Which OpenTherm boilers work well with Evohome?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts
  • DBMandrake
    Automated Home Legend
    • Sep 2014
    • 2361

    #16
    Originally posted by paulockenden View Post
    And another which says the opposite!!!
    I think the contrary article can be pretty easily debunked, just by looking at my own loop monitor graphs and using a bit of critical thinking. They claim:

    To answer this question we need to consider average heating loads across the UK. Many dwellings in our country are old buildings with relatively poor insulation, and this means an output of around 8–10kW is typically required to keep a house warm in winter.
    First of all, this assumes that you're trying to keep the entire house warm all day or evening. Those of us on the zoning bandwagon know that this is not our situation. So this argument can be dismissed immediately if you have a fully zoned system like Evohome and have a programmed schedule (and door closing habits!) to make the most of zoning. I don't heat bedrooms during the day, and in the late evening the living room (and bedroom warming up) are about the only radiators going at any significant strength, even in winter. Spare rooms are off, dining room is off, kitchen, hallway and bathroom are down to 16 degrees which in all but the coldest weather means radiators off.

    Looking at my loop usage in the late evening at this time of year it seems to be averaging about 5kW - this is to maintain the existing living room temperature and heat and then maintain the bedroom - which is in a not particularly well insulated loft bedroom. Our boiler is a 23kW unit so that would be a modulation ratio of 4.6 - just barely within 5:1, but that is with two rooms heating.

    Now consider night time - the bedroom radiator is the only one scheduled to be on, this is averaging approximately 2kW, although it does vary quite a bit from day to day - this would be well outside the modulation ratio of a 5:1 boiler so would result in the boiler cycling on and off all through the night.

    Their article also doesn't take into account spring and autumn weather where heat demands are considerably less, and higher modulation ratios become even more useful.

    So I think its fair to say that if you have a well insulated house (we don't!) or make use of zoning to have only one or two zones active at some times of the day (especially a bedroom radiator at night) then a higher modulation ratio is definitely useful, especially in moderate weather!

    Firstly, with a high ratio, the flame in the boiler is very small, and therefore weak. If it’s disturbed (e.g. by additional air movement), the flame might become unstable and unreliable, or even go out. Resulting in poor flame detection and/or nuisance tripping, the boiler will need to restart, which is what modulation is designed to avoid.
    If a manufacturer releases and promotes a boiler with a high modulation ratio I think they will have tested it properly to make sure the flame doesn't blow out at low outputs. And detecting a gas flame is not rocket science, thermocouples have worked just fine for decades and there are optical approaches too. I think this is just ill informed speculation on the part of the writer to try to bolster their position, not based on any hard data.

    Secondly, higher modulation ratios don’t necessarily mean lower energy usage. The primary efficiency consideration of the boiler is the return water temperature – the lower the better. The gains made in energy efficiency by moving a modulation ratio from 5:1 to 10: 1 with the same return water temperature are negligible.
    Which completely misunderstands what modulation ratio is about! The point of it is to prevent the boiler short cycling under low loads so that it can run continuously at a lower burn. Every time the boiler stops and starts some energy is wasted, which adds up. (Some CO is produced for a few seconds too, which does add up in the environment)

    Given that this article is promoted by Baxi and the other one by Vokera, I think this can be regarded as an opinion piece on Baxi's part.

    The first article is the one I would place faith in.
    Last edited by DBMandrake; 1 February 2017, 11:33 AM.

    Comment

    • paulockenden
      Automated Home Legend
      • Apr 2015
      • 1719

      #17
      Originally posted by DBMandrake View Post
      The first article is the one I would place faith in.
      Like you said, they are both PR pieces rather than serious technical journalism. But the first one does explain why even with OpenTherm your boiler might cycle, which I think surprises some people.

      P.

