Vaillant flow temperature control for HW

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  • Symbolics
    Automated Home Lurker
    • Feb 2019
    • 6

    #16
    Originally posted by mtmcgavock View Post
    You are mis understanding. The 65oc is the temperature of the water in the cylinder. When using the VR66 you have to use the Vaillant NTC probe into the cylinder. This then measures the temperature of the HW, which you set in your Vaillant controller, or on the front of the boiler. You then set which flow temperature you require for heating DHW in the Diagnostic menu along with what KW output you require for the cylinder.

    Settings D.020, D.077 and D.078 is what you require.
    Sorry, yes I did understand that using the NTC probe was one way to go. But I thought that the VR66 can alternatively take, as its input, the cylinder temperature dry contact that simply signals a demand for more hot water generation. This is the operating mode I am using. The primary benefit of this operating mode is that it allows me to site the Megaflo tank in the middle of the house, and the Vaillant boiler about 20 metres away in a boiler house. This keeps the noise levels down. Obviously the hot water has to travel over than 20 metres with very little heat loss - which it does, in my set up. I have Uponor district heating style insulated underground pipes and I lose less than one degree over that 20 metres.

    Comment

    • mtmcgavock
      Automated Home Legend
      • Mar 2017
      • 507

      #17
      Originally posted by Symbolics View Post
      Sorry, yes I did understand that using the NTC probe was one way to go. But I thought that the VR66 can alternatively take, as its input, the cylinder temperature dry contact that simply signals a demand for more hot water generation. This is the operating mode I am using. The primary benefit of this operating mode is that it allows me to site the Megaflo tank in the middle of the house, and the Vaillant boiler about 20 metres away in a boiler house. This keeps the noise levels down. Obviously the hot water has to travel over than 20 metres with very little heat loss - which it does, in my set up. I have Uponor district heating style insulated underground pipes and I lose less than one degree over that 20 metres.
      Even when using the standard Stat on the VR66 the flow temperature will revert to the D.078 setting. The Hot water setting on the boiler stays active as it's a software limit, even when just using a Switch Live rather than a VR66 it's still adjustable.

      Comment

      • Symbolics
        Automated Home Lurker
        • Feb 2019
        • 6

        #18
        Originally posted by mtmcgavock View Post
        Even when using the standard Stat on the VR66 the flow temperature will revert to the D.078 setting. The Hot water setting on the boiler stays active as it's a software limit, even when just using a Switch Live rather than a VR66 it's still adjustable.
        Yes, I have D.078 set at 80 degrees. Regardless, what happens when there is heating demand is that the boiler goes to about 65 degrees on the flow temp, and then goes into pump overrun mode. So even when there is no NTC connected allowing the boiler to figure out what the cylinder temperature is, the output DHW is modulated down to 65, and the actual temperature varies between 60 and 65.

        It is also interesting that the boiler seems to look at the return temperature and set a flow temperature based on the incoming water temperature. If the return temperature is (say) 52 degrees, the flow will be about 60. Maybe this is to allow condensing to happen, but in this mode the boiler is modulating right down to 8kW. This causes a very slow heating of the cylinder, which in turn causes long run times for heating mode.

        All in all, far from ideal, and I regret using the Vaillant boiler for this application. I may still buy a dedicated DHW boiler with a simple DHW temperature control that can just heat the water to the commanded temperature without too much additional undocumented “smart” behaviour getting in the way.

        Comment

        • harrym1byt
          Automated Home Lurker
          • Feb 2019
          • 3

          #19
          I don't know if this might help...

          Vaillant ECOfit Pure 418, VR 65 Control Module & VR10 cylinder sensor, VRC 407F Controller + VR 21 Outdoor sensor + Radio Receiver in boiler + Sunvic SDM 1901 3-port

          In a fully open vented system. With just basic clock, mechanical stats for controls, the boiler only allowed one temperature output of 60C (I think). I added the above Vaillant controls and can now set separate output temperatures for HW and CH, up to 80C. On CH the controls decide how hot the flow needs to be for CH, which usually means it stays below the maximum condensing operation temperature most of the time. It also now modulates it output much better than before, avoiding the on full/ completely off of the older system. Much less creaking of pipes as they expand and contract.
          Last edited by harrym1byt; 4 February 2019, 11:22 AM.

