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replacing room thermostat

Discussion in 'Electrics UK' started by blackdog1969, 10 Jan 2015.

  1. blackdog1969

    blackdog1969

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    Hi there members
    I am looking for a little help replacing an old Satchwell TLX2259 with a Timeguard TRT033. I thought it would be a straight swap but being a total novice I am finding it a tad tricky. Using a testing screwdriver I found that the yellow gave me a constant live, the red was live when the thermostat switched on and the blue held no current. So based on that I wired the yellow to the COM terminal, the red to the red to the NC and the blue to the NO, this resulted in the boiler not firing when the new thermostat called for heat then when I switched the red to NO and the blue to NC this resulted in the boiler firing constantly even when not called for....any help is very much appreciated. Thanks
     
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  3. bernardgreen

    bernardgreen

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    First of all the neon screwdriver does not give reliable results

    The old thermostat required

    Permanent Live into terminal 3
    Neutral into terminal 4

    Switched Live from terminal 1


    Most often the red is Permanent Live. Yellow is Switched Live and Neutral blue

    The new thermostat does not need Neutral.

    Permanent Live to Common
    Switched Live from NO

    Neutral must not be connected to anything in the new thermostat.

    You may have been very lucky Had you connected the red and yellow the other way round the contacts in the thermostat would have shorted Live to Neutral whicc would have blown the fuse and damaged / destroyed the contacts in the thermostat.
     
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  4. blackdog1969

    blackdog1969

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    Thank you for the reply. I will give this a try tomorrow first thing.
     
  5. eveares

    eveares

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    The blue is most likely neutral and so disconnect it from the thermostat as you don't need the neutral wire.

    Wire the permanent live wire (usually red or brown, but likely yellow in your case) to the COM terminal and wire the remaining red wire to the NO terminal.

    NO = Normally open (Open circuit when off / Closed circuit when on)

    NC = Normally closed (Open circuit when on / Closed circuit when off)

    Regards: Elliott
     
  6. EFLImpudence

    EFLImpudence

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    Not according to the pictures.
     
  7. eveares

    eveares

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    Your correct, but I was going on the following text

    If the above text is true, then he has likely got the permanent live wired to terminal 1, and the switched live wired to 3.

    Also the blue wire/neutral in the old stat is used for the shunt accelerator heater what ever that does.

    http://www.tlc-direct.co.uk/Technical/DataSheets/Sunvic/TLX.pdf
     
  8. sparkwright

    sparkwright

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    Blue should go in a seperate connector block (so may have to buy one if you haven't got any at home, get a small one - 5 amp).

    The wiring diagram on the old stat appears to show terminal 3 should be perm live, and terminal 1 should be switched live.

    It would seem the cable may be correctly the 'incorrect' way at the other end, wherever that might be, as one would expect the red to be the perm live.
     
  9. bernardgreen

    bernardgreen

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    A mechanical ( bi-metal ) thermostat switches ON at one temperature but switches OFF at a higher temperature. ( called hysteresis ) The difference can be several degrees and would result in noticeable temperature changes in the room. The "accelerator" compensates by adding a bit of warmth inside the thermostat when the heating is switched ON This means the bi-metal switches OFF at a room air temperature that is closer to the temperature at which the thermostat switched ON.

    Electronic thermostats have a hysteresis of less than a degree so do not need the "accelerator"
     
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  11. blackdog1969

    blackdog1969

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    Hello again, I have now tried both of these wiring combinations and both resulted in the boiler firing even when the thermostat was not calling for heat...I even had a friend of a friend who is a spark look at it and he could not get it working properly, he said that the model may not be compatible? Any further ideas?
     
  12. AndyPRK

    AndyPRK

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    Although a little dangerous , what happens when you leave the front off?
    Ie all wires disconnected?

    Maybe u damaged it when u connected the blue upto it.
     
  13. blackdog1969

    blackdog1969

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    The boiler just remains awaiting a call for heat (orange light).
     
  14. ericmark

    ericmark

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    I will admit it is a little confusing calling terminals normally closed “NC” and normally open “NO” as one does not know which is normal. However with my thermostat heating is NO and cooling is NC so I will expect most to be wired that way so unless you have air conditioning then NO is not used.

    The old bi-metal strip type thermostat would give a temperature span of around 2 -3 degrees which was really too much so it included a heater so it would switch off quicker but the new digital thermostat can be set to what ever is required without need of a heater so often factory set to 1ºC. So old heater always needed a neutral and would only work correct with 230 volt but new ones only need a neutral when not battery operated and with battery can switch 24 or 230 volt.

    Where a mercury tilt switch was used the old also had to be level when mounted.

    Old thermostats could only be set to a single temperature the new ones often have a built in clock so you can set night, morning, day, and evening with four different temperatures so removing need for frost thermostat and programmer in many cases.

    So blue need some tape on it and is never used. The Yellow to com and Red to NO and before you gave it a direct short that would have worked. Likely now it needs a new thermostat and a fuse renewing or MCB resetting because of the direct short.

    One hopes nothing blown inside the boiler but there could be if not fused to correct size.

    The permanent line and switch line (Com and NO) with electronic thermostats can be swapped it does not matter. However with the old bi-metal strip type is does matter. So your description of tests shows at the boiler yellow is permanent line and red is switched line and it would seem thermostat has been wired the other way around with red being taken as permanent line.

    The result would be the little heater would have been on all the time not just when there was a call for heat. So likely there was a marked difference between when heating switched off and when it switched back on again. If your tests are correct the old thermostat would have worked far better if the yellow and red wires were swapped.

    I fitted the predecessor to Horstmann DRT2 Programmable Digital Room Thermostat which has worked very well. It says batteries last two years but I change every year, with good quality batteries it can fail on flat batteries before the battery light comes on with standard zinc batteries the indicator works OK.
     
  15. ericmark

    ericmark

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    It seems wired correct but likely burnt out do to short circuit likely you will need a new one now.
     
  16. AndyPRK

    AndyPRK

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    Sounds like u damaged it when u connected the blue up.

    What happened ? Did a fuse blow ?
     
  17. blackdog1969

    blackdog1969

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    3 amp fuse blew in fused switch for the central heating. How do I know if it's damaged? And which wire combination are we assuming is the correct one in case I buy a new one. Could be a costly experience if I keep guessing at it.
     
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