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Dunkirk VLT Heat Exchanger Rumbling

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We had a Dunkirk VLT boiler with indirect water heater installed about 6 years ago. This morning we woke up to some rumbling in the basement. The boiler runs fine during domestic heating in low power, but as soon as central heating kicks in, the rumbling starts while in ignition power and high power. High power only lasts for about 10 seconds before powering down and recycling again and again. There are no failure errors on the screen. I tested the CH pump in low power, ignition power and high power. The only thing I can think of is scale. Is there anything else that can be wrong? Any other troubleshooting I can do? Thanks!

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  • hot_rod
    hot_rod Member Posts: 22,505
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    Yes, scaled HX surfaces can create that type of noise. It is usually a gradual build up and noise. Good news is that it is fairly easy to add a cleaner, flush and add good quality water. Efficiency should bump up also. Make sure the circulator is running during any boiler call.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HVACNUT
    HVACNUT Member Posts: 5,895
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    Shouldn't a domestic call set the boiler to high fire?
    When was the last maintenance done? Is there a copy of the combustion report? 
    Just because there's no error code and it hasn't gone into lockout doesn't mean there's proper combustion. 
    JoeMagrino
  • JoeMagrino
    JoeMagrino Member Posts: 2
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    Since posting this question, another nib was added so the boiler could be properly flushed. It didn't help. The heating crew was here for 5 hours trying to figure it out, calling ECR for technical help. Apparently the 3 times my unit was serviced in 6 years, it was never serviced according to the manual. At then end of the day, the only solution suggested was to start changing parts. Unfortunately a new igniter is about $100, a new flame sensor is about $100 and a new burner is about $400. I purchased the $38 inspection kit, the igniter and flame sensor, but I don't it's the igniter because it always ignites. It could possibly be the flame sensor since the flame goes out as it goes into high power. My fear is that it's the burner. Domestic hot water heats fine, no rumbling at all, but as soon as it switches over to central heating, that's when the thunder noise begins and the flame goes out, only to cycle over and over with no error codes. Could it be as simple as changing all the gaskets and o-rings in the inspection kit. The sound still sounds like air, but only in the exchanger. I have spent a lot of money on just troubleshooting. Any advice would be greatly appreciated.
  • Mcmonster508
    Mcmonster508 Member Posts: 19
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    I’m having the exact same issue. Have you had the gaskets replaced and the burner inspected? My issue was when it was first installed it was air bound for longer than it should have been and ended up burning a hole in the burner, which resulted in the loud noises I’m getting and the cycling till it reaches temp. 

    Is this the same sounds you’re getting?

    https://youtu.be/4AJBl5WqH4I
  • hot_rod
    hot_rod Member Posts: 22,505
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    Rumbling could be coming from inside the boiler if it has scale build up. Sometimes running a cleaner through the system will eliminate noises. They should be able to tell a fluid rumble from a burner noise?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mcmonster508
    Mcmonster508 Member Posts: 19
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    hot_rod said:
    Rumbling could be coming from inside the boiler if it has scale build up. Sometimes running a cleaner through the system will eliminate noises. They should be able to tell a fluid rumble from a burner noise?
    In my case I saw the hole in the burner with my own eyes, 
  • Ironman
    Ironman Member Posts: 7,400
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    It definitely sounds like a heat transfer issue.

    Scale build up, lack of circulation, air bound are some of the basics to check out.

    That boiler has an internal circulator with built in primary/secondary piping. But the valves must be set to match the system piping or you may not be getting proper flow.

    Some pics of the boiler with the near system piping and also of the inside of the boiler showing how the valves are set would be very helpful.


    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Mcmonster508
    Mcmonster508 Member Posts: 19
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    Ironman said:
    It definitely sounds like a heat transfer issue.

    Scale build up, lack of circulation, air bound are some of the basics to check out.

    That boiler has an internal circulator with built in primary/secondary piping. But the valves must be set to match the system piping or you may not be getting proper flow.

    Some pics of the boiler with the near system piping and also of the inside of the boiler showing how the valves are set would be very helpful.


    I’ve had the system descaled. I’ve had the system maintenance done yearly, earlier this year I had a company come in and do a heat and exchanger cleaning where they went to clean the burner and showed me there is a quart sized hole in the burner itself(the cylindrical tube inside the exchanger) and was told that was my problem. 

    It just seems like the most obvious. 
  • Ironman
    Ironman Member Posts: 7,400
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    Well yeah, you can’t expect proper combustion with a glaring defect like that.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.