Evohome + Opentherm v Evohome + Boiler Outside Weather Compensation

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  • megakid
    Automated Home Lurker
    • Oct 2021
    • 3

    Hi all - great thread, thanks

    I currently have a valliant ecofit plus 631 and a fairly standard S plan system with evohome w/hw kit. It all sounds rather straightforward to get it opentherm enabled but I have one outstanding question specific to my setup and wondered if anyone had an idea💡 :

    My S plan has 3 valves
    1 HW
    1 CH Radiators (all rads have HR92s)
    1 UHF valve (and pump) for a single large room zone (kitchen diner extension)

    If any of the valves orange wires is live (eg open) the boiler is on (firing). Each of these 3 valves are currently controlled via its own BDR.
    The first 2 valves are standard evohome HW and CH.
    The UFH valve BDR is configured as an electric underfloor heating zone within evohome, paired with its own Y87 on the wall. When the zone drops below Y87 set temp, the BDR fires which starts UFH pump and opens UFH valve which in turn fires the boiler (via orange wire)

    So my question is what does my OpenTherm (OT) enabled setup look like? The OT bridge will be wired to boiler and I will disconnect the orange wires feeding the boiler. Evohome should demand the correct flow temps (high for HW and variable for CH demand) but how do I get the UFH zone to be part of this? Ideally I want it controlled via a HR92, then it would be simple (eg whole floor = 1 radiator valve) but that’s not my setup.

    Anyone running a similar setup got any advice?

    Comment

    • bruce_miranda
      Automated Home Legend
      • Jul 2014
      • 2307

      I have exactly that same S plan set up. Your UFH zone's BDR has been configured incorrectly. An electric UFH zone does not produce a heat demand, so if your UFH zone was the only zone requesting heat, in your current setup the boiler wouldn't fire the Boiler Relay. It needs to be set up as a Zone Valve zone. It's only working in your current set up because you are using the Orange wire to fire the boiler and not a Boiler Relay.
      But back to your question. Because of the issue I mentioned earlier, when you move to OT (with a VR33 for your boiler) your UFH won't fire the boiler, so definitely fix that. In your setup the UFH's Y87RF is the sensor and the BDR91 is the actuator. And will act the same way as a HR92 zone would, once you make it a Zone Valve zone. So you should see variable flow temperatures even for the UFH zone. The Evohome is not seeing or treating it any differently to the other radiator zone.

      Comment

      • megakid
        Automated Home Lurker
        • Oct 2021
        • 3

        Music to my ears. Thanks for confirming.

        I really appreciate the value you’ve added to this thread and others I’ve found during my research- I had one question specifically for you @bruce_miranda, based on your other posts, my valiant boiler with evohome OT bridge will call for 90C heat during HW priority, I assume I can configure max flow temp on the boiler itself which caps this at a more sensible 65C or similar - why did you go further than this and install VR65? Is there any value in this except for tighter control on the boiler kw output settings with the separate HW & CH boiler “modes” (something evohome OT doesn’t use)?

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          My Vaillant boiler has the ability to use a different flow temperature for CH and HW demands. The problem is when you use the OTG+VR33 the boiler is not told what is being heated. The OTG doesn't distinguish between CH and HW, it only requests a flow temperature. 90C when heating HW and anything from 90C downwards for CH. So you use the knob on the boiler to clamp the max flow to whatever you want. But now you have a new problem, if you clamp to say 50C, this might be enough for CH and UFH, but is insufficient for HW. Now previously what I did was had a complicated system that used a combination of a HGI80+Domoticz+eBus interface that basically detected when there was HW demand and would alter the boiler knob max from 50C to 70C. This worked well for many years until I discovered the hack that involves using the VR65 wiring centre on its own. When the VR65 is connected to the boiler, it can tell the boiler when HW demand is present, best of all HW priority is the default. You then use the CYL terminals inside the VR65 and either use the Orange wire from your HW zone valve or loop the
          CYL terminals through the zone valve microswitch. The CYL terminals is actually designed for the Cylinder's thermostat. When the stat is closed, the VR65 requests a HW demand, when the stat is open the boiler reverts to CH demand. Now you can have dual flow temperatures for HW and CH. The knob in the front is still used for CH, and you use the boiler settings menu to set the desired flow temperature when there is a HW demand. Works brilliantly and with so many fewer things in the chain. Just don't connect anything to the VR65 because the VR33 has a very low ebus priority, so any other ebus thermostat will render the VR33 dormant. This is what stumped me in the early days, until I discovered the VR65 could be used on its own.

