Honeywell evohome and OpenTherm integration

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  • The EVOHOME Shop
    Site Sponsor
    • Dec 2014
    • 483

    #76
    Originally posted by the crooner View Post
    My recent experience suggests that this is not the whole truth for several reasons:

    Modern windows are far more thermally efficient

    Folk will have upgraded the scant provision of loft insulation

    Cavity wall insulation may have been retrofitted

    Extensions and alterations may have taken place

    Rooms may have been repurposed and target temperatures may be incorrect

    And most importantly the system may not have been designed in the first place.

    So just upping radiator sizes by 36% willy nilly may be exactly the wrong thing to do.

    A more considered, holistic approach is required. Then tune your properly functioning system with Evohome for best results.

    Tony
    I didn't suggest in my previous post that 'other' factors had changed - I was comparing like for like and if you do the calculations, my suggestion is based on simple physics used for heating design and is irrefutable.

    Room heat loss is based on the sum of its factors - if things have changed to prevent heat loss then these factors are then taken into consideration.

    Comment

    • G4RHL
      Automated Home Legend
      • Jan 2015
      • 1580

      #77
      As I have a new boiler coming and am considering OpenTherm (Richard’s firm has already delivered the boiler - its in my garage all ready!) last night I read through the whole of this topic and reread a more recent section that I had started. I am not an expert by any means but three things come out of what I read and the advices:

      1. A return flow of about 50C is optimum for a modern condensing boiler.

      2. Radiators have changed little in the past 20 to 30 years but with the latest boilers some may find their existing radiators undersized putting a new boiler in. But amongst the experts helping in the forum the jury is out as to whether undersize is an issue. To my mind I think it could be as logic says if you have an adequate set up with an old boiler where the return flow is higher than say 50C and then you install something that is more efficient with lower temperatures then something better at the distribution end is needed. But it seems from what is said in the forum the difference is not massive. Perhaps it would take a long time to recover the capital cost of changing all the radiators thus making the issue more a theoretical one for the average user.

      3. Size. Size matters! But what is meant by size? Reading the postings it is easy for a layman to think if he needs a bigger radiator it is bigger in height and width and then the concern because it means alterations to pipework which may or may not be feasible and if it is it may be unsightly. But in fact by size what is meant is single panel, single panel convector, double panel, double panel convector etc.and for the layman reading this if your existing radiators are metric in size (height and width) then the equivalent if upgrading to a double panel etc. is available.

      As well as researching boilers, and I admit with a bias toward Intergas, I also measured every room and worked out what the house may need to heat it and what each room needs. I checked the calculations on a number of sites and all more or less come back with similar figures for BTUs and kWs. I found that but for two rooms my existing radiators (23 years old) are fine. My existing boiler (an old Baxi) has a flow of 65C and return of 55C at the boiler. That is not too far away from recommendations to achieve 70/ 50.

      I will therefore keep my radiators and see what happens first, other than change one that does need to be changed.

      I think also I may initially just connect the existing BDR91 relay to the new boiler thermostat connection and let it run and see how it goes before removing it and replacing with the OpenTherm Bridge. Fortunately, provided I can identify the right wire that feeds from my airing cupboard to the boiler downstairs I can leave the BDR91 in the airing cupboard and do a simple swap with the Bridge at a later date. I have an abundance of wires in the box in the airing cupboard (of similar colours!) my existing set up being boiler plus separate DMW.

      Comment

      • HenGus
        Automated Home Legend
        • May 2014
        • 1001

        #78
        Originally posted by G4RHL View Post
        As I have a new boiler coming and am considering OpenTherm (Richard’s firm has already delivered the boiler - its in my garage all ready!) last night I read through the whole of this topic and reread a more recent section that I had started. I am not an expert by any means but three things come out of what I read and the advices:

        1. A return flow of about 50C is optimum for a modern condensing boiler.

        2. Radiators have changed little in the past 20 to 30 years but with the latest boilers some may find their existing radiators undersized putting a new boiler in. But amongst the experts helping in the forum the jury is out as to whether undersize is an issue. To my mind I think it could be as logic says if you have an adequate set up with an old boiler where the return flow is higher than say 50C and then you install something that is more efficient with lower temperatures then something better at the distribution end is needed. But it seems from what is said in the forum the difference is not massive. Perhaps it would take a long time to recover the capital cost of changing all the radiators thus making the issue more a theoretical one for the average user.

