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Thread: Evohome and electric underfloor

  1. #1
    Automated Home Jr Member
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    Default 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.

  2. #2
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    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.

  3. #3
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    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?

  4. #4
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    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.

  5. #5
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    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.

  6. #6
    Automated Home Legend paulockenden's Avatar
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    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.

  7. #7
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    The HC60NG is getting increasingly rare to find.

  8. #8
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    Quote 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.

  9. #9
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    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?

  10. #10
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    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.

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