Heat only (conventional) boiler temperature settings

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Please, help me to understand flow temperature settings.

I have a Vaillant heat-only (conventional) condensing boiler. The system is an open vent. I have a cylinder with a hot water thermostat set at 55 degrees.

I appear to be able to set the hot water and radiator heating separately on the Vaillant.

Am I really setting separate temperatures? Does the boiler deliver different hot water temperatures for hot water and heating?
I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
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  • BUFF
    BUFF Posts: 2,185 Forumite
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    it might help if you state exactly whih Vaillant boiler you have (controls also might help)
  • EssexHebridean
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    I’m pretty sure the boiler we have at work allows separate settings for heating and water flow temperatures - I thought that was pretty much standard these days, hence the advice about turning flow temperature down for radiators? 
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  • Sterlingtimes
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    BUFF said:
    it might help if you state exactly whih Vaillant boiler you have (controls also might help)
    Thank you. It is a Vaillant Ecofit Pure 425.

    The controls are as follows:

    a thermostat on the hot water cylinder set to 55 degrees

    a Wiser iTRV system fitted to all but one radiator set to various temperatures.

    A signal from any one iTRV will call for heat.

    So does this boiler discriminate in flow temperature between calls from the hot water thermostat and calls from an iTRV?
    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
  • Rodders53
    Rodders53 Posts: 2,187 Forumite
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    A combi boiler will definitely have separate HW and CH circulating water temperature control.

    A system boiler usually doesn't.
    BUT(as I understand it) some modern system boiler installs, with the correct controls, can have HW coil flow temperature increased when HW heating is demanded and revert back to a lower CH flow temperature when the HW tank is satisfied.
    Known as "hot water priority" it doesn't permit both CH and HW to be called for at the same time (aiui). 
    It's quite possible that with a recent boiler install this has resulted in an upgrade?

    OP needs to seek advice from the installers.

    HW on the Vaillant would need setting higher temp than the HW cylinder stat or it'll never be satisfied and the CH won't run.  CH should be set as low as one can go and still have comfortable room temperatures to ensure the boiler is in condensing mode.


  • [Deleted User]
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    BUFF said:
    it might help if you state exactly whih Vaillant boiler you have (controls also might help)
    Thank you. It is a Vaillant Ecofit Pure 425.

    The controls are as follows:

    a thermostat on the hot water cylinder set to 55 degrees

    a Wiser iTRV system fitted to all but one radiator set to various temperatures.

    A signal from any one iTRV will call for heat.

    So does this boiler discriminate in flow temperature between calls from the hot water thermostat and calls from an iTRV?
    Not necessarily. If, for example, you have a ‘S’ plan heating system with two zone valves and you have a HW and CH demand at the same time, then the boiler will default to the HW set temperature until the HW zone valve closes.
  • Sterlingtimes
    Sterlingtimes Posts: 2,407 Forumite
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    edited 25 October 2023 at 9:41PM
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    Not necessarily. If, for example, you have a ‘S’ plan heating system with two zone valves and you have a HW and CH demand at the same time, then the boiler will default to the HW set temperature until the HW zone valve closes.
    This might be the answer that I require; thank you. It is an S plan system.

    Just using extreme measures for illustration, let us say that the hot water thermostat at the cylinder is set to 50 degrees and the boiler is set water to 60 degrees, and the heating is set to 70 degrees, if the water zone valve water opens then the boiler says I must heat to 60 degrees and if the heating zone valve alone opens then the boiler says I must heat at 70 degrees. But if both zone valves open then 60 is the default. 
    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
  • Sterlingtimes
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    Rodders53 said:

    HW on the Vaillant would need setting higher temp than the HW cylinder stat or it'll never be satisfied and the CH won't run.  CH should be set as low as one can go and still have comfortable room temperatures to ensure the boiler is in condensing mode.

    Thank you, Rodders; this seems consistent with Dolor's posting below. I think that you have provided me with the likely answer.

    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
  • Rodders53
    Rodders53 Posts: 2,187 Forumite
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    edited 12 February 2023 at 6:52PM
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    S plan does not (cannot) signal two temperature demands to a boiler.  
    There's only the one call for heat wire in a conventional S-plan and all motorised valves are wired in parallel for that 'call'.

    Modified wiring of an S-plan system may be able to do two 'demand' signals to suitable boilers... but it is no longer S-plan in that case.

    Talk to the installer and or consult any wiring instructions and/or manuals left.

    EDIT https://lhsplumbing.com/blogs/news/priority-domestic-hot-water-pdhw describes a HW priority install, which I'm now studying on more detail.

    Edit 2:  https://www.vaillant.co.uk/for-installers/products/ecofit-pure-open-vent-boiler-26116.html#downloads manuals are rather vague and confusing to a mere mortal.

    Certainly I cannot see any HW and CH demand inputs (though Vaillant proprietary low volt bus kit might be able to do such) via their special wiring centre?

    S20 should appear in display on HW heating mode as I read the user manual.

    The panel / display / controls may be generic between all system and combi boilers in the range?

    The installer should have explained all as part of the commissioning and handover process.
  • BUFF
    BUFF Posts: 2,185 Forumite
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    BUFF said:
    it might help if you state exactly whih Vaillant boiler you have (controls also might help)
    Thank you. It is a Vaillant Ecofit Pure 425.

    The controls are as follows:

    a thermostat on the hot water cylinder set to 55 degrees

    a Wiser iTRV system fitted to all but one radiator set to various temperatures.

    A signal from any one iTRV will call for heat.

    So does this boiler discriminate in flow temperature between calls from the hot water thermostat and calls from an iTRV?
    do you also have a room 'stat(s) [perhaps Wiser]? do you know which Wiser hub you have & how many channels it has?
  • Sterlingtimes
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    Rodders53 said:

    EDIT https://lhsplumbing.com/blogs/news/priority-domestic-hot-water-pdhw describes a HW priority install, which I'm now studying on more detail.


    Thank you for an excellent posting with good referencing. I need to read and reflect upon this before I respond to you. This is not my engineering area, so I am a "mere mortal". However, the engineer inside me leads me to agree that the manual is vague and confusing. I will check on your S20 conjecture.
    I have osteoarthritis in my hands so I speak my messages into a microphone using Dragon. Some people make "typos" but I often make "speakos".
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