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Baxi Duo Tec 40 HE A Fluctuating Hot Water Temperature

Discussion in 'Plumbing and Central Heating' started by Master Mech, 16 Jun 2020.

  1. bernardgreen

    bernardgreen

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    I have had reason to look up the range of temperature for mains water supply in the London area.

    "Mains water is distributed across London through a network of pipes, and varies in temperature between 5 and 20°C during the year."

    That led to some thoughts about boilers.

    With primary flow set for 60°C the delta T varies from 55°C ( in winter ) down to 40° ( in summer )

    Delta T varies from 38°C ( winter ) and 23°C ( summer )

    It would seem sensible that the flow restrictor in the boiler should allow enough flow for the 23°C delta to be achieved before the modulation reaches its bottom limit and rapid cycling has to happen.
     
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  3. DP

    DP

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    Primary flow is not set to 60. When boiler runs full tilt to deliver hot water, flow temperature hovers around 70
     
  4. DP

    DP

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    Combi is manufactured To raise the cold water temperature by 35 degrees by restricting flow through the boiler. This temperature can then be adjusted by the end user by adjusting the control on the boiler ( if one is fitted)
     
  5. bernardgreen

    bernardgreen

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    Not talking about "full tilt"...... It is about minimum "tilt" being too high for the DHW flow rate.
     
  6. DP

    DP

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    Bernard, end user does not control the primary temperature. OP is getting 60 degrees due to a fault condition. This fault is causing the hot water temperature to swing about instead of steady temperature because boiler is shutting down when 60 degrees is reached for primary temperature As has been stated by OP, fan starts to slow down and then stop ( which then causes delivered hot water temperature to fall). Then boiler starts and hot water temperature rises to primary temperature reaches 60

    For correct operation, burner should not shut down.
     
  7. bernardgreen

    bernardgreen

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    What option has the control system got if the amount of heat being produced at minimum burn rate cannot be taken away from the boiler ?.
     
  8. DP

    DP

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    Fan runs slower to match heat generated to heat used:whistle:
     
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  9. nwgs

    nwgs

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    How would that happen ? Everything increase And decreases with the flow of water. You know a boiler that can supply 16 pm will manage to adjust its self if only 8lpm is available. As DP said, fan speed, which reduces gas demand, which reduces heat used, which reduces the load.

    40kw would be capable of causing CH issues, but given you live in the dark ages, it’s really not an issue for you

    The mind boggles
     
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  11. Master Mech

    Master Mech

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    Sorry guys this is all way over my head so what is my next move?

    Today I removed the strainers from the mains supply cock & the one in the hydraulic inlet assembly where the flow restrictor is. Both were immaculately clean.
     
  12. bernardgreen

    bernardgreen

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    So the fan could in the most extreme case could slow down to. ? You tell me what is the lowest percentage of maximum fan speed that the boiler can safely operate at.

    I assume you are aware that almost all combustion chambers have a minimum gas rate, below this gas rate the combustion becomes unstable and the boiler has to be shut down
     
  13. DP

    DP

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    Bernard, let us leave this discussion where it is as we are not making any progress here
    You are talking as a layman, me as a service person who works with these type of boilers.

    Combustion is set for exact values at low and high fire, anything out of spec causes boiler to shut down
     
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  14. nwgs

    nwgs

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    Why extreme cases ? Fan speeds change constantly

    You’re obviously thinking of a 240v standard speed fan, not a premix boiler.
     
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  15. bernardgreen

    bernardgreen

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    No I am thinking of a variable speed fan.

    What is the "exact value" for low fire compared to the amount of heat energy required for the minimum specified water flow rate and mains water at the high end of temperatures to be expected in summer.

    It does seem that in some boilers the low fire produces more heat than can be removed from the boiler when delta is low and there is a restricted flow of water. The restriction being due to the boiler having a flow restrictor as part of it's design.
     
  16. nwgs

    nwgs

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    Just give up Bernie.
     
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  17. dilalio

    dilalio

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    = the square root of fu¢k knows! :LOL:
     
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