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Tankless Boiler Harmonics

I read a similar discussion but I have more information. In 2018 the Alaskan 7.0 earthquake hit and it murdered our boiler. We replaced it with a Bausch tankless boiler (gas.)The harmonic started when we got the new one. Another item: the exhaust vent wasn’t in code. A prior owner installed a second floor deck, and the pipe could not exhaust under the deck, so, they added about 10 feet. The sound builds up fairly quickly, stays loud wherein it can be heard both inside and outside, and loud enough that the neighbors can hear it. Then it slows down and stops.The harmonic isn’t constantly on. It’s definitely a thing with water source and it only happens when we turn the water on. A toilet will have this shorter harmonic and a shower will have a longer harmonic, maybe 15 - 20 seconds. It also doesn’t happen again if we use an overlapping water source, such as having a shower on and someone flushes the toilet. I thought it might have something to do with the 10 foot exhaust vent but my boiler has a sensor that if the right exhaust pressures doesn’t hit a certain #, it turns off. That problem was ice forming at the end up the pipe. I’d put a heat gun to end of the pipe, reset the unit, and it was fine. I solved that problem one summer by insulating the pipe so it’s stays hot enough for the condensation to empty the pipe before it freezes. None of the HVAC guys who services it can figure it out. Any ideas?


    HVACNUT Member Posts: 5,829
    You have a tankless water heater or a combi boiler?
    Natural gas or propane?
    What elevation?
    What model Bosch and what are the combustion numbers?
  • Big Ed_4
    Big Ed_4 Member Posts: 2,783
    Do you have a thermal expansion tank on domestic water ? You may have to recharge tank .
    I have enough experience to know , that I dont know it all
  • GroundUp
    GroundUp Member Posts: 1,907
    Is it a tankless or is it a boiler? Why is a toilet getting hot water? Was the combustion ever checked and/or adjusted?
  • Dyaeger
    Dyaeger Member Posts: 4
    Thank you for the follow-up questions. The model # is T8700SE 199. It’s natural gas. It’s on city (domestic) water. Don’t know the combustion #’s but the tempeture is 140 (had to increase it from the standard 120 because we have a couple of scorch proof sink filters.) It gets maintenance every 6 months. No, the toilet isn’t getting hot water, but thanks for the chuckle.
  • Dyaeger
    Dyaeger Member Posts: 4
    Oh, and the elevation of Eagle River, AK is around 950 feet.
  • Kickstand55
    Kickstand55 Member Posts: 110
    Is the gas pipe or exhaust corrugated? That could do it, especially on high fire.
  • hot_rod
    hot_rod Member Posts: 22,118
    Does the installation manual discus checking and adjusting the combustion? Did the installer use a combustion analyzer to set it up?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dyaeger
    Dyaeger Member Posts: 4
    I will check these questions and follow up. I appreciate your help so far.
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,908
    I had a Buderus GB-142 Wall Hung Mod Con do this.  It was a bad air intake/gas valve issue less than 1 yr old.  It was whistling. Loud.   It was covered .  Thats was the end of it  mad dog
  • GroundUp
    GroundUp Member Posts: 1,907

    You said that there are harmonics in the water heater when a toilet is flushed. If the toilet is not being fed hot water, the water heater will do nothing whatsoever when a toilet is flushed.