          Comment

          • mtmcgavock
            Automated Home Legend
            • Mar 2017
            • 507

            #20
            Originally posted by Symbolics View Post
            Yes, I have D.078 set at 80 degrees. Regardless, what happens when there is heating demand is that the boiler goes to about 65 degrees on the flow temp, and then goes into pump overrun mode. So even when there is no NTC connected allowing the boiler to figure out what the cylinder temperature is, the output DHW is modulated down to 65, and the actual temperature varies between 60 and 65.

            It is also interesting that the boiler seems to look at the return temperature and set a flow temperature based on the incoming water temperature. If the return temperature is (say) 52 degrees, the flow will be about 60. Maybe this is to allow condensing to happen, but in this mode the boiler is modulating right down to 8kW. This causes a very slow heating of the cylinder, which in turn causes long run times for heating mode.

            All in all, far from ideal, and I regret using the Vaillant boiler for this application. I may still buy a dedicated DHW boiler with a simple DHW temperature control that can just heat the water to the commanded temperature without too much additional undocumented “smart” behaviour getting in the way.
            To me it sounds like you've got some form of circulation issue on the cylinder circuit, at a guess it sounds like it is returning too quickly or your pump is on it's way out. It's not a problem with the VR66 or the boiler setting at 65oc.

            Comment

            • harrym1byt
              Automated Home Lurker
              • Feb 2019
              • 3

              #21
              Originally posted by Symbolics View Post
              Yes, I have D.078 set at 80 degrees. Regardless, what happens when there is heating demand is that the boiler goes to about 65 degrees on the flow temp, and then goes into pump overrun mode. So even when there is no NTC connected allowing the boiler to figure out what the cylinder temperature is, the output DHW is modulated down to 65, and the actual temperature varies between 60 and 65.
              I recently replaced my timeclock plus mechanical stats with a VR65 VRC470F etc. on my existing Vaillant 418. For four days I struggled to get my system to properly respond to a water heating request, despite working perfectly for room heating. I tried lots of settings and nothing worked, but after four days I decided to try resetting the boiler to factory defaults via the d.?? menu, then set it up from scratch. It then heated water absolutely fine. What was wrong with the original settings, I never found out, other than it is fixed now.
              Last edited by harrym1byt; 4 February 2019, 08:57 PM.

              Comment

              • Symbolics
                Automated Home Lurker
                • Feb 2019
                • 6

                #22
                Originally posted by mtmcgavock View Post
                To me it sounds like you've got some form of circulation issue on the cylinder circuit, at a guess it sounds like it is returning too quickly or your pump is on it's way out. It's not a problem with the VR66 or the boiler setting at 65oc.
                Thanks all. It turns out that the boiler pump needed to be set at a fixed high speed. With the automatic pump speed, the pump was going slowly which then made the boiler modulate its output and it never got past 60 degrees. A fixed high speed pumping scheme from the boiler is ok in my setup because I have a low loss header into which the boiler feeds, which hydraulically decouples the boiler from the Megaflo.

                Comment

                • Vikings
                  Automated Home Sr Member
                  • Jan 2021
                  • 55

                  #23
                  I was wondering if there was a further improvement that could be made to solving this problem.

                  I have a very similar situation. My Vaillant has Y-plan wiring (i.e. CH and HW heating fed with mid-position diverter valve) and I've implemented my own thermostat with software to control the room heating using TPI (with a relay to switch the CH demand signal). I am planning to move this to Opentherm to regulate the circulation temperature instead of TPI. However, when the hot water tank needs to be heated I would need to switch to a high enough circulation temperature.