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        #18
        Originally posted by paulockenden View Post
        Like you said, they are both PR pieces rather than serious technical journalism. But the first one does explain why even with OpenTherm your boiler might cycle, which I think surprises some people.
        Yes indeed. I linked to a very similar article in another thread not long ago - in fact the graphs look identical so it was probably also from Vokera, maybe the same or similar article direct from their own website.

        Unless it adds significant cost a greater modulation range can only be a good thing in my opinion, especially if you have a build with an oversized boiler, which I suspect is quite common!

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          #19
          I think people assume bigger houses need bigger boilers. In fact, heating probably won't stress even a tiny boiler. It's hot water (either the speed of heating a hot tank, or the flow available to fill a bath / power of a shower if you have a combi) which are the real determining factors of boiler size.

          Comment

          • rotor
            Automated Home Guru
            • Aug 2015
            • 124

            #20
            Very enlightening reads. I had no idea about modulation ratios. So... how do we find out what boiler to buy? Needs to be:

            - Reliable
            - Efficient
            - Excellent modulation ratio
            - Full opentherm / Evohome support

            Not an easy ask!

            Comment

            • dty
              Automated Home Ninja
              • Aug 2016
              • 489

              #21
              I'm about to replace my boiler from the 15th century with a Vaillant EcoFit Pure system boiler and attempt to OT it. Where are you guys sourcing the VR33s from?

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #22

                Comment

                • dty
                  Automated Home Ninja
                  • Aug 2016
                  • 489

                  #23
                  Clearly going to have to brush up on my Dutch...

                  Comment

                  • DBMandrake
                    Automated Home Legend
                    • Sep 2014
                    • 2361

                    #24
                    Google translate ?

                    If you use Chrome just click the translate button in the address bar.

                    Comment

                    • bruce_miranda
                      Automated Home Legend
                      • Jul 2014
                      • 2307

                      #25
                      In fact on that site if you just keep translate on you can complete the entire ordering process without any issues. And payment is via PayPal any way.

                      Comment

                      • HenGus
                        Automated Home Legend
                        • May 2014
                        • 1001

                        #26
                        Just found this:

                        What is OpenTherm? Boilers Boilers – Unofficial Controls OpenTherm Boilers and Controls Shown below are a list of OpenTherm capable / compatible manufacturers and boilers. These OpenTherm Boi…


                        Not sure that the list is 100% accurate as I know the Viessmann 200 is not an Opentherm boiler.

                        Comment

                        • G4RHL
                          Automated Home Legend
                          • Jan 2015
                          • 1580

                          #27
                          Can someone please explain, probably for me in words of one syllable, what open therm does? When I look it up it seems to be no more than the boiler being told what to do when the control panel deems it necessary. But that's what mine does now with Evohome and my boiler is not opentherm. It's 20 years old this year.

                          Comment

                          • dty
                            Automated Home Ninja
                            • Aug 2016
                            • 489

                            #28
                            Evohome only tells your boiler to come on or go off. If it decides it needs only 50% of the boiler capability, it will turn it on for 10 minutes, then off for 10 minutes. So your boiler is constantly turning on and off which is inefficient and also causes thermal stress and wear and tear on the boiler and other components such as the pump, valves, etc.

                            With OpenTherm, Evohome can tell the boiler it wants 50% load, and the boiler will modulate its output to match and, if you're lucky, stay on permanently at a lower load rather than cycling.

                            Comment

                            • G4RHL
                              Automated Home Legend
                              • Jan 2015
                              • 1580

                              #29
                              Thanks. Much appreciated.

                              Comment

                              • HenGus
                                Automated Home Legend
                                • May 2014
                                • 1001

                                #30
                                Originally posted by G4RHL View Post
                                Can someone please explain, probably for me in words of one syllable, what open therm does? When I look it up it seems to be no more than the boiler being told what to do when the control panel deems it necessary. But that's what mine does now with Evohome and my boiler is not opentherm. It's 20 years old this year.
                                This might help:

                                Comment

                                Working...
                                X