Evohome S Plan conversion from 3 relay to 2 with no bypass

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  • John Cooper
    Automated Home Jr Member
    • Oct 2015
    • 15

    Evohome S Plan conversion from 3 relay to 2 with no bypass

    This is sort of connected to the S-Plan vs Y-Plan for Evohome thread which is ongoing, but this is separated for clarity.

    I am replacing an old boiler with new Worcester Bosch 40 CDi (LPG not that it matters).

    My current controls are Evohome with three BDR91 relays controlling CH pump, HW pump and ancient boiler, no valves.

    The installers aren't really happy with this and would prefer conventional S plan with one pump and two MOVs controlled by CH and HW signals (they don't really want to know that these come from two BDR91s).

    I am happier with valving CH and HW separately anyway so I have agreed. I have a problem with Evohome losing contact with some remote radiators and can re-deploy the spare BDR91 as a second CH trigger controlled by another Evohome controller sited so that it can see both the missing radiators and the relay, but that's another matter.

    At the moment a bypass loop for overrun pumping is not necessary as Evohome takes care of that, having control of both boiler and pumps (correct me if I'm wrong).

    As I understand it, reading the manual, the new boiler does not have a dedicated overrun output signal, and just runs the pump when it wants. So I would need an automatic bypass loop after the pump to stop it pumping against closed valves.

    I don't like that and would prefer the overrun to pump via the DHW loop when the demand for both CH and HW drops. I have an aversion to dissipating heat in a bypass loop and would rather see it do some useful work.

    I think the way to do this is via an extra changeover relay, which switches the boiler input (Common contact) between the output from the HW MOV (Normally Closed contact) and disconnected (Normally Open contact), controlled by the boiler pump signal. The HW BDR91 switches the HW MOV to either live (Normally Open) or the boiler pump signal.
    The net effect of this (ignoring CH which operates normally) is that the boiler runs when there is HW demand, and the MOV is forced open when there is pump demand, regardless of HW demand.

    Is this a reasonable approach or should I just let the installers have their bypass loop?
  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    #2
    You'll still need an ABV for times when the boiler needs to cool using over-run but all of the TRVs are closed.

    This might happen, for example, when your schedule changes your zones to overnight temperatures.

    Comment

    • MrB
      Automated Home Sr Member
      • Oct 2015
      • 80

      #3
      I have a WB 30CDi and I seem to remember the WB installer manuals show all the safety requirements to be plumbed in for this boiler.
      Pressure release, air bleed, and ABV.

      There are so many different overrun scenarios that the high pressure release valve and ABV are the final safety items.
      Play with them/remove them and you could see your roof jump 10ft in the air ... :-)

      Comment

      • John Cooper
        Automated Home Jr Member
        • Oct 2015
        • 15

        #4
        Originally posted by paulockenden View Post
        You'll still need an ABV for times when the boiler needs to cool using over-run but all of the TRVs are closed.

        This might happen, for example, when your schedule changes your zones to overnight temperatures.
        I think you miss my point, if the boiler needs to cool it will still run the pump, and open the DHW MOV instead of a bypass, but the extra relay isolates the MOV from re-enabling the boiler.

        Comment

        • John Cooper
          Automated Home Jr Member
          • Oct 2015
          • 15

          #5
          Originally posted by MrB View Post
          I have a WB 30CDi and I seem to remember the WB installer manuals show all the safety requirements to be plumbed in for this boiler.
          Pressure release, air bleed, and ABV.

          There are so many different overrun scenarios that the high pressure release valve and ABV are the final safety items.
          Play with them/remove them and you could see your roof jump 10ft in the air ... :-)
          Surely the only overrun scenario is when the boiler is or has recently been firing and the demand stops - whether timed, or by temperature it runs the pump for a while. You just need to make sure it is not pumping against two closed valves.

          Comment

          • John Cooper
            Automated Home Jr Member
            • Oct 2015
            • 15

            #6
            OK, scratch all the above, it won't work the way I think it will work.

            Need to think about all the transitions. Need to look at the logic state of the pump being run by the boiler when the boiler itself is not being demanded.


            I don't think it can be done with a simple relay.

            I can see the attractiveness of a simple bypass.