          Comment

          • G4RHL
            Automated Home Legend
            • Jan 2015
            • 1580

            Originally posted by bruce_miranda View Post
            My Vaillant boiler has the ability to use a different flow temperature for CH and HW demands. The problem is when you use the OTG+VR33 the boiler is not told what is being heated. The OTG doesn't distinguish between CH and HW, it only requests a flow temperature. 90C when heating HW and anything from 90C downwards for CH. So you use the knob on the boiler to clamp the max flow to whatever you want. But now you have a new problem, if you clamp to say 50C, this might be enough for CH and UFH, but is insufficient for HW. Now previously what I did was had a complicated system that used a combination of a HGI80+Domoticz+eBus interface that basically detected when there was HW demand and would alter the boiler knob max from 50C to 70C. This worked well for many years until I discovered the hack that involves using the VR65 wiring centre on its own. When the VR65 is connected to the boiler, it can tell the boiler when HW demand is present, best of all HW priority is the default. You then use the CYL terminals inside the VR65 and either use the Orange wire from your HW zone valve or loop the
            CYL terminals through the zone valve microswitch. The CYL terminals is actually designed for the Cylinder's thermostat. When the stat is closed, the VR65 requests a HW demand, when the stat is open the boiler reverts to CH demand. Now you can have dual flow temperatures for HW and CH. The knob in the front is still used for CH, and you use the boiler settings menu to set the desired flow temperature when there is a HW demand. Works brilliantly and with so many fewer things in the chain. Just don't connect anything to the VR65 because the VR33 has a very low ebus priority, so any other ebus thermostat will render the VR33 dormant. This is what stumped me in the early days, until I discovered the VR65 could be used on its own.
            I don’t have knowledge of the Vaillant but what you report sounds strange plus the temperature settings seem awfully high. My boiler is a condensing boiler, which may be why it sounds strange to me. My HW I set at 60C which is more than enough. The flow temperature for the CH at 65c which is fine plus the power rating for CH in the boiler is set to 6.4kw.

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              Originally posted by G4RHL View Post
              I don’t have knowledge of the Vaillant but what you report sounds strange plus the temperature settings seem awfully high. My boiler is a condensing boiler, which may be why it sounds strange to me. My HW I set at 60C which is more than enough. The flow temperature for the CH at 65c which is fine plus the power rating for CH in the boiler is set to 6.4kw.
              I don't know why you think things are strange in my setup. I have a condensing boiler too. Don't confuse what OT is requesting, those are stupid temperatures e.g. 90C. My boiler is clamped to a 55C max for CH and 60C max for HW. This ensure the boiler stays in condensing mode as much as possible.
              So even when OT is requesting a 90C for HW, my boiler will only go to 60C to bring the HW up to 50C in the Unvented Cylinder. My Solar panels then top up the temperature in the Cylinder and takes the water up to 65C to avoid the bacteria build up.

              Comment

              • MadeTomatoHue
                Automated Home Jr Member
                • Nov 2022
                • 47

                I'm interested in the "Evohome + Boiler Outside Weather Compensation" part of the title, but most of the thread seems to be about the OpenTherm side. Feel free to point me to the relevant posts if I missed them .

                In case it helps... I'm considering getting the outside temperature via a web service, and using it to configure the flow temperature on a Worcester Bosch boiler via EMS. As per starter post, I'm wondering if EvoHome will get confused about the space heating taking longer / shorter times than it expected, causing heat demand / TPI to get out of kilter.

                Comment

                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

                  The trouble with using the Web Services for outside temperature is the lag. And also it doesn't take into account the orientation of your house. In theory, if you vary the flow temperature constantly then it will upset the Evohome learning. In practice, I think the Evohome learning is overrated and minor adjustments in the flow temperature aren't going to bother it too much. But since you have the ability and skills to implement Weather Compensation, why not simply do away with TPI and constant modulation via OT.

                  Comment

                  • MadeTomatoHue
                    Automated Home Jr Member
                    • Nov 2022
                    • 47

                    Originally posted by bruce_miranda View Post
                    The trouble with using the Web Services for outside temperature is the lag. And also it doesn't take into account the orientation of your house. In theory, if you vary the flow temperature constantly then it will upset the Evohome learning. In practice, I think the Evohome learning is overrated and minor adjustments in the flow temperature aren't going to bother it too much. But since you have the ability and skills to implement Weather Compensation, why not simply do away with TPI and constant modulation via OT.
                    Summary from other threads is that Worcester Bosch doesn't talk OpenTherm, it uses a proprietary protocol called EMS. There was an OT to EMS conversion device previously available, but no longer. What I would like to do is see if a service that pretends to be the EvoHome OT bridge could be constructed on one of the not-HGI80 boards, and it could then turn OT into EMS. But I read that the OTB has its own quirks, so not sure replicating its behaviour in a way that EvoHome would understand is really a good endpoint. I may look at that more if the below doesn't get to where I'd like to be.

