How to control secondary pump from Evohome

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  • fezster
    Automated Home Sr Member
    • Dec 2017
    • 72

    #46
    Originally posted by dty View Post
    when I look at my modulation power and my dT, they are directly proportional to each other, as would be expected with a fixed speed pump.
    So as power goes down, so does dT?

    Flow rate=KW/(4185 * dT)

    Which is precisely my concern because it means when the boiler is modulating (most of its life), it is not running at the design 20 degree dT to be able to condense. This seems to contradict the manufacturers recommendations on a LLH being the most efficient way to run a boiler.

    Comment

    • dty
      Automated Home Ninja
      • Aug 2016
      • 489

      #47
      I've not thought about it too closely, to be honest. I have the LLH simply because the boiler's integrated pump doesn't have enough head after the losses in the heat exchanger to cope with my system.

      Given the manufacturers recommend this (and it seems to be standard practice in the US and Europe), I'm assuming it's still sane. Do you really need a dT of 20 to condense? I thought you'd get condensation with a reasonably low (< 50C or 55C) return?

      Comment

      • bruce_miranda
        Automated Home Legend
        • Jul 2014
        • 2307

        #48
        The Vaillants are designed and speced for a 20C dT. Bit most run a 11C drop. Trying to achieve a 20C dT is quite hard, in practice especially with a large boiler.

        Comment

        • fezster
          Automated Home Sr Member
          • Dec 2017
          • 72

          #49
          Originally posted by dty View Post
          I've not thought about it too closely, to be honest. I have the LLH simply because the boiler's integrated pump doesn't have enough head after the losses in the heat exchanger to cope with my system.

          Given the manufacturers recommend this (and it seems to be standard practice in the US and Europe), I'm assuming it's still sane. Do you really need a dT of 20 to condense? I thought you'd get condensation with a reasonably low (< 50C or 55C) return?
          True - if your flow temps are low, then you'll get condensing. I guess it's only an issue once your flow temps are over 65. I, too, have the issue that the combined head loss of both the boiler and system are quite high, so am having to use a semi-commercial pump. I'm hoping the LLH will allow me to use 2 domestic pumps, and have the boiler operating more happily with constant flow rate through it (i.e. no more ABV, which I could never quite get right anyway).

          I have an added issue of having 2 heating zones (upstairs and downstairs), and if they are running at significant temperature differentials, the boiler gets a sudden rush of cold water meaning it hits S53. Sometimes this ends up in a perpetual cycle, meaning the boiler is in S53 mode for over an hour, and the heating running lukewarm only. I guess in your case, as you have EvoHome, you wouldn't have this problem, as hot water is circulating your entire system, even if it isn't actually entering all of the radiators.

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #50
            Actually its worse with Evohome. Water doesnt actually flow unless its going through a radiator. So depending on how many are actually calling for heat and when you actually might have a situation where only one rad comes on first thing and so you get S53 because the hot water isn't leaving the boiler fast enough from the flow.

            Close Coupled Tees might be another option.

            Comment

            • fezster
              Automated Home Sr Member
              • Dec 2017
              • 72

              #51
              That's where the ABV should kick in to maintain the minimum flow rate. Slightly different problem to the one I described, which is inherent in any cold zone flushing water to the boiler return (this could be because DHW kicks in, instead of another radiator zone).

              A LLH would (hopefully) aid in both instances, as the hot and cold water from different zones should dilute at a slower pace in the header, allowing the boiler to sense the return temp dropping and have enough time to modulate down.

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #52
                Sorry I meant that you get an S53 when the next set of cold rads call for heat, which means that now a lot of cold water is suddely rushing to the boiler. Similar to your situation.

                Comment

                • fezster
                  Automated Home Sr Member
                  • Dec 2017
                  • 72

                  #53
                  Originally posted by bruce_miranda View Post
                  Sorry I meant that you get an S53 when the next set of cold rads call for heat, which means that now a lot of cold water is suddely rushing to the boiler. Similar to your situation.
                  Ah - actually good to hear I'm not the only one experiencing that issue.

