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replacing analog with digital room thermostat

Discussion in 'Electrics UK' started by dxtans, 3 Oct 2018.

  1. dxtans

    dxtans

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    Hello,
    I wish to replace my current room thermostat ( WS4) with a digital one. (Honeywell DT90E)
    But am confused with the wiring. I need help please. What needs to be wired into the new one.

    Thanks
    Dxtans IMG_1690.JPG

    Current: British Gas WS4
    N: Black
    L: Brown
    3: Grey
    ( plus earth)
    The DT90E has the following terminals, ( is it like for like) ie: N to N and L to L ?
    L and N.
    IMG_1695.JPG

    thanks any guidance welcomed
     
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  3. SFK

    SFK

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    DXtans
    Manual is here: http://www.free-instruction-manuals.com/pdf/pa_457920.pdf
    Your Setup is Diagram A.

    Your new thermostat uses Batteries and so does not need any connections to actually power it.
    It is simply providing a switch to switch on and off a signal (in this case 240V) to the boiler.

    Your wiring is:
    Live: Brown
    Neutral: Black
    Switched Live: Grey
    Earth

    So on your new Thermostat
    A: Live In: Your Brown Wire
    B: Live Out (Switched Live): Your Grey Wire
    C: No Connection: no wires connected to this
    Neutral - put in a termination block and place safety away from other cables.
    Earth - put in a different termination block and place safety away from other cables.

    Now when Thermostat calls for heat, the Switch in the thermostat is moved from C to B, and the Live IN cable is connected to the Switched Live cable, making it live and activating the boiler.

    SFK
     
    Last edited: 3 Oct 2018
  4. EFLImpudence

    EFLImpudence

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    The N on the diagram is after the rectangle which represents the boiler or valve - whatever it turns on.
     
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  5. Echo the husky

    Echo the husky

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    I wonder how many people have put a neutral wire in terminal B and had a loud bang when the stat called for heat.
     
  6. DetlefSchmitz

    DetlefSchmitz

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    ...and has to buy another thermostat...
     
  7. dxtans

    dxtans

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    Thanks SFK,
    Your description and explanation is very lucid.
    I will attempt the replacement tomorrow and post back the results
    Thanks
    dxtans
     
  8. dxtans

    dxtans

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    Now replaced the analogue with the digital one. As per SFK instructions. Tested the temp settings. All working as expected.
    Thanks for your help SFK.

    thanks
    dxtans
     
  9. SFK

    SFK

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    Dxtans,
    My pleasure.
    Always great to hear how it went.
    SFK
     
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  11. Mottie

    Mottie

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    I have the DT90E and what I don't like about it is that when the heating first comes on it never goes straight to the set temperature. It normally switches off 1 degree before the set temperature for a few minutes, then starts again until .5 of a degree away from temperature, then switches off again then switches on. Finally when it gets to the set temperature it stays at that temperature but switches on for a minute or so every 10-15 minutes. I phoned Honeywell up at they say that is normal. Is it?
     
  12. SFK

    SFK

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    Motman,
    Never used this thermostat, but looking at the details on page 8 of the manual and the following document, yes it is normal. Presumably this is to minimise Overshoot.
    If you wish to change this you need to modify the parameters including the "Proportional band width" and the "Minimum On/Off time".

    Manual is here: http://www.free-instruction-manuals.com/pdf/pa_457920.pdf

    Proportional band width: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwip8YvF1-zdAhWoL8AKHXuGCyEQFjAAegQICRAC&url=https://www.honeywelluk.com/documents/All/pdf/TPI%20Functionality%20explained.pdf&usg=AOvVaw3cbVsv9wfDRixp-I1TsdpH

    And some details of all the Parameters and possible settings, (mostly suggesting that you should not change these parameters in normal use) is here:
    https://www.diynot.com/diy/threads/honeywell-dt90e-thermostat-setting-up-help-needed.217838/
     
    Last edited: 4 Oct 2018
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  13. ban-all-sheds

    ban-all-sheds

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    As many as those who fall short of the competence requirements in the installation instructions.
     
  14. Scot Maclachlan

    Scot Maclachlan

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    My existing stat looks like this. Can anyone tell me how I should replace it with a DT90E1012?

    [​IMG]
     
  15. AndyPRK

    AndyPRK

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    Not seen one of them before
     
  16. EFLImpudence

    EFLImpudence

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    What model is the old thermostat?

    Is there a wiring diagram on the inside of the cover?
     
  17. EFLImpudence

    EFLImpudence

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    It looks very complicated for just a thermostat.
     
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