Evohome + Opentherm v Evohome + Boiler Outside Weather Compensation

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  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    #31
    Thats a shame. Because that would have even given weather compensation too. Am still wondering if I should go down the Opentherm route or not. Given that I will be taking a risk of installing a VR33 in my Vaillant boiler which I am "not supposed" to in the UK.

    Comment

    • The EVOHOME Shop
      Site Sponsor
      • Dec 2014
      • 483

      #32
      If you look at ErP directive - evohome + OpenTherm + 3 or more heating zones ranks higher than Weather Comp. I am not really sure why people get so hung up on WC - it is just a modulating control system based on outside air temperature. OpenTherm is a modulating control system based on internal temperature and actual demand. I would hedge my bets that modulation based on internal reference (actually what is required) would save more than influence based on external temperature (which can fluctuate massively).

      Comment

      • rotor
        Automated Home Guru
        • Aug 2015
        • 124

        #33
        Originally posted by The EVOHOME Shop View Post
        If you look at ErP directive - evohome + OpenTherm + 3 or more heating zones ranks higher than Weather Comp. I am not really sure why people get so hung up on WC - it is just a modulating control system based on outside air temperature. OpenTherm is a modulating control system based on internal temperature and actual demand. I would hedge my bets that modulation based on internal reference (actually what is required) would save more than influence based on external temperature (which can fluctuate massively).
        I agree with this. I've never understood the weather compensation argument. You need whatever you need indoors, not out. OpenTherm seems way more useful in making the system run optimally.

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          #34
          The one advantage weather compensation could give is to not heat south facing zones on a spring morning if solar gain is likely to heat them anyway. A kind of optimisation++

          But it would need to be based on a weather FORECAST rather than an outside temperature sensor.

          I already do something /almost/ like this using IFTTT.

          Comment

          • DBMandrake
            Automated Home Legend
            • Sep 2014
            • 2361

            #35
            Originally posted by rotor View Post
            I agree with this. I've never understood the weather compensation argument. You need whatever you need indoors, not out. OpenTherm seems way more useful in making the system run optimally.
            On a more conventional system there is a good argument for weather compensation.

            The warmer it is outside the less heat loss through the walls and the less heat the radiators need to put into the rooms. Manual wax-pellet TRV's are not the most accurate things around (and suffer from problems such as Offset error among others) and in this case reducing the flow temperature when the there is less heat loss helps to avoid large temperature oscillations. On a condensing boiler it will also improve efficiency not having the flow temperature any higher than you need for the amount of heat lost from the rooms.

            I also find it more comfortable if rooms are maintained at their set temperature with as low a flow temperature as possible - again with manual TRV's a high flow temperature when the heat loss through the walls is low tends to result in large temperature oscillations as the TRV overshoots, completely closing and then opening again in ~30 minute cycles, and I find the radiant heat from a high panel surface temperature during the "on phase" in warmer weather can be quite uncomfortable.

            I think most of us probably turn our flow temperatures down manually a bit in warmer weather for the reasons above.

            With an evohome system with OpenTherm control most (but not all) of the problems above are negated. Once rooms are up to temperature it will drop the flow temperature as low as it can while still satisfying the needs of the most demanding room, this means you get the comfort of a low panel temperature and minimal temperature oscillation/overshoot, and condensing boiler savings even in the winter. (Which you wouldn't normally get as you would be forced to turn your static flow temperature up high to get a satisfactory warm up time, and weather compensation would also keep the flow temperature high due to the cold outdoor temperature)

            What evohome+opentherm doesn't address as far as I can see, is that during the "100% demand" warm up phase it will still ramp the flow temperature up to the boilers user-set maximum even in warm weather, reducing condensing boiler efficiency and possibly causing a little bit of initial overshoot and discomfort. Once the rooms are warmed up it will then throttle back the flow temperature however.

            In this case, manually turning the "maximum" flow temperature down a bit (or having weather compensation) should still make the warm up phase a bit more efficient in warmer weather and less likely to have rooms initially overshooting until the flow temperature is ramped back. (It only takes a single zone with a high demand to trigger maximum flow temperature, which can easily happen when optimal start staggers the start times of different zones, or just from having a complex schedule)
            Last edited by DBMandrake; 31 March 2016, 02:03 PM.

            Comment

            • The EVOHOME Shop
              Site Sponsor
              • Dec 2014
              • 483

              #36
              Originally posted by paulockenden View Post
              The one advantage weather compensation could give is to not heat south facing zones on a spring morning if solar gain is likely to heat them anyway. A kind of optimisation++

              But it would need to be based on a weather FORECAST rather than an outside temperature sensor.

              I already do something /almost/ like this using IFTTT.
              Weather Compensation cannot account for solar gain and the rooms are heated according to outdoor air temperature. The WC outdoor sensor is normally located on the north face of the building away from solar energy to prevent inaccuracies.

              IFTTT is more adventitious in this scenario using 'OFF' or 'AWAY' in 'Zone X' if 'SUNNY' (using local weather information).

              Comment

              • paulockenden
                Automated Home Legend
                • Apr 2015
                • 1719

                #37
                Exactly, that's why I said it needs to be a forecast rather than a sensor. It would still, technically, be "weather compensation".

                Comment

                • DBMandrake
                  Automated Home Legend
                  • Sep 2014
                  • 2361

                  #38
                  Personally I think the Evohome zoning will automatically take care of solar gain on the sunny side of the house, as long as the zone temperature measurement points in the rooms are well chosen. (No direct sunlight on the sensors and the measurement location being representative of the room etc)

                  Now that we're starting to get some sunny weather I'm already seeing it deal well with afternoon sun in the living room versus the colder side of the house at the same time of day, with no changes to schedules or set points.
                  Last edited by DBMandrake; 31 March 2016, 05:36 PM.

