Room Thermostat Wiring

Discussion in 'Electricians' Talk' started by TinkerCharlie, Jul 31, 2017.

  1. TinkerCharlie

    TinkerCharlie New Member

    Morning folks,

    Need wiring advice changing over a very old room thermostat to a Honeywell DT90E model.

    Here's some pics of the current set-up:

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    New Honeywell Room Thermostat back-plate:

    [​IMG]

    Can someone guide me as to where the existing wires go into the new back-plate please? Current to wires will be off!

    Cheers,

    Charlie.
     
  2. dobbie

    dobbie Well-Known Member

    Someone has joined the live and switched live in that stat therefore it would not have worked.
    The blue in the old stat is a neutral,put it in a piece of connector strip to make safe as it is not required.
    The red which should be live goes to A in the new stat.
    The yellow which should be the switched live goes to B,also put a piece of red tape on this to show it is a live cable.
     
    TinkerCharlie likes this.
  3. TinkerCharlie

    TinkerCharlie New Member

    You're a star Dobbie, thanks. I didn't think it looked quite right.

    Does the yellow/green wire still need to be earthed?
     
  4. dobbie

    dobbie Well-Known Member

    Put a piece of connector strip on it the same as the neutral to make safe.
     
  5. TinkerCharlie

    TinkerCharlie New Member

    Great stuff. Many thanks for advice.
     
  6. TinkerCharlie

    TinkerCharlie New Member

    Quick question: if a room thermostat has batteries, will it only work with them in, as when I took them out the display went dead.

    Leads me to think that perhaps there is no AC current to the unit if so.
     
  7. TinkerCharlie

    TinkerCharlie New Member

    Scrap that. Should have tested it first with the heating on. Seems to work fine...
     
  8. Devil's Advocate

    Devil's Advocate Well-Known Member

    As you've sussed out, it'll only work with batteries in - it does NOT use the power that's in the cables.
     
  9. TinkerCharlie

    TinkerCharlie New Member

    Cheers Devil's. Interesting it only uses the battery power. Learnt another new thing!
     
  10. Devil's Advocate

    Devil's Advocate Well-Known Member

    All the room stat does when it decides the boiler needs to come on is to close a switch inside it - and that joins the two wires you have there, the red and the yellow.

    As Dobbie says, tho', your old stat was seemingly wired incorrectly. The Red should be the 'live' and the Yellow should be the 'switched live', so the fact they'd been wired together suggest that the stat wasn't doing anything as it was always sending a 'switch on' signal to the boiler.

    So I'm guessing you controlled the boiler and CH by using the separate timer control? And basically as along as the timer was timed to be 'on', the boiler was firing - regardless of the temp of the room.

    (Psssst, anyone considering fitting a better room stat like the DT90E should also be seriously considering fitting a programmable stat instead. This will do both jobs - timings and temps - in a much more useful fashion.)
     
  11. TinkerCharlie

    TinkerCharlie New Member

    Oh I have a programmable one upstairs. Just moved in to the place so discovering new and interesting electrical adventures!!
     
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  12. MGW

    MGW Active Member

    The neutral in old stat goes to a heater to reduce the hysteresis, but with the electronic type it needs around a 0.5 C hysteresis or the boiler would be switching on/off every 5 minutes, with a condensate boiler where the flame height is variable the thermostatic radiator valve can reduce the hysteresis so it can hardly be detected, specially using eTRV heads, which is why on modern boilers the wall thermostat is only there to switch off the boiler to reduce cycling.

    With old boilers however the wall thermostat is still king, and the TRV is only to stop in the main bedrooms from getting too hot. So with an old boiler the programmable thermostat is very good, but with new boiler in most cases pointless, it is either super simple, or go all the way with something like EvoHome.
     
  13. Colin Key

    Colin Key New Member

    I am also changing to new DT90E. I need advice on wiring. I have red, blue and yellow/green wires from the old thermostat. I understand this a 2 wire system though. Can somebody advise which ones are required please. Many thanks.
     
  14. Devil's Advocate

    Devil's Advocate Well-Known Member

    Hi Colin.

    Post your info an here too so's we can all have a gander.

    Is the old 'stat still in place? If so, take photos of where these wires go to.

    If not, did you note - or can you remember - which terminal they went to on the old 'stat?

    If you look now at your old 'stat, can you say for certain "The red wire went here, the black one went..."

    REALLY importantly, where did the green/yellow (earth) wire go to on the old stat?
     
  15. Colin Key

    Colin Key New Member

    Re-wired now. Found out old cable had earth as the switched live!
     
    Devil's Advocate likes this.
  16. Devil's Advocate

    Devil's Advocate Well-Known Member

    Jeepers!

    Well sorted.

    (Er, did you do it properly? RED = LIVE, BLUE? = SWITCHED LIVE (with a ring of red tape around it) and YELLOW/GREEN = EARTH, safely terminated behind the 'stat. Ie - did you adjust the wiring at BOTH ends?)
     
  17. dobbie

    dobbie Well-Known Member

    It sounds like he had a 3 core cable,Red,yellow and blue with the yellow as switch wire which would be correct.
     
    Devil's Advocate likes this.
  18. Devil's Advocate

    Devil's Advocate Well-Known Member

    Which is why trying to help some folk on here with only written descriptions can be a 'mare...
     

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