How to control secondary pump from Evohome

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  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    #16
    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.

    Comment

    • bruce_miranda
      Automated Home Legend
      • Jul 2014
      • 2307

      #17
      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; 28 May 2017, 09:20 AM.

      Comment

      • paulockenden
        Automated Home Legend
        • Apr 2015
        • 1719

        #18
        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!

        Comment

        • mtmcgavock
          Automated Home Legend
          • Mar 2017
          • 507

          #19
          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?

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #20
            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.

            Comment

            • mtmcgavock
              Automated Home Legend
              • Mar 2017
              • 507

              #21
              I have to say 38kw for a heat only boiler is ridiculously big, out of interest why on earth did you have one so large?

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #22
                I was stupid and listened to my plumber...he just said it's easier to wind a boiler down than realise it's too small. If there were models between 28Kw and 38Kw we would have had a bit more discussion. Also at the time it was my first new boiler and I just went on blind faith.

                A few other plumbers since then have told me that it wasn't a bad choice as a 438 has a bigger and better heat exchanger so works better than a 428, even with the 438 wound down to 28Kw.
                Last edited by bruce_miranda; 28 May 2017, 07:15 PM.

                Comment

                • fergie
                  Automated Home Sr Member
                  • Mar 2017
                  • 92

                  #23
                  Originally posted by bruce_miranda View Post
                  How I wish I had not listened to my plumber and installed the system boiler that I wanted, rather than the heat only that he recommended. Ofcourse he did manage to convince me at the time, that keeping the boiler to the bare minimum was best and allowed other parts to be changed with ease. And since then I have changed the pump to a quiet Alpha2, would not have been able to do that with a system boiler.
                  My Atag installer was the opposite- sold me on the system boiler by explaining it meant the 10y warranty would also cover the internal pump etc. Also solved the problem of the loud external pump in the airing cupboard by getting it removed.

                  Like you I was between sizes - 24kw too small and 32kw possibly too big, but I'm planning to extend the house further and Scottish winters can be pretty cold!

                  Comment

                  • dty
                    Automated Home Ninja
                    • Aug 2016
                    • 489

                    #24
                    Meanwhile, back in the land of secondary pumps, the decision has been taken to drive the pump off the boiler. Vaillant say I need a VR40 (2-in-7 multi-function module) for this purpose. To solve the over-run issue, I just need to leave a rad open to act as a bypass until my heating guy can fit a proper ABV.

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      #25
                      So are you planning to run the secondary pump off the VR40? Or just in parallel with the primary pump.

                      Comment

                      • dty
                        Automated Home Ninja
                        • Aug 2016
                        • 489

                        #26
                        Electrically, the first thing. Logically, the second thing! :-)

                        There's nowhere on the PCB to run the secondary pump from. I could bodge it by wiring the secondary to the same connector as the primary, but I don't know if the board is capable of driving that load. Or I could bodge it by wiring the secondary pump directly to the primary pump, but again I don't know if the board can drive that, plus the existing wire would need to cope with the load. I could use a traditional external relay, but I'd have to cut into the wires for the primary pump. All of these things are bodges. Also warranty.

                        So, VR40. It has two relays on board each of which can be configured in one of several modes (more than 7, I think), one of which is "external pump". So the secondary pump will come on with the primary, which is what I wanted. And I really should have an ABV on the secondary side, so fixing that solves the overrun problem.

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #27
                          Interesting bit of kit. I finally learnt what D.27 and D.28 are used for. I wonder if the internal and external pump can have different over run times. I doubt it but would be good to know.

                          Comment

                          • dty
                            Automated Home Ninja
                            • Aug 2016
                            • 489

                            #28
                            I doubt it. There's nothing in the parameters about separate overruns. In fact, d.27 and d.28 are the only things in the parameters that reference this board.

                            Comment

                            • mtmcgavock
                              Automated Home Legend
                              • Mar 2017
                              • 507

                              #29
                              Surely this means your pump will run whether the boiler is pumping whether that be a HW or CH demand. So why not just wire the pump into the S/L on the boiler and then it wouldn't over run?

                              Comment

                              • dty
                                Automated Home Ninja
                                • Aug 2016
                                • 489

                                #30
                                Because I'm using OpenTherm and don't have a switched live. I mean... the wire is still there, but there's nothing connected to the other end of it. If I could bind a boiler BDR, that would work fine. But I can't... because OpenTherm.

                                Comment

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