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Old Honeywell Thermostat to new Smart stat

Discussion in 'Electrics UK' started by Zee86, 9 Sep 2021.

  1. Zee86

    Zee86

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    D8405AEF-BB54-41EE-8D7A-869279F18742.jpeg Hi, please can someone help, I want to replace my old thermostat With a new thermostat, please can you help with wiring? I will attach pictures.

    thanks

    A23DC8B8-C607-4060-9A00-EFA0D52DF0D9.jpeg

    462C289C-8609-4F81-93D8-350356A2E882.jpeg
     
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  3. CBW

    CBW

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    L to L (4)
    N to N (3)
    3 from Honeywell to terminal 2 in new stat
    Link L (1) and 4 in new stat.

    Hopefully that’s a yellow wire as opposed to the cpc (aka earth).
     
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  4. Zee86

    Zee86

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    Thank you, where can I buy a small wire to link L? If it is Earth what will happen?
     
    Last edited: 9 Sep 2021
  5. CBW

    CBW

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    If there’s no excess from you fitting the new stat, you may (although I stand to be corrected) have to purchase a small piece of the same size 3 Core and Earth (3C&E), I think Wilko sell it by the metre.
     
  6. Taylortwocities

    Taylortwocities

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    Note that your new stat is probably designed gif underfloor heating. It has a connection for a floor probe. (Terminals 5&6).
    Obviously that’s no good for detecting the room (air) temperature.

    Therefore, very important that you check the settings and (if you can) select only room temperature. Do not select floor temp options, or you’ll get an alarm because there will not be a floor probe.
     
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  7. Zee86

    Zee86

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    Thank you very much
     
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  8. Zee86

    Zee86

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    I’ll be sure to check that. Thank you very much
     
  9. ericmark

    ericmark

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    The old wiring does not seem right seems as if using 2 - neutral, 1 - line in, and 3 - which the plan shows as N/C but we would normally use the N/O contact, however it also seems as if the plasticiser is leaching out, that green grunge, this can make the insulation very brittle.

    I have not seen any domestic boiler using MODBUS, and you will not need the NTC sensor, that is to ensure the floor does not get too hot with underfloor heating, It seems you have the Moes GC version which is a simple on/off contact. It is not what I would call a "Smart" thermostat. The problem with gas boilers is today most have to carefully control the return temperature, and the flame height is altered to ensure the return water is cool enough to gain the latent heat. So every time the boiler turns off, the boiler looses all the heat in boiler through the flue, and when it restarts is does so at flat out setting, so either connection to e-bus or use of some cleaver algorithms is required to reduce how often the boiler switches off, that's the clever bit.

    So the smart thermostat works out how much the room temperature normally over shoots, and calculates when to switch off the boiler so it has the largest time possible before turning on again yet keeping the hysteresis to a minimum. Which with a little thought one realises will never really happen. Things like leaving a door open one day and closing the next will upset all these clever algorithms.

    So to do a good job one way or another the wall thermostat needs linking to TRV heads, so all the real work is done by the TRV head, there are thermostats which do not link to TRV heads, which are either made for open plan houses i.e. no doors, like Nest, or designed for use with motorised zone valves like EPH, but most link with TRV heads, so at least two zones, my house has 9 programmable TRV heads, unfortunately non link to Nest wall thermostat, and the boiler does not modulate so really not the best control.

    The Moes thermostat allows 1°C to 5°C dead zone temperature, this needs matching to the TRV temperature range, but it is rather a trail and error system. The Hive for example links to the TRV and the TRV sends a demand for heat, which keeps the boiler running even if thermostat is warm enough, be it Drayton Wiiser or Honeywell EvoHome these are truly smart thermostats, the Moes may look good and connect to internet, but you will not really gain much compared with old thermostat.

    Even the useless Nest has geofencing, the Moes you have to manually alter setting if in the house or not, it does not track your phone and turn on heating when you get near to home, or have a built in PIR to over ride the geofencing if it detects some one is walking around at home.

    If your boiler is not modulating then 1°C is rather a lot, most electronic thermostat are set to 0.5°C and for non modulating boilers you can get thermostats which use a mark/space ratio as they approach target to reduce the hysteresis.

    I would say not worth replacing the old thermostat if it still works, maybe add some programmable TRV heads, but the Moes is even worse than Nest, and that takes some doing.
     
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  11. Zee86

    Zee86

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    Thanks, Looks like that yellow wire on the other end goes nowhere? See pic
    Also its a decdeal smart thermostat see other pic, i just want to have the new thermostat on permanently at a certain temperature, i will be controling the timing and on/off from the Randall 102, i have an old boiler for Central heating only and a water tank to heat water for taps.
    i do not have underfloor heating.


    7CBAE988-E42D-4BB0-9DEC-5B86F84D8F10.jpeg


    6403E7B1-7CD9-4291-8F5E-DB4D4D3A050C.png
     
    Last edited: 10 Sep 2021
  12. CBW

    CBW

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    That terminal block (aka choc block) is where the yellow wire links back up somewhere. See how there’s 2 wires in it?
     
  13. Zee86

    Zee86

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    Im guessing it goes to the boiler?
     
  14. CBW

    CBW

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    Probably, but just wire it as I’ve suggested for now, and it should work ok.
     
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  15. Zee86

    Zee86

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    Hi, I finally installed the new thermostat, it turns on and buttons work but the heating it not coming on? Can someone help please.

    the new thermostat saying the temperature is 22.0 I have set it to 28 but for some reason not going up.

    the Heating symbol is ‘On’ on the thermostat. Is it because my whole set up is old?
     
    Last edited: 2 Oct 2021
  16. CBW

    CBW

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    No, if it worked before then it’s either wiring issue or something else overlooked. Post photo of wiring. Have you selected heating on at the boiler/timer?
     
  17. Zee86

    Zee86

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    Heating is switched on at the boiler/timer, let me see if i can get a picture of the wiring, its how you told me to wire it, give me 2 minutes thanks
     
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