Evohome on a Y plan system - should i add a boiler demand relay?

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  • richardjh
    Automated Home Jr Member
    • Nov 2017
    • 11

    Evohome on a Y plan system - should i add a boiler demand relay?

    Hello,

    We moved to a new house and I replaced the faulty CH/HW time controller with an Evohome setup and HR92 on most radiators. The system is a Y plan and i have two BDR91 relays controlling the zone valve as i also have the hot water kit. I currently have it configured with no boiler demand relay. Once the system is up to temperature i get a lot of short 'on' cycles, sometimes so short the boiler is only on for a few seconds. I understand why this happening, but wonder if it would be better if I added a boiler demand relay so i can set the max. number of cycles and minimum on time?

    Anyone done this?


    thanks,
    Richard.
  • HenGus
    Automated Home Legend
    • May 2014
    • 1001

    #2
    Page 43 of the Evohome Installation Guide might be of help.

    Comment

    • top brake
      Automated Home Legend
      • Feb 2015
      • 837

      #3
      sounds like there may be a binding error so recommend that you contact Honeywell support
      I work for Resideo, posts are personal and my own views.

      Comment

      • Edinburgh2000
        Automated Home Guru
        • Dec 2016
        • 134

        #4
        Originally posted by richardjh View Post
        ...........if I added a boiler demand relay so i can set the max. number of cycles and minimum on time?
        You don't need a boiler demand relay to be able to adjust the cycle rate. I use my Evohome to control zone valves with no boiler BDR. Each of my zone valves is controlled by its own BDR and I have set the cycle rate to 3 per hour. As I understand it, that sets the period for which each BDR gives a heat demand. For '3 per hour' that means 20 minute periods. If the zone is calling for, say, 10% heat demand, then the BDR will be off for 18 minutes and on for 2 minutes. I have valves that take about a minute to open and a minute to close so anything more than '3 per hour' cycle rate causes my valves to be spinning constantly which is a bit pointless. Even at '3 per hour' I still get valves closing and then reopening one minute later, which has no material effect on the zone, given that I have some large heavy cast iron radiators. I assume that is for a 95% heat demand in the zone.

        Try setting your cycle rate to 3 per hour and see if you get fewer short cycles.

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          #5
          You shouldn’t see those settings unless you've bound a boiler relay.

          Comment

          • mtmcgavock
            Automated Home Legend
            • Mar 2017
            • 507

            #6
            Originally posted by paulockenden View Post
            You shouldn’t see those settings unless you've bound a boiler relay.
            You can see those settings when a boiler relay isn't bound and altering them seems to apply them to the heating relay. My parents have the same set up (Without the boiler relay) and you can alter the cycle rates and minimum on time. And on altering these it appear it adjusted it accordingly.

            Comment

            • paulockenden
              Automated Home Legend
              • Apr 2015
              • 1719

              #7
              The only time I could see those settings was when my installers messed up and double bound my heating relay as a boiler relay too.

              Once I’d corrected the configuration and removed the boiler relay binding those parameters went away.

              Comment

              • bruce_miranda
                Automated Home Legend
                • Jul 2014
                • 2307

                #8
                I can confirm that if you have any BDR91 within the CH heating system you get those settings. I use OT but one of my UFH zones uses a BDR91, I can see those settings. But I have no idea if those settings affect the BDR91 of the UFH zone or not. Haven't tried.

                Comment

                • paulockenden
                  Automated Home Legend
                  • Apr 2015
                  • 1719

                  #9
                  How odd. You don’t see them on a conventional Y or S plan (with no electric controlled zones).

                  I might try binding a phantom BDR91 to a fake zone and see if they appear!

                  Comment

                  • richardjh
                    Automated Home Jr Member
                    • Nov 2017
                    • 11

                    #10
                    That is what i thought i would try - either on a fake zone or as a boiler control relay that isn't actually connected to the boiler!

                    Comment

                    • DBMandrake
                      Automated Home Legend
                      • Sep 2014
                      • 2361

                      #11
                      Originally posted by mtmcgavock View Post
                      You can see those settings when a boiler relay isn't bound and altering them seems to apply them to the heating relay. My parents have the same set up (Without the boiler relay) and you can alter the cycle rates and minimum on time. And on altering these it appear it adjusted it accordingly.
                      I ran as a two relay S-Plan system for a while (CH and HW zone valve relays only) and I did NOT have access to those settings until I added a boiler control relay. This is on the Wifi version of the controller.

                      So I don't know how you see them without a boiler control relay. Are you sure one isn't double bound ?

                      Many posts on here over the last couple of years saying the same thing.

                      Comment

                      • mtmcgavock
                        Automated Home Legend
                        • Mar 2017
                        • 507

                        #12
                        Originally posted by DBMandrake View Post
                        I ran as a two relay S-Plan system for a while (CH and HW zone valve relays only) and I did NOT have access to those settings until I added a boiler control relay. This is on the Wifi version of the controller.

                        So I don't know how you see them without a boiler control relay. Are you sure one isn't double bound ?

                        Many posts on here over the last couple of years saying the same thing.
                        Definitely not double bound (They have two boilers so can't have the Boiler relay, and use the Microswitches on the valves to fire the boilers). All I can think is that they have an additional zone onto the S plan using a T87RF and BDR91, but the current way that is it set up adjusting the settings on the controller work. I was having trouble with the boiler cycling at the 6 cycles per hour and adjusted it to 3 which made a difference.

                        And from memory my system showed the same options before I bound the boiler relay at a later date. I also have an additional zone, so perhaps it's this?

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #13
                          my BDRs are also not double bound. I have always cleared all bindings. but I too have a UFH zone that uses a BDR91+T87RF

                          Comment

                          • richardjh
                            Automated Home Jr Member
                            • Nov 2017
                            • 11

                            #14
                            I have now bound another BDR91 as a boiler control relay, though it is a dummy - nothing is connected to the relay contacts.
                            I can now see the cycles per hour setting and minimum on time settings. I have set the cycles to 3 and min. on to 5 minutes.

                            Can't say it behaves as expected. I sometimes see the CH heating valve relay come on and not the boiler demand relay. Sometimes I see the boiler demand relay on and neither the CH or DHW relays on. This doesn't make sense. (well to me at least!)

                            Comment

                            • paulockenden
                              Automated Home Legend
                              • Apr 2015
                              • 1719

                              #15
                              Originally posted by richardjh View Post
                              Sometimes I see the boiler demand relay on and neither the CH or DHW relays on.
                              That sounds like a comms issue.

                              Comment

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