Combi boiler making noise when running hot water

[belgianwaffle]

Whenever I turn the heating on or simply run hot water (from any tap), within 5 seconds of the burners igniting, my Viessman Vitopend 100 boiler gets loud, like an airplane taking off. This lasts for about 5 seconds, then it stops.

You can hear it here: https://streamable.com/9v8h97. (At the 20-second mark, the burner LED goes on, and about 5 seconds later you can hear the noise.)

My insurance sent a repair guy over who claims it's limescale buildup in the heat exchanger and proposes replacing the heat exchanger. (A new one is 600€, which is probably more than what my entire boiler's worth.) This would not be covered by my insurance (it's only for smaller repairs), so I asked if we could try descaling it, but he refused (without an explanation).

Is it the heat exchanger? If so, could I descale it myself?

Full disclosure: My boiler has also been having pressure problems so I will be replacing the expansion vessel and the pressure relief valve, but I don't know if this could be related.

 

[cross thread]

Think it would be the secondary heat exchanger causing the issue,not the main

 

[belgianwaffle]

Think it would be the secondary heat exchanger causing the issue,not the main

If I understand correctly, the primary heat exchanger heats the central heating system, while the secondary one transfers heat to the domestic water supply.

Given that the boiler is making the noise both when requesting hot water from a tap and when turning the central heating on, I am now under the impression that it is unlikely this is a heat exchanger issue. Or did I get this all wrong?

 

[cross thread]

Sorry didn’t read your issue properly, you could try fernox F2

 

[belgianwaffle]

Given that the boiler is making the noise both when requesting hot water from a tap and when turning the central heating on, I am now under the impression that it is unlikely this is a heat exchanger issue. Or did I get this all wrong?

Would Fernox F2 help in this case (assuming there are 2 heat exchangers and the problem also occurs when simply running hot water using any tap)?

 

[old duffer]

It's called thermal resonance.....a sudden large volume of hot air going down a tube makes a noise. I could explain the actual science but that would just bore you. Its normal and natural and does not mean anything sinister.
I can also hear a lot of air in the water flow through the pipework...

 

[belgianwaffle]

It's called thermal resonance.....a sudden large volume of hot air going down a tube makes a noise. I could explain the actual science but that would just bore you. Its normal and natural and does not mean anything sinister.
I can also hear a lot of air in the water flow through the pipework...

Thanks. Is there a reason it has started doing this all of a sudden? Anything I can do to fix it?

 

[old duffer]

Thanks. Is there a reason it has started doing this all of a sudden? Anything I can do to fix it?

there is nothing to fix, its what happens.
the second law of classical thermodynamics, heat may only go from higher to lower temperatures. Colder external temps, warmer internal gases (higher burn rate) and a mixture of both. What you are experiencing is you notice it more.
My huge diam flue from a continuos water heater makes this sound regardless of the time of year but its less so in the summer months

 

[ianmcd]

It isnt the heat exchanger, you need a proper gas engineer , your burner is resonating at low flame and the gas valve needs set up properly, not for a poke and go neewbie

 

[belgianwaffle]

What you are experiencing is you notice it more.

Are we both referring to the sound you hear at the 25-second mark? The actual sound is much louder than what you hear in the video. (Think of an airplane flying overhead.) There was really nothing to notice before as it only started recently.

 

[belgianwaffle]

It isnt the heat exchanger, you need a proper gas engineer , your burner is resonating at low flame and the gas valve needs set up properly, not for a poke and go neewbie

The guy the insurance sent over was a gas engineer (and he's the one who said it was the heat exchanger).

 

[ianmcd]

The guy the insurance sent over was a gas engineer (and he's the one who said it was the heat exchanger).

well sorry but he is wrong

 

[old duffer]

The guy the insurance sent over was a gas engineer (and he's the one who said it was the heat exchanger).

the term "engineer" is used very loosely in capita plc awarding university lol

 

[old duffer]

a

Are we both referring to the sound you hear at the 25-second mark? The actual sound is much louder than what you hear in the video. (Think of an airplane flying overhead.) There was really nothing to notice before as it only started recently.

the sound of a jet engine overhead will alter depending upon pressure in the atmosphere
sound is a pressure wave and mechanical wave.
I hear a space being filled rapidly with hot air and I also air running through with the water i.e. a lower volume of water until the pipe fills its available diam

 

[martygturner]

Have you checked that the condensate drain is actually passing water, gunked up traps cause the boiler to backfill and you get a whooshing sound as the boiler lights and modulates up.