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HTP MC80 foghorning on low fire; CO, CO2 good

wcs5050 Member Posts: 131
Anyone have any ideas? Burn oscillating on low fire to the point of extinguish. Burner is kept clean with regular service.


  • Zman
    Zman Member Posts: 7,572
    What are the numbers?
    Sometimes you need to be on the high side of the range on both of those numbers to make it quite.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • happydave
    happydave Member Posts: 79
    Adjust the gas valve
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,037
    edited February 2017
    I've had the same problem with both a Munchkin and a Lochinvar Knight. Both have good gas pressure and combustion numbers. Both boilers will complain if the flue has negative slope and thus, standing water in the exhaust.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Ironman
    Ironman Member Posts: 7,379
    Set co2 to the richest allowed by manufacturer. Remove or enlarge any CSST tubing within 10' of the appliance.

    I know folks don't wanna hear the latter recommendation, but the corrugation in CSST definitely causes harmonic combustion noise. I removed 15' of CSST from an ATMOSEPHERIC boiler last week and replaced it with black iron. It stop the harmonic resonance completely.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.