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Thread: How to control secondary pump from Evohome

  1. #11
    Automated Home Legend paulockenden's Avatar
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    That's just modulation, surely?

    Perhaps Mr Vaillant meant that there's a boiler model in their System range that has a greater modulation ratio than your heat only boiler?

  2. #12
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    Quote Originally Posted by bruce_miranda View Post
    Why not run a single loop with two pumps?
    In theory, pumps in series increase pump head, which is what I need. (Pumps in parallel increase flow.) But you can't really run pumps in series unless they are 100% matched. Slight variations in start up speed will cause them to fight each other - especially if they are modulating. Imagine too having one pump with one flow rate and the next pump with a very slightly different flow rate. How does that work? What speed will the water move through the entire system? At least one of the pumps will be highly stressed plus you'll have high or low pressure in the zone between them which will further confuse them if they're modulating.

    Now consider that one of the pumps is built-in to the boiler. What are my chances of sourcing a 100% identical pump without even the tiniest manufacturing discrepancy?

    Pumps in series - good in theory, disasterous in practice.

  3. #13
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    Quote Originally Posted by paulockenden View Post
    That's just modulation, surely?

    Perhaps Mr Vaillant meant that there's a boiler model in their System range that has a greater modulation ratio than your heat only boiler?
    Modulation tends to be used when you consider moving down from a position. What I experience is that I have to manually set a maximum, from which point the boiler modulates down. the system boiler can apparently figure out what it can crank It's maximum to, before modulating down from there.

  4. #14
    Automated Home Legend paulockenden's Avatar
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    Colour me sceptical on that one!

  5. #15
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    Atleast this was the explaination given to me when I kept getting the S.53 status code on my boiler that was kept at the factory default of 38Kw. Hence needing to be manually wound down to 28Kw.

    P.S. just checked and it appears to be true. On a System boiler the maximum heat output can be set to Automatic. That option isn't available on a Heat only boiler.
    Last edited by bruce_miranda; 27th May 2017 at 09:06 PM.

  6. #16
    Automated Home Legend paulockenden's Avatar
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    From the Vaillant system boiler commissioning manual:

    "The boilers are fully modulating for central heating, and
    it is therefore not necessary to range rate the boiler.
    However, if desired, it is possible to range rate the boiler,
    as follows:"

    And from the open vent manual:

    "The heating partial load of the product is set to Auto at the
    factory. The product independently determines the optimum
    heating output depending on the current heat demand of
    the system. You can retroactively change the setting in the
    Diagnostics menu."

    Looks quite similar to my non-heating-engineer eyes!

    P.

  7. #17
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    As I said, it could be a Vaillant 438 shortcoming then. There is no Automatic setting on that Open Vented boiler and they are set at max from the factory. But on the system boilers there is an Automatic setting and therefore (apparently) they don't run into the S.53 status that my boiler was running into when it was set too high.
    Last edited by bruce_miranda; 28th May 2017 at 10:20 AM.

  8. #18
    Automated Home Legend paulockenden's Avatar
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    Ah - just googled the error code and it's the delta-t being more than 20 degrees.

    Apparently other boiler manufacturers ramp up the boiler output so that this doesn't happen, but Vaillant starts up at full pelt.

    Also interesting to read that this error code can also happen if a boiler is too SMALL for the system. If there's loads of rads and a cold house then the water on return will be quite cold and so delta-t will be greater than 20!!!

    So it sounds like this particular error can hit you from both ends!

  9. #19
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    S.53 can also be an issue with your pump. Had a few issues with the Vaillant Open Vents with this issue that if your pump is not performing well you will get the S.53 or S.54 error. It can also mean a variety of issues so don't just assume it means one thing.

    I tend to agree with Paul though, it shouldn't make any difference whether it is open vent or system. The boiler should realise that 38kws aren't needed on the demand and then modulate down from that. On the new Open Vent EcoTec Plus ones there is a setting asking you what you want the boiler set on for max output. What really is the difference between the Open Vent and the system other than it can modulate the pump speed?

  10. #20
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    So according to Mr Vaillant on a System boiler because the heat and the pump are being controlled by the boiler itself, it is able to vary the heat output and pump speed to adjust itself. But on my 438 the boiler starts at the max and the pump and pipework were not able to shift that hot water out fast enough, so I get the S.53. My previous pipework was 22mm and I couldn't go past 22Kw before I hit that status code. After I changed to a 28mm heating loop and the new Alpha2 pump, I can crank the boiler up past 30Kw without a problem. But the 38Kw is seriously OTT for my house, so It's currently set up as a 28Kw boiler. And all is fine.

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