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #39
                    Ok I am taking a plunge into the unknown now. Richard seems to have convinced me to jump in at OpenTherm. So have ordered it from the Evohome shop. I've also ordered the VR33 from Netherlands. I'll let people know how I get on.

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      #40
                      My current setup uses a normal timer for the hot water. Will i need to move to the hot water kit if I plan to use OpenTherm? Otherwise how will Opentherm know there is demand for Hot Water too?

                      Comment

                      • top brake
                        Automated Home Legend
                        • Feb 2015
                        • 837

                        #41
                        Originally posted by bruce_miranda View Post
                        My current setup uses a normal timer for the hot water. Will i need to move to the hot water kit if I plan to use OpenTherm? Otherwise how will Opentherm know there is demand for Hot Water too?
                        Yes you'll need the hot water kit if you want to use VR33 and OpenTherm bridge. This is what I have.
                        I work for Resideo, posts are personal and my own views.

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #42
                          Oh sugar...some more expense. Ok so I have an S plan system. I understand that the Opentherm bridge will trigger the boiler for radiators and hot water. But i then have a further zone valve for UFH currently controlled via a Y87RF and a BDR91. What happens to the BDR91 of that and the Hot Water kit?

                          Also how are the zone valves wired when you use the Opentherm bridge instead of using the BDR91 for boiler demand?

                          Comment

                          • top brake
                            Automated Home Legend
                            • Feb 2015
                            • 837

                            #43
                            Originally posted by bruce_miranda View Post
                            Oh sugar...some more expense. Ok so I have an S plan system. I understand that the Opentherm bridge will trigger the boiler for radiators and hot water. But i then have a further zone valve for UFH currently controlled via a Y87RF and a BDR91. What happens to the BDR91 of that and the Hot Water kit?

                            Also how are the zone valves wired when you use the Opentherm bridge instead of using the BDR91 for boiler demand?
                            How have you wired it at present?

                            Wiring diagrams for the BDR91 are in the appendix but you need to decide how you're gonna control the system. If you can take some pictures and or do a sketch I can advise.
                            I work for Resideo, posts are personal and my own views.

                            Comment

                            • bruce_miranda
                              Automated Home Legend
                              • Jul 2014
                              • 2307

                              #44
                              Currently I use only 1 BDR91 which is used to control the UFH zone valve via the Y87RF. The Y87RF is paired to the evohome as a Zone Valve sensor and the BDR91 is pair as the actuator. So for the UFH there is automatic boiler demand via the evohome even though I don't have boiler demand relay set up in the System Devices. The BDR91 switches on the zone valve, which in turn switches on the boiler whenever the UFH needs it.

                              I currently use no Boiler demand for either HW or radiator CH because my current Honeywell timer clock seemed to serve the purpose. The timer clock is wired in a standard S plan format.

                              I can see the benefit of using Opentherm to control the boiler flow temperature hence am prepared to change the way things work. But the bit I am confused about is how are all the zone valves wired in an OpenTherm setup.

                              I will add the HW kit to the evohome. But before I pour more money into my set-up I'd like to understand the wiring.

                              I know the OpenTherm bridge receives the Boiler Demand from the evohome controller, switches the boiler on and controls the flow temperature based on the demand. That bit I understand. However in a standard S plan wiring it's the zone valves that open first and then they fire the boiler up. With Opentherm, who or what opens the different zone valves?

                              In the evohome controller under Boiler Demand, I have to choose between Boiler Relay or Opentherm. Hence my questions.
                              Last edited by bruce_miranda; 3 April 2016, 10:05 PM.

                              Comment

                              • top brake
                                Automated Home Legend
                                • Feb 2015
                                • 837

                                #45
                                Originally posted by bruce_miranda View Post
                                Currently I use only 1 BDR91 which is used to control the UFH zone valve via the Y87RF. The Y87RF is paired to the evohome as a Zone Valve sensor and the BDR91 is pair as the actuator. So for the UFH there is automatic boiler demand via the evohome even though I don't have boiler demand relay set up in the System Devices. The BDR91 switches on the zone valve, which in turn switches on the boiler whenever the UFH needs it.

                                I currently use no Boiler demand for either HW or radiator CH because my current Honeywell timer clock seemed to serve the purpose. The timer clock is wired in a standard S plan format.

                                I can see the benefit of using Opentherm to control the boiler flow temperature hence am prepared to change the way things work. But the bit I am confused about is how are all the zone valves wired in an OpenTherm setup.

                                I will add the HW kit to the evohome. But before I pour more money into my set-up I'd like to understand the wiring.

                                I know the OpenTherm bridge receives the Boiler Demand from the evohome controller, switches the boiler on and controls the flow temperature based on the demand. That bit I understand. However in a standard S plan wiring it's the zone valves that open first and then they fire the boiler up. With Opentherm, who or what opens the different zone valves?

                                In the evohome controller under Boiler Demand, I have to choose between Boiler Relay or Opentherm. Hence my questions.
                                The zone valves will be powered and opened by separate BDR91 relays. You'll need three if you want to retain your S Plan heating valve.

                                Alternatively you can create a hot water priority system by wiring the heating zone valve BDR91 from the C terminal on the hot water zone valve BDR91. This will ensure that the boiler runs at the lowest possible flow temperature for space heating.

                                You won't need to permanently power the zone valve grey wires and you won't need the orange wires to fire the boiler any more. The wiring diagram for individual zone valve is in the appendix.

                                Hope this is starting to make sense.
                                I work for Resideo, posts are personal and my own views.

                                Comment

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