                  Though slightly concerned that the mix of cold water with the hot in the pipework isn't sufficient for the boiler to sense the cooling temperature fast enough. Hope the LLH achieves that.

                  Comment

                  • dty
                    Automated Home Ninja
                    • Aug 2016
                    • 489

                    #54
                    Originally posted by bruce_miranda View Post
                    Close Coupled Tees might be another option.
                    That's actually what I have rather than an all-in-one LLH with deaerator and magnetic trap thingy. Same thing though, hydraulically.

                    Comment

                    • JohnH
                      • Dec 2017
                      • 1

                      #55
                      Evohome and Vaillant S53

                      Originally posted by fezster View Post
                      Ah - actually good to hear I'm not the only one experiencing that issue.

                      Though slightly concerned that the mix of cold water with the hot in the pipework isn't sufficient for the boiler to sense the cooling temperature fast enough. Hope the LLH achieves that.
                      Hi, I realise this is an old thread, but I am researching why my Vaillant (637 EcoTec Plus) is hitting so many S53 faults and then eventually F24's when it switches off.

                      The Vaillant has been happy for about 10 years running with 2 external pumps for upstairs/downstairs zoning.
                      I have just added an UFH heating loop (4 manifolds and about 300metres) and removed about 10 rads.
                      We have also just upgraded to the EvoHome with digital TRVs and the UFH control.

                      To stop hot water circulating around the entire 3 story house looking for 1 rad that might be calling for heat, we fitted 2 EH control units.
                      1 controls bedrooms and has 12 zones on it.
                      The other controls the UFH, 3rads and the DHW.

                      Since upgrading I am getting S53 errors continuously. I have flushed the system several times to ensure there is no gunk (we have a Magna anyway).

                      The flow/return differential can be as great as 30degrees. This is normally at night when most of the rads are set to 15degrees. UFH set to 21degrees.
                      When something calls for heat, the return flow temperature drops dramatically and forces the S53 issue.

                      We used to have 26+rads so the 37kw boiler was recommended at the time.
                      We now have 15 rads, UFH and a 300litre storage tank.

                      If anybody still active on this thread, were any solutions found?

                      Thanks in advance

                      Comment

                      • fezster
                        Automated Home Sr Member
                        • Dec 2017
                        • 72

                        #56
                        Do you have an Automatic Bypass Valve? It needs to be set correctly to maintain the minimum flow through the boiler.

                        Honestly, I would advise just biting the bullet and having a Low Loss Header installed. Everything seems to work much more predictably with no need for expensive Magna pumps.

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #57
                          Either that, derate the boiler. I too had no end of pain with my 38KW boiler that was sized for radiators but then when we put in UFH, the heating requirements dropped. My boiler now runs at 18KW and because all my radiators aren't calling for heat at the same time, everything seems to work perfectly.
                          I use a normal Alpha pump, not even a LLH.

                          Comment

                          • fezster
                            Automated Home Sr Member
                            • Dec 2017
                            • 72

                            #58
                            The boiler size isn't dictated by the UFH and / or number of radiators, though. It's dictated by the heat loss of the house (in the depths of winter). The UFH/rads are then sized to meet that minimum heat loss, and preferably rads are oversized to allow lower flow temps.

                            It's always a compromise because there are only a handful of days where the outside temperature is -1. I think the newer boilers are able to modulate down much further, though.

                            Comment

                            • bruce_miranda
                              Automated Home Legend
                              • Jul 2014
                              • 2307

                              #59
                              True, but besides heat loss, you also need to check what is the heat demand going to be like. When you have zoned systems like Evohome, very rarely are more than a handful of radiators demanding heat. So regardless of what you r whole house heat loss calculations say, at that moment, your heat loss is only limited to the zones being heated.

                              Comment

                              • fezster
                                Automated Home Sr Member
                                • Dec 2017
                                • 72

                                #60
                                Totally agree, hence my point about a LLH making everything behave much more predictably. The LLH handles the minimum flow rates through the boiler, without having to have the compromise of an ABV which seldom works as expected.

                                Range rating the boiler is really only an option if your boiler is genuinely oversized (which admittedly often happens).

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