                    Also as in other threads I may steadily work towards load compensation by remapping a (say) 50% boiler TPI cycle where EvoHome is assuming 70C flow to ~50C flow temp fulltime by overriding TPI on BRD via multi-binding.

                    For now I have a few simpler things to try out for effectiveness and comfort:
                    • Use fixed CH flow (~55C) unless outside temperature is below a threshold, so EvoHome can hopefully learn what usually works and get a bit of help on colder days
                    • Choose per-day flow temp based on mean / high / low (TBD) outside temp for day, so EvoHome can try and settle into a rhythm for that day
                    • Adjust flow temp every N hours based on forecast for current time, which I would expect to confuse EvoHome
                    • Rather than override BDR TPI see if simply adjusting flow temp based on zone or system heat demand causes TPI to have higher duty cycle, and steer this to 100% if possible. Might also confuse EvoHome.


                    Do you mean the lag between what outside temperature is now and that house fabric gives a few hours delay before that is felt inside? I'm undecided on whether this matters. If goal is to get house to current desired temperature then EvoHome will hopefully adjust heat demand accordingly depending on the trends it sees. As long as it isn't too confused by the chnaging flow temp...

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      The lag I refer to is the actual temperature vs what the web services say it is. I have my own outdoor temperature and it is almost consistently about 30-45mins ahead of any Web service reading for my area.

                      Comment

                      • MadeTomatoHue
                        Automated Home Jr Member
                        • Nov 2022
                        • 47

                        Originally posted by bruce_miranda View Post
                        The lag I refer to is the actual temperature vs what the web services say it is. I have my own outdoor temperature and it is almost consistently about 30-45mins ahead of any Web service reading for my area.
                        I was planning to use a once per day download of hourly forecasts and then just pick something "representative" when choosing a flow temp, so there'll be a margin of error.

                        Comment

                        • MadeTomatoHue
                          Automated Home Jr Member
                          • Nov 2022
                          • 47

                          I've put together a first version of this to add simple weather compensation. It runs a couple of scripts on my OpenWrt router that...
                          1. Downloads a set of temperature forecast data from open-meteo.com each day
                          2. Every four minutes (because boiler seems to revert to default after 5 minutes)...
                            1. Looks up forecast outside temperature for current hour
                            2. Selects a flow temperature between 25C to 75C according to where outside temperature sits on a -5C to 20C line (inverse linear relationship)
                            3. Sends that flow temperature to Worcester Bosch boiler via BBQKees EMS interface
                          3. Overrides things if DHW heating is scheduled (6AM and 4PM) to use 65C flow temperature

                          It's currently selecting a 54C flow temperature with 5.5C outside.

                          Next steps are to use HGI80 plugged into router USB to eavesdrop on messages and...
                          • Add some monitoring for when DHW heating is active (rather than just scheduled)
                          • See if boiler TPI extends out to compensate for lower flow temperature - hoping it will
                          • See if room temperatures look like they struggling - hopefully not
                          • See if BDR91 call for heat matches what boiler is doing, or whether they seem to be fighting each other leading to excessive firing cycles
                          Last edited by MadeTomatoHue; 4 December 2022, 12:36 AM.

                          Comment

                          • Stevedh
                            Automated Home Guru
                            • Mar 2017
                            • 177

                            May or may not be relevant, I started using Domoticz with a 'USB 868MHz wireless dongles for RAMSES HVAC systems' so that I could use the new evohohome alexa 'smart heat' skill and that allows you to replace the forecast based temperature evohome uses with an actual sensor. I haven't done that yet, although I have setup a number of rooms to use zigbee temperature sensors instread of the ones built into the HR92 and those work really well.

                            Comment

                            • lloyd
                              Automated Home Guru
                              • Oct 2020
                              • 160

                              Originally posted by Stevedh View Post
                              May or may not be relevant, I started using Domoticz with a 'USB 868MHz wireless dongles for RAMSES HVAC systems' .... that allows you to replace the forecast based temperature evohome uses with an actual sensor.
                              I think that is only available with the old style, non Wi-Fi versions of the controller. Real shame.

                              Comment

                              • bruce_miranda
                                Automated Home Legend
                                • Jul 2014
                                • 2307

                                Originally posted by lloyd View Post
                                I think that is only available with the old style, non Wi-Fi versions of the controller. Real shame.
                                I was curious too. Because the Evohome V2 controller can use an actual outdoor sensor, but does nothing more than display it on the screen. The Evohome V3 contllers use the Internet weather services for outdoor temperature, which it uses for smart weather compensation, but it cannot use a local external sensor.

                                Comment

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