Evohome and electric underfloor

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  • Meh
    Automated Home Jr Member
    • Mar 2020
    • 23

    Evohome and electric underfloor

    Hi all,

    So I am about to install evohome upstairs in my home to all radiators. Also on the towel rails in the bathrooms.

    I also have electric underfloor in each bathroom and was planning to control that with a bdr91 that supplies power to the existing thermostat. The thermostat remembers its last setting after power is cut, so I can safely set it to a high temperature and leave it there. The bdr91 would turn it on when needed and off when not.

    My question was can I use the hr92 on the towel rails in the bathroom as the thermostats that turns on both the towel rail and the bdr91?

    Basically if I set the bathroom to 25 degrees I want both the underfloor and towel rail to turn on until it hits that temperature. This is mainly because the towel rails and wholly inadequate to heat up some of these bathrooms on their own!

    OR Is there any better way to do this? Any advice appreciated.
  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    #2
    This is an interesting use case. I know you can have two actuators using a single sensor, but then in your case the zone types will be different. What should happen if the Electric zone needs heat but the towel rad circuit doesn't.
    But please make sure you check if your UFH circuit load can directly be controlled by a BDR91. You might need a contacter.

    Comment

    • Meh
      Automated Home Jr Member
      • Mar 2020
      • 23

      #3
      Thanks for the reply.

      Basically they are in the same room, so they both need to be on when it’s too cold and both off when it’s too hot. I understand they will both have different heat up and cool down times but as a combined system it should be able to work it out.

      Is it possible tho? Can I tell evohome to turn on the bdr91 whenever the radiator trv turns on?

      Comment

      • Meh
        Automated Home Jr Member
        • Mar 2020
        • 23

        #4
        Oh and regarding the 5a limit I have yet to check, but even if I need to add a contractor or another relay I could bury it in the back box behind where I put the bdr91 I guess.

        Comment

        • bruce_miranda
          Automated Home Legend
          • Jul 2014
          • 2307

          #5
          As long as you are happy for the boiler to fire up, should the electric zone need heat, I think you can do what you are trying. Just pair the BDR91 as another actuator, like you would pair two HR92s in a single zone.
          The Electric Zone type was introduced to stop the boiler from firing when there was heat demand.
          The contactor can be quite a big chunk of metal, so don't assume it will get behind a back box.

          Comment

          • paulockenden
            Automated Home Legend
            • Apr 2015
            • 1719

            #6
            Isn't the a big brother for the BDR91, not very well publicised, but which takes a bigger load? Can't remember the name....

            P.

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #7
              The HC60NG is getting increasingly rare to find.

              Comment

              • Meh
                Automated Home Jr Member
                • Mar 2020
                • 23

                #8
                Originally posted by bruce_miranda View Post
                The HC60NG is getting increasingly rare to find.
                I managed to find it in a pack with a thermostat, this might end up looking ugly though. Maybe I can find a better underfloor thermostat with some kind of call for heat/cut off external input instead.

                Will investigate.

                Comment

                • Meh
                  Automated Home Jr Member
                  • Mar 2020
                  • 23

                  #9
                  Hmm ok so these thermostats I have only have power in, power out (to the electric mat) and a sensor.

                  I assume higher temperature means lower resistance in the sensor. Maybe shorting the sensor with the bdr91 when OFF would turn it off, while unshorting it when ON would make it turn on again (until it hit the temperature).

                  This way I wouldn’t need to worry about load. I could hide the bdr91 in the wall cavity if possible?

                  Comment

                  • bruce_miranda
                    Automated Home Legend
                    • Jul 2014
                    • 2307

                    #10
                    I definitely wouldn't do that! Most UFH controllers have 2 sensors. One that measures the floor temperature and one that measures the air temperature. The temperature probes can vary from digital 3 pin probes to passive 2 pin probes. But in any case, I wouldn't just short the temperature sensor because it's quite a critical safety feature of the UFH controller. Try and do it properly. But first you must see what is the maximum draw the UFH element can have and then work from there.
                    Looking ugly is better than your house catching fire because the heating element has just been left to heat continuously. Finding a UFH controller that allows a wiring centre input is best because it is only looking for the switch on condition and will handle the element load itself.

                    Comment

                    • Meh
                      Automated Home Jr Member
                      • Mar 2020
                      • 23

                      #11
                      Ok, I think you misunderstood what I meant. Shorting the probe could only ever turn it off (it’s a passive probe).

                      This thermostat had no air sensor on it.

                      Another option is to replace it with a Neo stat (I have loads of these laying around).

                      These have 2 inputs, one for air and one for floor. I could leave the floor one as is and relay the air one through two resisters to simulate on/off temperatures.

                      On a side note tho, I just read that these electric underfloor only really take the chill out the floor and aren’t that great at heating the room.

                      Is it really expensive to leave it set at 21 or 24 all day?

                      Comment

                      • bruce_miranda
                        Automated Home Legend
                        • Jul 2014
                        • 2307

                        #12
                        Many have the air sensor built into the controller and then you chose which temperature sensor you want to use as your thermostat. My WarmUp 4 has a nice little display to show me how much energy is being used to heat up the floor, so I can then work out how much it costs. Electric UFH should almost always be used as secondary comfort heating, as you say to take the chill off the cold tiles.

                        Comment

                        • bruce_miranda
                          Automated Home Legend
                          • Jul 2014
                          • 2307

                          #13
                          You sound like the handy type. Why not open one of the UFH thermostats and see how the relay in that is being switched on. Then all you need to do is use your BDR91 to switch the UFH's relay on, so it continues to take the load and your BDR91 is simply providing the contact free ON OFF signal.

                          Comment

                          • Meh
                            Automated Home Jr Member
                            • Mar 2020
                            • 23

                            #14
                            Originally posted by bruce_miranda View Post
                            You sound like the handy type. Why not open one of the UFH thermostats and see how the relay in that is being switched on. Then all you need to do is use your BDR91 to switch the UFH's relay on, so it continues to take the load and your BDR91 is simply providing the contact free ON OFF signal.
                            That’s a good idea . Might do that.

                            I think my main aims are:

                            1) be able to control the UFH from evohome.
                            2) be able to see somehow from the stat if evohome has turned it on or off
                            3) be able to override evohome if from the stat.

                            The only way I can see to do this is to setup a fake air probe. With the bdr91 on, it switches the relay to a resister which registers as 20 degrees, with the bdr91 off it switches to 30 degrees.

                            Leave the stat set to 25 degrees and now evohome controls it. Set it to below 20 and it overrides to on, set it above 30 and it overrides to off.

                            That’s not very intuitive tho! So now I’m thinking to not bother and just setup some timers on the underfloor and just control the bathroom rad and be done with it.

                            Comment

                            • bruce_miranda
                              Automated Home Legend
                              • Jul 2014
                              • 2307

                              #15
                              There is another way. Something I use. If you buy a Smart UFH controller like the Warmup 4ei that I use. You can use an automation platform to keep the bathroom zone running on Evohome in sync with the UFH zone in that room.

                              Comment

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