was turned on. I noticed the noise after performing a boiler-flush operation during the previous week. The noise was loud enough to hear on the second floor of the house.
         I suspected (hoped) air in the boiler was causing the noise. I had probably re-filled the boiler improperly after the flush operation last week, so I drained and re-filled the boiler. This time (unlike last week's hurried job), I read and followed instructions in the manual: "Before filling the boiler, remove the air vent cap to allow the system to fill properly. Replace the cap when the system is full." --from Navien Combination Boiler Installation & Operation Manual Version: 1.00(Oct. 02. 2013)
         As a precaution, I attached a neoprene tube over the air vent nozzle (where the cap had been) to drain any escaping water into a bucket. I turned on the water-supply valve and waited until the sound of water flowing into the boiler had stopped.
         I replaced the air vent cap, switched the boiler back on, and opened (one at a time) all hot-water valves in the house, letting each run for 60-90 seconds, in order to purge them of any air that might have been in their supply pipes. During this process, I heard no "airplane engine" noises from the boiler.
         Conclusion: Navien NCB boilers might make unusual and loud noises if there's air in the boiler. This condition can exist in spite of the built-in air vent. Draining and re-filling the boiler according to the instructions in the boiler manual might allow the boiler to operate more quietly. A homeowner familiar with basic plumbing (one who knows which valves to close/open) should be able to do the needed work, and save the expense of a service call.           2019aug19(10:18)-sls
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