                  This might conflict with the room heating requirement so I was wondering if I could have two modes:
                  a) HW demand OFF - CH is regulated using the boiler temperature OT control.
                  b) HW demand ON - set OT temperature to max and control CH circulation using the TPI signal (i.e. modulating the CH demand signal into the Y-plan wiring as before)

                  Would this work?

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #24
                    Evohome now comes with HW priority, so it will switch off the CH circuit when it switches to a HW heating cycle. OT jumps to a 90C set point when HW heating is required.

                    Comment

                    • Vikings
                      Automated Home Sr Member
                      • Jan 2021
                      • 55

                      #25
                      I don't have Evohome as I'm using my own control system. I was hoping that I could have CH heating at the same time as HW but not have the radiator feed running at full temp (which is currently 65C on my boiler setting). I was thinking that reverting to TPI would then avoid the rooms overheating. The diverter valve would be operating the TPI on/off switching in the same way as it does in my existing (non-OT) configuration.

                      Comment

                      • bruce_miranda
                        Automated Home Legend
                        • Jul 2014
                        • 2307

                        #26
                        Native OT has the ability to distinguish between CH mode and DHW mode. You could use that to alter the behaviour of your two circuits.

                        Comment

                        • Vikings
                          Automated Home Sr Member
                          • Jan 2021
                          • 55

                          #27
                          I'm not sure if that would help me - but apologies if I've missed something. My boiler has only one circuit with one demand signal connected to it (terminal 4 on the circuit board) and is driving a single pump. There is a 3-position valve in the airing cupboard that can divert the feed to CH or to HW or to both. That is controlled simply by the demand signals from the two thermostats (CH and HW) i.e. the Y-plan wiring setup. I've intercepted those two signals and and am able to drive them directly.

                          Comment

                          • kevinsmart
                            Automated Home Ninja
                            • Sep 2018
                            • 257

                            #28
                            Originally posted by Vikings View Post
                            I was wondering if there was a further improvement that could be made to solving this problem.

                            I have a very similar situation. My Vaillant has Y-plan wiring (i.e. CH and HW heating fed with mid-position diverter valve) and I've implemented my own thermostat with software to control the room heating using TPI (with a relay to switch the CH demand signal). I am planning to move this to Opentherm to regulate the circulation temperature instead of TPI. However, when the hot water tank needs to be heated I would need to switch to a high enough circulation temperature.

                            This might conflict with the room heating requirement so I was wondering if I could have two modes:
                            a) HW demand OFF - CH is regulated using the boiler temperature OT control.
                            b) HW demand ON - set OT temperature to max and control CH circulation using the TPI signal (i.e. modulating the CH demand signal into the Y-plan wiring as before)

                            Would this work?
                            Yes, this should work and would at least limit CH overshoot on DHW heating

                            Comment

                            • bruce_miranda
                              Automated Home Legend
                              • Jul 2014
                              • 2307

                              #29
                              You've just given me an idea to try something else. My Vaillant 438 doesn't have a HW sensor input, but it can have a HW setpoint separate from CH. The HW set point dial is useless unless used in conjunction with a Vaillant wiring centre and Vaillant controls. I'm going to have a play with a Vaillant wiring centre and see if I can get the HW demand to register and make the HW dial operational. It will allow me to remove a lot of custom HA that I use to detect a HW heating cycle and then performing various actions, one of them being raising the max Flow temperature setpoint.

                              Comment

                              • Vikings
                                Automated Home Sr Member
                                • Jan 2021
                                • 55

                                #30
                                Originally posted by kevinsmart View Post
                                Yes, this should work and would at least limit CH overshoot on DHW heating
                                That was certainly my thinking.

                                I'm assuming that the demand signal into the boiler will override what the OT control might be doing so - in HW mode - if HW thermostat has reached temperature and CH TPI control say OFF then switching off the demand signal to the boiler (terminal 4 on my Vaillant board) will stop the circulation.

                                Comment

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