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.

Pulsing Harmonic Hum

Had a new boiler installed, Lockinvar Knight KHN155.
We now experience a pulsing harmonic hum. We were told it has to do with the air temperature which is why it does not occur everyday the heat runs. A "suppressor" was installed on the intake side to correct the problem though it only dampened the noise a little.
i have to believe there is a way to fix this to where the hum is gone?


    HVACNUT Member Posts: 5,476
    I would get the I&O manual and make sure its vented to the manufacturers specs.

    Was a combustion test done?
    Incomplete combustion will give mod cons a humming/whistle noise.
    Ask to see the print out of the test? Or have them come back and do it again.
    The I&O manual will show the acceptable tolerances.
  • Ironman
    Ironman Member Posts: 7,223
    Search this site and you'll find we've posted extensively about this.

    More than likely, you have CSST supplying the gas to the boiler which is the root cause of the harmonic distortion. There are two things you can do to eliminate this:
    1. Have a COMPETENT tech that knows gas and that has a digital combustion analyzer set the co2 to the richest value allowed in the manual.
    2. Enlarge or remove the CSST tubing.

    Whatever was put in the air intake should be removed.

    Trust me on this, I've got hundreds of hours dealing with this issue on various brands.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • DZoro
    DZoro Member Posts: 1,048
    Agree with the guys above. It is running too lean which will worsen the noise with warmer outdoor temps. The only way to fix is to do combustion analysis, while checking gas pressures.
    The "suppressor" is not going to work at all.