Honeywell evohome and Vaillant ecotec plus 838

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  • freddyq
    Automated Home Jr Member
    • Oct 2016
    • 40

    Honeywell evohome and Vaillant ecotec plus 838

    Hi all, I'm new to these forums but am looking to do lots with home automation so hoping I can learn from all the experts here I'm looking at installing the Honeywell evohome system at home and have the above-mentioned boiler which is less than a year old. Some people have told me that before getting the system installed I might want to check the wiring guide for my boiler to ensure it won't get "upset or confused" at constantly being turned on and off by the evohome. Does anyone have any knowledge or experience about this?

    The other thing which occurs to me from a simplistic view is that if the boiler is being turned on and off much more frequently, is that not going to put more stress on it thereby ageing it quicker? Or making it prone to break down? It might be totally nonsensical but just wanting to check.

    Any advice or guidance will be much appreciated!

    TIA
  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    #2
    Should work without a problem. The entire point of evohome is that every room can satisfy it's own heating needs independently of each other. ALL the other single thermostat solutions including the so called smart ones like NEST etc, are smart about the single zone they are installed in. So you can still have cold rooms when the room with the stat is heated enough. Does this result in the boiler coming on more frequently....perhaps. But it's all about general comfort. And once the house is evenly heated, the boiler should turn on less often rather than more.

    Comment

    • DBMandrake
      Automated Home Legend
      • Sep 2014
      • 2361

      #3
      Originally posted by freddyq View Post
      The other thing which occurs to me from a simplistic view is that if the boiler is being turned on and off much more frequently, is that not going to put more stress on it thereby ageing it quicker? Or making it prone to break down? It might be totally nonsensical but just wanting to check.
      When you use a boiler relay with the Evohome it uses TPI - "Time Proportional integral" that cycles the boiler on and off in a regular cycle if there is only a small heat demand. By default the cycle period is 10 minutes so it will come on from some amount of time from 1 minute out of 10 to 10 minutes out of 10 depending on the demand.

      Most modern digital thermostats these days use TPI because it is more energy efficient and gives a much better control of the temperature as it is able to maintain a much more constant temperature compared to a conventional thermostat that runs the boiler constantly until the temperature is about 1 degree too high, then turns it of until the temperature is 1 degree too low. (Which can be a long time on some systems)

      An alternative if your boiler supports OpenTherm is to use the OpenTherm bridge instead of a boiler relay - now instead of cycling the boiler on and off the Evohome "asks" the boiler for a lower flow temperature when there is less heat demand, so the boiler runs constantly but at a reduced burn. OpenTherm is more efficient and gives better control than TPI but not many boilers support it or require add in modules to support it.

      Normal gas fired boilers should be fine with TPI. Mine is over 20 years old and it has been running with the Evohome with TPI for over a year without any problems.

      Comment

      • freddyq
        Automated Home Jr Member
        • Oct 2016
        • 40

        #4
        Thanks both for your replies - very useful insight and I'm feeling more satisfied about taking the plunge

        @DBMandrake - you mention "When you use a boiler relay with the Evohome" - do you mean an additional relay installed to the boiler or the BDR91 relay used by evohome?

        Also one other thing I've recently been trying to understand relating to the bypass point, I got told by one person that I should have an automatic bypass installed if I'm getting the evohome but (and I guess this might be why they suggested it) I didn't mention that although I was asking about 8 TRVs (so two packs) I actually have 9 radiators and 2 towel rails. So the additional radiator which is a small one in the hallway and the towel rails will not be controlled. I get the impression that as a result, the need for a bypass is negated/reduced because even if the boiler was asked to fire up when all TRVs are closed (I understand the evohome does have a tendency to do this) there will be somewhere for the heat/hot water to go. If later I decide to control the towel rails and last radiator then I'd need to get a bypass installed. Is that all correct?

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          #5
          Are you saying you have no motorised zone valves?

          Comment

          • freddyq
            Automated Home Jr Member
            • Oct 2016
            • 40

            #6
            I don't have anything installed yet - due to be installed on the 12th. I will be setting up 8 TRVs on radiators around the house so yes, I will have the motorised zone valves. I'm just trying to figure out exactly what DBMandrake meant.

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #7
              Sorry, I meant before you even start putting in evohome. Do you currently have motorised zone valves to divert the boiler's hot water to either CH or HW circuits? The reason I ask is, if you do, you will need to figure out how to turn those on when you put evohome in. Putting the radiator controls in is fine and putting the boiler relay will fire the boiler up when the radiators want heat, but if you have a zone valve that's shut those radiators aren't going to get hot. This question is also related to your ABV question.

