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MZ Boiler Knocking Noise Steam Venting Overheat?

Matt71 Member Posts: 1
I woke up this morning to a loud knocking noise from the burner. Model is a MZ 25S. Water temperature was nearing max temperature and steam was venting to the outside of the house. It's 80 C now in the summer months so I don't know why steam would vent to the outside like that. Condensation during the winter would make sense. Something isn't right.

Anyone have any ideas what would cause this knocking noise and steam venting to the outside of the house?


  • MZ

    Is the steam coming out of the flue?  Could be a bad heat exchanger, i.e. water leaking into the combustion chamber.

    Knocking and high temperatures would indicate reduced or no flow through the heat exchanger, but the MZ has a flow switch that only allows the burner to fire when there's proper flow.  I'd removed and clean the strainer, check the pump to make sure it's working and check the operation of the flow switch.  I'd also open up and clean the heat exchanger and check for any leaks.

    Do you have a technician that's familiar with Monitor boilers in your area?
    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
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