How to control secondary pump from Evohome

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  • dty
    Automated Home Ninja
    • Aug 2016
    • 489

    #31
    Just to wrap this up, the VR40 has been installed and seems to work perfectly. Shame it required another add-on board, but at least it all works now. I've left a rad valve slightly open until my heating guy can come and install an ABV on the secondary circuit. Happy days.

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    • bruce_miranda
      Automated Home Legend
      • Jul 2014
      • 2307

      #32
      And do both pumps run for the same length of time?

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      • dty
        Automated Home Ninja
        • Aug 2016
        • 489

        #33
        Yes, hence why I really need a bypass on the secondary side. Really, it's no bad thing as the primary loop is scarcely 3 ft long, so not really enough to dissipate any significant heat on the overrun.

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        • fezster
          Automated Home Sr Member
          • Dec 2017
          • 72

          #34
          Sorry for reviving this thread, but did you fit an ABV on your secondary side?

          I'm having a LLH installed and all of the installers have said I can do away with the ABV, but I'm thinking ahead to installing HR92s on all my rads, in which case is it possible for all of the TRVs to be closed, yet the boiler still firing? The difference in my case will be that the secondary pump will be driven from the zone valve on the secondary side. Not knowing how evohome works, is there any scenario where all of the HR92s are closed, but the zone valve is still open? Or is that impossible?

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          • fergie
            Automated Home Sr Member
            • Mar 2017
            • 92

            #35
            Hot water demand.

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            • fezster
              Automated Home Sr Member
              • Dec 2017
              • 72

              #36
              DHW has its own zone valve - my system is S Plan plus.

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              • dty
                Automated Home Ninja
                • Aug 2016
                • 489

                #37
                Yes there is. It seems odd to me, but when closing down, Evohome shuts the HR92 valves before it shuts off the zone valves, boiler, etc. Plus there’s always the scenario of missing radio messages. You need a bypass of some sort. Period.

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                • paulockenden
                  Automated Home Legend
                  • Apr 2015
                  • 1719

                  #38
                  Originally posted by dty View Post
                  Yes there is. It seems odd to me, but when closing down, Evohome shuts the HR92 valves before it shuts off the zone valves, boiler, etc.
                  It's because of the way the system works. Heat demand comes from the HR92s. So when a zone is hot enough the HR92s shut down, then tell the controller that there's no demand. The controller then adjusts the heat demand (which can be anywhere between zero and 100 percent) that it sends to the boiler controls (BDR or OT).

                  P.

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                  • fezster
                    Automated Home Sr Member
                    • Dec 2017
                    • 72

                    #39
                    Originally posted by dty View Post
                    Yes there is. It seems odd to me, but when closing down, Evohome shuts the HR92 valves before it shuts off the zone valves, boiler, etc. Plus there’s always the scenario of missing radio messages. You need a bypass of some sort. Period.
                    I thought this may be the case. Obviously, with a LLH, the boiler has a pathway to expel it's heat, but I was thinking about the pump on the secondary side not having anywhere to pump to, perhaps leading to premature failure. I'm loathed to install an ABV as they often seem to pass water when not intended, so am thinking perhaps a modulating pump is the answer.

                    Comment

                    • fergie
                      Automated Home Sr Member
                      • Mar 2017
                      • 92

                      #40
                      My boiler is in the garage along with an open rad which acts as an overflow. I’ve also got 3 towel rails in bathrooms which aren’t controlled by EH and therefore left partly open all the time.

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                      • dty
                        Automated Home Ninja
                        • Aug 2016
                        • 489

                        #41
                        Originally posted by fezster View Post
                        I thought this may be the case. Obviously, with a LLH, the boiler has a pathway to expel it's heat, but I was thinking about the pump on the secondary side not having anywhere to pump to, perhaps leading to premature failure. I'm loathed to install an ABV as they often seem to pass water when not intended, so am thinking perhaps a modulating pump is the answer.
                        If you can find a modulating pump that is happy to be deadheaded, please let me know. If you do go for a modulating pump, be aware that you can't then use an ABV at all as they end up fighting each other.

                        Comment

                        • fezster
                          Automated Home Sr Member
                          • Dec 2017
                          • 72

                          #42
                          Originally posted by dty View Post
                          If you can find a modulating pump that is happy to be deadheaded, please let me know. If you do go for a modulating pump, be aware that you can't then use an ABV at all as they end up fighting each other.
                          @dty Could you tell me a little about your LLH setup? Do you have a fixed speed pump on your primary/boiler side? And if so, is it rated to overcome the boiler's hex resistance at max KW output?

                          I wanted to try and understand what happens on the primary side of an LLH when the boiler modulates down? As the flow rate is fixed, the delta T at the boiler must surely reduce. Is this what you see in reality, though?

                          The secondary side flow rate will vary according to heat demand (assuming a variable speed pump), but the primary side is fixed. Trying to get my head around why a LLH is often cited as the best way to separate primary and secondary flows - is it more about convenience than efficiency?

                          Comment

                          • dty
                            Automated Home Ninja
                            • Aug 2016
                            • 489

                            #43
                            I have a system boiler, so it has a built-in pump that I don’t know a lot about other than it didn’t have enough remaining head to reach the extremities of my large system.

                            Comment

                            • fezster
                              Automated Home Sr Member
                              • Dec 2017
                              • 72

                              #44
                              With a single zone calling for heat, have you ever examined the flow/return temps on the boiler? Are they still at a differential close to 20, even though the boiler is probably running at it's lowest modulation setting?

                              Comment

                              • dty
                                Automated Home Ninja
                                • Aug 2016
                                • 489

                                #45
                                Checking the installer manual, it specifically says to set the pump to one of the fixed speed modes if using the boiler with a LLH. And, indeed, 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. The secondary side, however, is running on variable speed.

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