        3. Size. Size matters! But what is meant by size? Reading the postings it is easy for a layman to think if he needs a bigger radiator it is bigger in height and width and then the concern because it means alterations to pipework which may or may not be feasible and if it is it may be unsightly. But in fact by size what is meant is single panel, single panel convector, double panel, double panel convector etc.and for the layman reading this if your existing radiators are metric in size (height and width) then the equivalent if upgrading to a double panel etc. is available.

        As well as researching boilers, and I admit with a bias toward Intergas, I also measured every room and worked out what the house may need to heat it and what each room needs. I checked the calculations on a number of sites and all more or less come back with similar figures for BTUs and kWs. I found that but for two rooms my existing radiators (23 years old) are fine. My existing boiler (an old Baxi) has a flow of 65C and return of 55C at the boiler. That is not too far away from recommendations to achieve 70/ 50.

        I will therefore keep my radiators and see what happens first, other than change one that does need to be changed.

        I think also I may initially just connect the existing BDR91 relay to the new boiler thermostat connection and let it run and see how it goes before removing it and replacing with the OpenTherm Bridge. Fortunately, provided I can identify the right wire that feeds from my airing cupboard to the boiler downstairs I can leave the BDR91 in the airing cupboard and do a simple swap with the Bridge at a later date. I have an abundance of wires in the box in the airing cupboard (of similar colours!) my existing set up being boiler plus separate DMW.
        As you know, it was a couple of years since I last had a home with Evohome; however, I did install at Atag boiler with Opentherm. In my view, boiler sizing is important: big is not always best with a condensing boiler. What is important is the modulating ratio. My experience over a couple of Winter seasons showed that it is difficult to get the return floe to <56C if zoning has closed off most of the heating system. The flow from the boiler has nowhere to go other than through the radiator that is open and the ABV (if you have one). Unless the boiler can get down to a low output by modulating then you will end up with high flow return temperatures (ie; no condensing) and the boiler cycling ON and OFF. Opentherm control will smooth out some of these bumps in the road but it cannot overcome the physics of the boiler; ie, it can only reduce the boiler to minimum output and that's it. That said, the problem in sizing the boiler is to some extent determined by how quickly want their homes to heat up, and HW demand.

        FWiW, even with the existing radiators in my old 5 bed home my installer calculated that I could sensibly get away with a 18kW boiler. After much head scratching, I compromised on 24kWs.

        PS. In my new build home with a WB 24k condensing boiler, I have gone for Tado rather than Evohome. It just works.

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          #79
          Moving from single to double (or even triple) convector rads also needs pipe alterations, because the radiator centre shifts.
          Also traditional boiler sizing is also quite outdated with a zoned system like Evohome. How often are all of your zones calling for heat at the same time? Because if they are, you aren't using Evohome for what it was designed.
          My house was sized for a 38Kw boiler which I have derated to 18Kw. It's amazing what you can get away with if you plan your house heating strategy well. And if you have a storage DHW then if you heat the HW after most of the zones are satisfied it takes away the need to have a massive boiler even for DHW. Because again the way boiler sizing is done, is they add the heating coil after all the rooms are added up.
          Last edited by bruce_miranda; 5 February 2021, 11:54 AM.

          Comment

          • G4RHL
            Automated Home Legend
            • Jan 2015
            • 1580

            #80
            Thanks. I suspect for CH I will be able to have the boiler running quite low intend initially too set the maximum for CH at 19.3kW (one of the options in the manual, the others are 25.7, 12.8, 9.0 and 6.4). Some experimenting to be done.

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #81
              Infact go lower and only raise it, if you feel the zoned rooms aren't heating up fast enough. I heat my house based on live usage patterns but if you do that based intended patterns, you will notice that at any time only a couple of rooms are really being heated up, the rest are either waiting to be heated, or just being topped up.

              Comment

              • G4RHL
                Automated Home Legend
                • Jan 2015
                • 1580

                #82
                Originally posted by bruce_miranda View Post
                Infact go lower and only raise it, if you feel the zoned rooms aren't heating up fast enough. I heat my house based on live usage patterns but if you do that based intended patterns, you will notice that at any time only a couple of rooms are really being heated up, the rest are either waiting to be heated, or just being topped up.
                I connected the OpenTherm Bridge this afternoon in place of the BDR91. I suppose the system goes into learning mode over the next few days or weeks. However, the boiler, an Intergas Xclusive 36kW, now shows something it has not before. The new boiler was installed last week. At present I have set the flow temperature to 70c but will in time reduce that. However, when I now look at the panel it shows a temperature reading of say 35c at times the boiler is not firing. Is this OpenTherm working and showing me the return temperature?