            Comment

            • DBMandrake
              Automated Home Legend
              • Sep 2014
              • 2361

              #7
              Originally posted by John Cooper View Post
              At the moment a bypass loop for overrun pumping is not necessary as Evohome takes care of that, having control of both boiler and pumps (correct me if I'm wrong).
              Unless you have a bypass radiator in your central heating loop, then yes you are wrong. In your current config the pump that supplies the cylinder will always be able to flow freely, however the pump that supplies the central heating loop must have one bypass radiator otherwise if all TRV's are closed it will be pumping into nowhere, which will put a lot of strain on the pump.

              You can't rely on the pump only being triggered to run when HR92's are open, even if you don't have pump overrun, because the wireless communications are not 100% reliable, so it can happen at times that the heating relay (and thus your CH pump) would get switched on for a period of time as much as 20 minutes when no HR92's are actually open. Also the HR92's may only be very slightly open and yet still call for some heat from the system and turn on your CH pump even though the flow restriction is too great for the pump and does not meet the boilers minimum flow requirements.

              Although you might have got away with it, unless you had a bypass radiator you should have already had an ABV on the output of the central heating pump if you want a trouble free system.
              As I understand it, reading the manual, the new boiler does not have a dedicated overrun output signal, and just runs the pump when it wants. So I would need an automatic bypass loop after the pump to stop it pumping against closed valves.

              I don't like that and would prefer the overrun to pump via the DHW loop when the demand for both CH and HW drops. I have an aversion to dissipating heat in a bypass loop and would rather see it do some useful work.

              I think the way to do this is via an extra changeover relay, which switches the boiler input (Common contact) between the output from the HW MOV (Normally Closed contact) and disconnected (Normally Open contact), controlled by the boiler pump signal. The HW BDR91 switches the HW MOV to either live (Normally Open) or the boiler pump signal.
              The net effect of this (ignoring CH which operates normally) is that the boiler runs when there is HW demand, and the MOV is forced open when there is pump demand, regardless of HW demand.

              Is this a reasonable approach or should I just let the installers have their bypass loop?
              Something to be aware of is an automatic bypass valve is actually mandated by building regulations for new installs now, so legally they can't NOT install one for you if they make major changes like converting it to conventional S-Plan, unless they are being naughty. (This probably explains their reluctance to not install it) And in general on an S-Plan system you must fit one after the pump and before the zone valves because no matter how clever your control system is there is always the chance that both zone valves will close at once due to fault or error - there has to be somewhere for that flow to go to protect both the pump and the boiler from damage.

              Also keep in mind that an ABV helps to regulate the differential pressure across the radiator loop - this helps to prevent noisy radiators when only one or two radiators are open, but still allow all the flow to go through radiators (and not the bypass) when all the radiators are open. Even aside from protecting the pump and boiler it makes the system run more smoothly and consistently as the number of open radiators changes.

              If you want to come up with a clever wiring scheme to dump the pump overrun into the hot water cylinder then having an ABV installed doesn't preclude that in any way - as soon as your zone valve to the cylinder opens the differential pressure will drop and the ABV will close and be removed from the equation - all the flow will go through the cylinder and none will be "wasted" by dissipation in the ABV loop.

              As you noted in my S-Plan thread I also wasn't happy with it flowing only through the bypass loop during every TPI cycle off period hence configuring the wiring to keep the heating zone valve open as much as possible, except where it doesn't make sense. I'm sure you could come up with something to dump excess heat into the cylinder, but keep in mind that if you dump the excess into the cylinder every TPI cycle the cylinder will very quickly go over temperature even though you're only transferring latent heat from the boiler.

              Comment

              • John Cooper
                Automated Home Jr Member
                • Oct 2015
                • 15

                #8
                Originally posted by DBMandrake View Post
                Although you might have got away with it, unless you had a bypass radiator you should have already had an ABV on the output of the central heating pump if you want a trouble free system.

                Something to be aware of is an automatic bypass valve is actually mandated by building regulations for new installs now, so legally they can't NOT install one for you if they make major changes like converting it to conventional S-Plan, unless they are being naughty. (This probably explains their reluctance to not install it) And in general on an S-Plan system you must fit one after the pump and before the zone valves because no matter how clever your control system is there is always the chance that both zone valves will close at once due to fault or error - there has to be somewhere for that flow to go to protect both the pump and the boiler from damage.
                Thanks for all your comments. Rapidly coming to the conclusion that I should leave well alone. Things are the way they are for a reason.

                The bypass loop I thought I had is a towel rail, and having finally traced through the pipework I find it is connected before the pump, so it isn't. There are several open radiators in the house without TRVs so I am not worried about things now. Just the future with a new boiler when all radiators will have HR92s and both circuits have MOVs

                Comment

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