Fuse keeps blowing....

Discussion in 'Getting Started FAQ' started by Richard Snow, Apr 29, 2018.

  1. Richard Snow

    Richard Snow New Member

    I recently changed my thermostat from Analogue to digital (Danfoss) and a day later the fuse blows stopping the boiler from working, coincidence??

    The wires I used to the new thermostat are:

    red - com
    black - 2
    blank - 3

    earth - not attached.
    Is this correct?
  2. rogerk101

    rogerk101 Well-Known Member

    Usually the thermostat looks to the system as just an on-off switch, which then means the thermostat needs its own battery. Other times, they also need a permanent 240V line and neutral, in which case they don't need a battery.
    Since not everyone on here knows what the pins on a Danfoss thermostat mean, please upload a photo of the wiring terminals or one of the wiring instructions that came with the new thermostat.
  3. Richard Snow

    Richard Snow New Member

    Thanks for replying, unfortunately I cannot post a [picture it says its too big....
  4. Richard Snow

    Richard Snow New Member

    Basically it has 3 points to use
    com ,1 and 2

    I have 3 wires, red, black and earth.

    Currently I have a red in common and the black in 2.
  5. Richard Snow

    Richard Snow New Member

    sorry the wiring diagram states point 1 is common
  6. 14th edition

    14th edition Well-Known Member

    You will have to edit the picture to reduce its size. But you need to check what the wiring does at the moment.... its possible that someone has used the earth as a switch line and the black as a neutral... So you need to check there is no neutral, i.e. the black wire! You will need a test meter, test between red and black. If you get a reading of 230v approx its a neutral which means when the stat operates it will short and blow the fuse.
    The original stat will give clues as most older stats needed a neutral and newer electronic ones don't. So test first...
    Sammi likes this.
  7. Richard Snow

    Richard Snow New Member

    Faulty room thermostat seems to be the issue. Will replace tomorrow.
  8. Sammi

    Sammi Member

    Richard, if you cannot post a photograph of how the old analogue roomstat was wired or cannot recall how it was connected, then we won't be able to tell you if your new wiring is correct.

    What 14th Ed has said above is not uncommon, and if you have wired up your new digi stat incorrectly, then it is quite likely it IS now faulty as you have been short-circuiting the relay inside. Please don't replace it until you are certain of the wiring.

    If you cannot remember how the old stat was wired, then try and trace where that cable goes. Is it straight to your boiler, or is it to a rectangular box, a wiring centre? Either way, cut the power, open up, and tell us where these three wires - red, black & 'earth' - go to.
  9. Richard Snow

    Richard Snow New Member

    right, managed to upload photo of wiring diagram for new digital thermostat.

    also put old thermostat back on and that works! it's not just that responsive to tempreture I.e few degrees out either way.

    Attached Files:

  10. Sammi

    Sammi Member

    This isn't a case of the new stat being wired incorrectly, but it's possibly a case of the wrong wires being used :).

    Yes, you wire to 1 and 2 (com and 'on')

    Ok, you've rewired the old stat and it hasn't blown a fuse and it does control the boiler - can you post a close-up pic of that please?
  11. Richard Snow

    Richard Snow New Member

    Hi Sammi
    thanks for taking the time to answer back.
    images of old thermostat.

    Attached Files:

  12. dobbie

    dobbie Well-Known Member

    You are using the earth wire (cpc) for the switched live which is dangerous and against regulations.
    For the new stat you need to reconfigure the wiring back at the boiler or wiring centre.
    The black which is a neutral at the moment needs to become a switched live sleeved red.
    The bare cpc needs to be connected to earth at boiler/wiring centre sleeved green &yellow and parked in new stat sleeved green &yellow.
    Sammi likes this.
  13. Richard Snow

    Richard Snow New Member

    Hi Dobbie,
    This was already in the house when I moved in!
    How does the earth connect to the new stat when it only requires 2 wires, red and black?

    I think it might be time to call in an electrician!!!
  14. Sammi

    Sammi Member

    It's as Dobbie has said.

    On your old stat 4 is the Neutral, the 1 is the Live and the 2 is the Switched Live. There is no 'earth' wire. So, your red is 'live' the black is 'neutral' and the earth is actually the 'switched live' and not an earth. Can you see how dangerous this was - a 'deadly' earth wire.

    The two wires you need for your new stat are the Live and Switched Live, and they can go to 1 and 2 (com and on) on your new stat any way around, although it's best to place the Live to 1.

    But please don't do this using the existing wires they way they are, because that would make you as bad as the person who wired your old stat, and that is very bad indeed.

    As Dobbie says, find out where these wires are going - it'll be to your boiler (if it's a combi) or your wiring centre if conventional.

    Report back where this is - post a photo ideally.

    Then move the 'earth' wire to a real earth terminal in there as this will protect the cable and sleeve it in green/yellow. Leave the Red where it is, and then move the Black to where the old earth wire used to go. You must then put a RED collar (tape will do) on that black wire to indicate that it becomes 'live' and also do this at the new stat end.

    Oh, there's a good chance you've fried your new digi stat, burned the relay contacts together. But try it first to see if you got away with it.
    dobbie likes this.
  15. dobbie

    dobbie Well-Known Member

    Whoever wired this carried out a dangerous bodge instead of running the correct cable.
    As I said in post 12 the wiring needs reconfigured, if you are not confident with this definately call an Electrician.
    It is a simple job for one and will not take long.
    Sammi likes this.
  16. Sammi

    Sammi Member

    If you had replaced your old stat having forgotten where the wires had previously gone, you may well have placed the bare 'earth' wire on an actual earth terminal in that stat (if there is one?). That would have made the metal parts of that stat go 'live' whenever the stat was turned up.

    Pour yourself a drink - you are still here.
  17. Richard Snow

    Richard Snow New Member

    Luckily I took a photo before removing so wires were returned to original state.........:)

    I will take a picture of the wiring centre.

    The wiring from the centre to the room thermostat must be about 15m (yes!) long as the boiler is in my garage.
    Sammi likes this.
  18. Richard Snow

    Richard Snow New Member

    now it gets interesting!!
    traced the wires to 14,15 &16 which are the new coloured wiring.

    Attached Files:

  19. dobbie

    dobbie Well-Known Member

    I would say that the wire from the stat is terminating into no's 2&3 with the black going into vertical connector strip.

    The whole thing is a mess with green & yellows getting used as live wires.

    It looks like the bare wire from the stat goes into 2 links to 4 and joins the white of the 3 port valve.

    The wire in 3 links to 9 and joins a green & yellow which would be a live from the programmer.

    The best idea is get an Electrician in to talk about putting things right.
    Sammi likes this.
  20. Richard Snow

    Richard Snow New Member

    thank you for helping me out.
    I think while I've got the electrician in I'll get the system updated to wireless.
    Sammi likes this.

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