              Comment

              • DBMandrake
                Automated Home Legend
                • Sep 2014
                • 2361

                #8
                Originally posted by freddyq View Post
                Thanks both for your replies - very useful insight and I'm feeling more satisfied about taking the plunge

                @DBMandrake - you mention "When you use a boiler relay with the Evohome" - do you mean an additional relay installed to the boiler or the BDR91 relay used by evohome?
                By boiler relay I mean the supplied BDR91
                Also one other thing I've recently been trying to understand relating to the bypass point, I got told by one person that I should have an automatic bypass installed if I'm getting the evohome but (and I guess this might be why they suggested it) I didn't mention that although I was asking about 8 TRVs (so two packs) I actually have 9 radiators and 2 towel rails. So the additional radiator which is a small one in the hallway and the towel rails will not be controlled. I get the impression that as a result, the need for a bypass is negated/reduced because even if the boiler was asked to fire up when all TRVs are closed (I understand the evohome does have a tendency to do this) there will be somewhere for the heat/hot water to go. If later I decide to control the towel rails and last radiator then I'd need to get a bypass installed. Is that all correct?
                If you have at least one radiator that is open all the time (lock shield valves on both sides left open, no TRV) then technically no, you don't need an automatic bypass valve.

                However if you ever decide to fit HR92's (or even manual TRV's) on all remaining radiators in the future you'd need an automatic bypass valve. It's good to have one anyway because it regulates the maximum differential pressure across the radiators.

                Without one you'll find that your radiators are quiet when they're all heating up but if there is only one or two active they might hiss quite a lot because the pressure increases. An ABV helps stop that happening as it lets you set the maximum differential pressure across the radiators. When enough radiators are open it doesn't flow at all, so there is no effect under those circumstances, but when not many radiators are open and the pressure gets higher it acts as a pressure relief valve.

                It's part of the building regulations for all new installations now I believe.

                Comment

                • freddyq
                  Automated Home Jr Member
                  • Oct 2016
                  • 40

                  #9
                  Thanks @bruce_miranda - I have no idea to be honest. How do I find out? Is it a boiler thing or an installation thing?

                  @DBMandrake - thanks I'm with you. Do you think I can get the evohome installed and setup now and then add the automatic bypass valve later? Or is there a reason why I should really get the ABV installed with or before the evohome?

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #10
                    Hang on, my bad, you have a combi boiler. So you need to check because many modern combi boilers have an ABV fitted inside them. Also you won't have any motorised zone valves too because you have combi boiler. The valves are all built into the boiler. Which does mean you can't have the evohome HW kit too. So just the boiler relay with the radiator controls should be fine. If all your radiators don't have the radiator controls, you'll need to work out how they demand for heat.

                    Comment

                    • freddyq
                      Automated Home Jr Member
                      • Oct 2016
                      • 40

                      #11
                      Yes sorry I have a combi boiler and I actually read another post on here from someone who had the same boiler but lower power who had checked up and confirmed from Vaillant that the boiler itself has an embedded diverter valve with bypass.

                      I've known all along I don't need the HW kit because that's all on-demand for us with the combi.

                      So the towel rails don't have any controls on them as far as I know, they always heat up when the heating is turned on. The radiator in the hallway does have a manual thermostat so can be turned on an off so I would ensure that is in the on position.

                      I'm just curious to understand if I have to have an ABV fitted now...

                      Comment

                      • bruce_miranda
                        Automated Home Legend
                        • Jul 2014
                        • 2307

                        #12
                        Currently how is heating controlled? Is it via a timer, room stat or both? You need to work out with evohome how will heat demand be requested for all radiators.

                        My towel rails also dont have trvs but they get heated whenever any of the other radiators in the house demand heat. And i'm fine with that. But you may not be

                        Comment

                        • freddyq
                          Automated Home Jr Member
                          • Oct 2016
                          • 40

                          #13
                          We have an analog timer on the boiler which we use. There is a very basic thermostat on the wall in the hallway but we don't use that. I deliberately kept everything basic because I always planned to install a smart system. So I'm guessing with evohome I will switch the boiler from timer to on and then let the evohome control boiler on off times.

                          And yea, I'm fine with the towel rails and hallway radiator being heated whenever any of the controlled radiators requests heat.

                          Comment

                          • bruce_miranda
                            Automated Home Legend
                            • Jul 2014
                            • 2307

                            #14
                            Then its a very simple installation. Just turn the timer clock to constantly on and replace the input of the room stat to the boiler with the bdr91, setup as the default boiler relay.

                            Comment

                            • freddyq
                              Automated Home Jr Member
                              • Oct 2016
                              • 40

                              #15
                              Wait so the boiler has three positions; on, off and timer. I thought I'd be leaving it to the on position with evohome but I actually need to leave it on the timer position but set all times to be on?
                              "replace the input of the room stat" as in replace the basic dial thermostat in the hallway with the BDR91?

                              It has also been suggested to me that I get the BDR91 wired between the power supply switch for my boiler and the boiler itself, set all timer times to be on. So the relay controls the power from the switch to boiler and it will only turn on when relay tells it to....?

                              Comment

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