CH rads alternate between running hot/cold

Discussion in 'Plumbing and Central Heating' started by old duffer, 16 Oct 2021.

  1. old duffer

    old duffer

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    I have had a full central heating system installed, all new pipes (that's every single inch of pipe has been replaced) and all new rads (16 off)
    The property is over 5 levels with rads on all levels and the boiler is at the highest point in the roof space

    The boiler circulating pump is on the return side (so water is effectively pulled back rather than pushed forward)
    Rads on level 1 (Top (2 of them) always get hot
    Rads on level 2 (1 of them) always get hot
    Rads on level 3 (5 of them) only 4 get hot, the fifth is on-off
    Rads on level 4 (2 of them) always get hot
    Rads on level 5 (6 of them) is where the big issues are.
    Some days a rad will be hot, then other days its cold. Then the same rad can run hot and another rad that was hot is now cold. So they seems to alternate.

    I've checked to ensure air removal from rads and added auto vent part way in circuit (boiler also has auto vent)
    I've balanced (as best I can) the system

    Anyone any suggestions as to what is happening?
     
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  3. Harry Bloomfield

    Harry Bloomfield

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    It needs balancing. If there are no TRV's fitted then you ought to have those too.
     
  4. old duffer

    old duffer

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    Balance, get the flow back to rads that had no flow, then next day, same things happens. So it has to be more than balancing at work here. Something is happening that if effecting the priority flow from the manifold to the rads as they alternate on the same manifold
     
  5. Harry Bloomfield

    Harry Bloomfield

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    With 5 floors, I would have thought it sensible to split the place up into heating zones.
     
  6. muggles

    muggles

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    Well first things first, it's up to your installer to ensure everything is balanced and working correctly, with everything heating up evenly throughout your home. You shouldn't need to do anything.

    What make & model boiler do you have? If it's an external pump, post details of that too please.

    Could be air in the pipework, could be that the circuit resistance is too high. Have you tried turning all the rads off except one of the cold ones downstairs to see what happens?
     
  7. old duffer

    old duffer

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    That made me laugh...
    No external pump
    How do you measure "circuit resistance" and what do you mean by that?
    When I isolate all rads then of course the target rads become hot as there is no where for the water to go other than the open circuit
     
  8. old duffer

    old duffer

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    How would that help?
     
  9. muggles

    muggles

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    Circuit resistance can't be measured, but can be calculated based on pipe sizing and length of run.

    What make & model boiler is it?

    Not sure why my post made you laugh. I was serious about the installer needing to rectify it (unless you are the installer of course...)
     
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  11. fixitflav

    fixitflav

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    What type of boiler - heat only, system or combi?
    If heat only, where is the pump? Do you have a F/E tank or an expansion vessel? Where in the circuit does the cold feed connect?
    A sketch of the layout would be useful.
     
  12. Harry Bloomfield

    Harry Bloomfield

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    You would then be able to prioritise the more important zones, to heat those first.
     
  13. old duffer

    old duffer

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    You can't measure it yet you can calculate it? Is it like a quark LOL
    its a viesman 200w 35kw

    I laughed at the idea of a plumber ever being able to diagnose a problem once they have been paid in full. I'm just one of the millions who have paid for an expectation rather than an outcome.
     
  14. old duffer

    old duffer

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    all my home is important to me..which parts of your home are not important to you?
     
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  15. JohnD

    JohnD

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    Spare bedrooms, utility room, workshop?
     
  16. muggles

    muggles

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    All good installers should return to sort these kinds of issues if asked, even after they've been paid. I certainly would - I want my customers to be happy with the the system I've installed. If yours won't come back and they're a Viessmann accredited installer, a quick call to Viessmann to complain should have them running back with their tail between their legs.

    As to your specific issue, your boiler's pump can be adjusted through the installer settings on the panel so I'd start by looking at what that's set to. I'll have a look at my pipe resistance tables and see whether your boiler's pump has any hope of circulating your system. It may well be that you need some additional work doing in the form of a low-loss header installation below the boiler, and a second pump.
     
  17. SpecialK

    SpecialK

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    What pump does the boiler come with? 15/70? 7meters of head?
     
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