                Comment

                • SteveFraser
                  Automated Home Lurker
                  • Sep 2020
                  • 7

                  #83
                  Originally posted by G4RHL View Post
                  I connected the OpenTherm Bridge this afternoon in place of the BDR91. I suppose the system goes into learning mode over the next few days or weeks. However, the boiler, an Intergas Xclusive 36kW, now shows something it has not before. The new boiler was installed last week. At present I have set the flow temperature to 70c but will in time reduce that. However, when I now look at the panel it shows a temperature reading of say 35c at times the boiler is not firing. Is this OpenTherm working and showing me the return temperature?
                  Even better. I believe it’s the flow temperature.
                  This can be adjusted on the Intergas X series boilers. I think the lowest temperature default is 30 C.

                  Comment

                  • kevinsmart
                    Automated Home Ninja
                    • Sep 2018
                    • 257

                    #84
                    I think it’s displaying the flow temperature request (control setpoint).

                    So if your current flow temperature exceeds this temperature, the boiler remains off but may continue to pump/overrun for a while.

                    Comment

                    • guyank
                      Automated Home Sr Member
                      • Sep 2015
                      • 73

                      #85
                      When my flow temperature is within the tolerance and no additional heat is needed it displays 40C and number 1 as the pump is running but the burner is off. Once the heat demand is satisfied, I.e. the requested flow temp is below 40C, the both displays turn off.

                      Comment

                      • G4RHL
                        Automated Home Legend
                        • Jan 2015
                        • 1580

                        #86
                        Originally posted by guyank View Post
                        When my flow temperature is within the tolerance and no additional heat is needed it displays 40C and number 1 as the pump is running but the burner is off. Once the heat demand is satisfied, I.e. the requested flow temp is below 40C, the both displays turn off.
                        Now a few days in with OT, yes I see this is what is happening. I am finding with OT that rooms get to their designated temperature a little slower than they used to but not such as to be an issue. More importantly as an experiment for a few days or so I have set downstairs rooms all to come on at 07:00 at 19C. They all reached that temperature more or less at the same time. One room was a little behind but not much. It is north facing with two outside walls one of which is exposed more than others. Often radiators seem cooler but the temperature is where is should be, the system being happy with what it has attained and/or been directed to do. The problem now is convincing the lady of the manor that whilst the radiator may seem cooler to touch it does not mean the room is cooler! Looking at the thermostat readout does not seem to convince!

                        Comment

                        • Wobbin
                          Automated Home Jr Member
                          • Apr 2017
                          • 11

                          #87
                          Originally posted by G4RHL View Post
                          Now a few days in with OT, yes I see this is what is happening. I am finding with OT that rooms get to their designated temperature a little slower than they used to but not such as to be an issue. More importantly as an experiment for a few days or so I have set downstairs rooms all to come on at 07:00 at 19C. They all reached that temperature more or less at the same time. One room was a little behind but not much. It is north facing with two outside walls one of which is exposed more than others. Often radiators seem cooler but the temperature is where is should be, the system being happy with what it has attained and/or been directed to do. The problem now is convincing the lady of the manor that whilst the radiator may seem cooler to touch it does not mean the room is cooler! Looking at the thermostat readout does not seem to convince!
                          I have a lady of the Manor like that too. If the heating is on, all the radiators should be hot. Why aren't they all hot !!??? <sigh>

                          Comment

                          • G4RHL
                            Automated Home Legend
                            • Jan 2015
                            • 1580

                            #88
                            Originally posted by Wobbin View Post
                            I have a lady of the Manor like that too. If the heating is on, all the radiators should be hot. Why aren't they all hot !!??? <sigh>
                            I also now get “Why is boiler always starting? I keep hearing a whoosh sound every few minutes. I thought these boilers were meant to be quiet?” To explain that the boiler was not firing up but she could hear the pump starting and stopping to balance the required heat seemed to fall on deaf ears!

                            Comment

                            • bruce_miranda
                              Automated Home Legend
                              • Jul 2014
                              • 2307

                              #89
                              Increase the pump over run time a bit and then you won't have the pump stopping and starting as much.
                              You gain a lot with OT but you do lose stuff like the load scaling.

                              Comment

                              • G4RHL
                                Automated Home Legend
                                • Jan 2015
                                • 1580

                                #90
                                Originally posted by bruce_miranda View Post
                                Increase the pump over run time a bit and then you won't have the pump stopping and starting as much.
                                You gain a lot with OT but you do lose stuff like the load scaling.
                                Thanks. According to the manual I can adjust this between 0 to 15 minutes. I will go into action. I have no idea what it is set at at present, presumably a default setting .

                                Comment

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