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Thread: How to control secondary pump from Evohome

  1. #61
    Automated Home Legend paulockenden's Avatar
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    And if you have a combi then the boiler size will be dictated by the simultaneous heat demand (how many bathrooms, etc.)

    But some boilers allow you to specify a different rating on the CH and DHW side - my Intergas certainly does.

  2. #62
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    My Vaillant is also capable of doing this but only in conjuction with a Vaillant wiring centre and some creative wiring. I can have different Flow Temperature but also different KWh load settings based on a CH or DHW heating schedules.

  3. #63
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    In theory, all modulating boilers should cope with differing heat demands. In practice, being able to reduce the KW rating (on a Vaillant especially) can aid with the boiler not overshooting.

    However, the biggest problem is the modulation ratio. On the 438, for example, the boiler can only modulate down to 6KW or so, which is still a large output if you only have one radiator calling for heat - which inevitably leads to cycling.

  4. #64
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    What I have found, much to my pleasant surprise, is that if you set the Vaillant 438's partial loading lower than an amount, (which I cannot remember!), it actually behaves like the lower rated boilers in that same series and then modulates down to the lower point as per those other boilers. I noticed this only because Boiler Modulation is one of the ebus parameters I monitor.

  5. #65
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    It'll only go so far. The actual gas valve assembly is different and the PCB setting needs to be adjusted in d.93:

    https://www.urbanplumbers.co.uk/vail...70-fault-code/

    The guy on diynot actually changed his, as the 438 was oversized. Essentially he has a 428 now:

    438 has gave valve 0020035222 that is sized to burn lots of gas and 428 will have 053574 that delivers much less gas.
    All other parts in these boilers are absolutely the same

    Look here
    https://www.wolseley.co.uk/wcsstore....8438_Parts.pdf

  6. #66
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    Any direct link to the diynot thread on this. Because my gas valve has started drifting a bit and needs readjusting each year. If I need to buy a new one, this option is quite attractive to me. Because now my boiler is definitely over sizer after the installation of new insulation and UFH.

  7. #67
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    No thread, it was a direct conversation. But you can reach out to notorious on there - just PM him.

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