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Hissing radiators

Discussion in 'Plumbing and Central Heating' started by Aloysius, 21 Mar 2021.

  1. Aloysius

    Aloysius

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    Hi

    I know there have been other threads on this but none quite resolved my query.

    we had a new system boiler fitted two weeks ago, keeping our existing radiators. All fine.

    however, yesterday we noticed (for the first time I think) quite loud hissing from our main ground floor radiators. This is consistent hissing immediately from the boiler coming on and does not change as long as the thermostat calls for heat. Altering TRV stops the hissing if it turns the rad off, but otherwise it is the same when on.

    the only thing that seems to affect the hissing is adjusting the lock shield valve in either direction. The noise also seems to come from this side of each rad. Tightening the valve reduces then stops the hissing if tightened fully, but of course just turns the radiator off and the hissing moves to other radiators in the house. Opening up the valve also reduces the hissing but I wasn’t sure that was the right thing to do as I felt a drop of water and I guess it also ruins the balance.

    the only other thing I have noticed from yesterday is when I turn the trv up on some of the rads which we keep low. Previously I wouldn’t have heard a noise but now I hear water gushing/gurgling as the radiator comes on.

    I have left it with the lock shield valves tightened enough to reduce the noise (which is still noticeable) but not quite enough to turn the rads off. This has made rads in other parts of the house hiss though.

    any ideas gratefully received.
     
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  3. D_Hailsham

    D_Hailsham

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    Make and model of original boiler?

    Make and model of new boiler?

    Were the radiators balanced when the new boiler was installed?

    Can you post a pic/scan of the Benchmark Commissioning page in the installation manual of the new boiler?
     
  4. Aloysius

    Aloysius

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    Hi

    Old boiler was Worcester Bosch Greenstar 24i system

    New boiler is Viessmann 100w system 35kw (although installer said he turned down to 28kw).

    Installer said he did balance radiators.

    Is the attached page what you wanted to see (I have excluded personal info)?

    As I said above, i haven’t noticed any noise from the system since it was installed more than two weeks ago until yesterday when it suddenly became very apparent. I didn’t do anything particular yesterday which could have caused this, although I have been gradually turning some of the TRVs in the house down over the last week or so given the milder weather. Just turned the trv up in the home office where I am writing this and the hiss is pretty noisy as the rad heats up. It sounds like someone running a bath in the next room. It’s not just like a hiss from one part of the rad or one valve.


    E33A7BC4-C4DF-44DF-AB3E-671A9705FF98.jpeg
     
  5. D_Hailsham

    D_Hailsham

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    You say you have a 35kW Viessmann 100-w system boiler; the current model is only 32kW. Can you provide the GC (Gas Council) number of your boiler - it's on the front of the installation manual.

    I see that the installer has balanced for a 10C CH temperature differential (65C-55C). Your boiler is designed to work with a differential of up to 20C. A 10C differential requires a flow rate twice that of a 20C differential, which means the pump has to work harder to overcome the considerably increased (4x) friction losses in the heat exchanger and the pipes. What you are hearing is the high flow required if balancing at a 10C differential.

    Please check the values of parameters 2 and 4 (pump settings).

    Do you have any problems with the hot water, e.g time taken to replenish the cylinder?
     
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  6. Aloysius

    Aloysius

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    Hi

    There is a whole sequence of GC numbers quoted on the front of the manual - photo attached.

    I haven’t noticed any major problems with the hot water up to now, although oddly enough my shower this morning struggled to get as hot as usual. Could be due to others using water before me, but it coincides with the noisy radiators starting only yesterday afternoon and it’s the first time I have noticed any issues with the water since the boiler was installed.

    I’m afraid I have no idea how to "check the values of parameters 2 and 4 (pump settings)."

    I have very little technical knowledge. I am wondering why the noise will have started only yesterday if the cause is the differential set at installation. The only things I have really changed since installation are the TRV settings on various radiators (turning a number of them down low) and I have done this gradually rather than all in one go yesterday.

    Thanks very much for taking the time to advise me.


    97032E39-2F7E-49CB-9514-A4D9C168EE52.jpeg
     
    Last edited: 21 Mar 2021
  7. D_Hailsham

    D_Hailsham

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    Thanks for the pic.

    Did you have weather compensation as an extra when the Viessmann was installed? It doesn't matter if you didn't, but the instructions for adjusting the pump speed (which I'm not suggesting) do mention weather comp. Follow the instructions on page 29 for "changing the maximum speed", but ignore step 8, i.e. go direct from step 7 to step 9. Make a note of the maximum shown at step 7 and let me know what it is.

    Have you spoken to the installer about your problems?
     
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