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Wiring for replacement thermostat

Discussion in 'Electrics UK' started by owe, 17 Mar 2019.

  1. owe

    owe

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    My wireless thermostat has packed up so have purchased a new one (updated model as the previous one is 12 years old). Thought that it would be a simple straight swap over of the receiver unit however the terminals are completely different.

    Old one
    [​IMG]

    I'm assuming that this is the correct wiring for the new one, but no connection for the black? New unit is a Salus RT510
    [​IMG]


    Does it just need a termination block, or does it need a whole new cable running from the boiler?
     
  2. EFLImpudence

    EFLImpudence

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    Do you actually require the NC for your system?

    Either you don't - so you can rewire the thermostat AND THE OTHER END OF THE CABLE so that you do not use the CPC(earth wire) as a live conductor,

    OR

    you do - therefore your new thermostat is not suitable.
     
  3. owe

    owe

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    No idea. Need to figure out if the thermostat is suitable for the job before trying to find someone to fit it... plus only have 14 days to this one back if it isn't
     
  4. Taylortwocities

    Taylortwocities

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    Go to the wiring centre (that means the other end of the cable) and find out where the black cable terminates. If it goes to a valve cable,etc, then you cannot use that stat and you'll need one that has an NC and a NO contact.

    ALSO>>>

    It is not permitted to use the green/yellow conductor for anything other than earth. So (while you are at it) you need to replace the whole cable with a cable (or cables) that provide 4 cores PLUS an earth conductor.
     
  5. owe

    owe

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    been in touch with the seller of the stat and they've informed me "use the terminals NO and COM ON on the back of the stat and take them to LS and LR in the boiler"
    doesn't mean anything to me, but I'm assuming to a spark it would??

    Not sure who did the wiring in the first place, if it was the boiler installer or the sparks who rewired... but will get it looked at
     
  6. EFLImpudence

    EFLImpudence

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    Well, it means something but it is not relevant to your situation as it is not the correct product for your needs.
    You may as well have asked them "How do I wire this light switch?".

    How can you think that your system, which presumably works properly, can be wired differently and still be expected to work properly.


    It doesn't matter who wired it - apart from the fact that they used the CPC as a live conductor because there aren't enough wires.
     
  7. ericmark

    ericmark

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    With the Y plan it is common to use both N/O and N/C terminals with the cylinder thermostat, however unusual to do it with the room thermostat, and the at rest position of the motorised valve is domestic hot water and fully over is central heating only, this is done as in the old days domestic hot water was often thermo syphon and even when not it means if the valve fails to open there is some where for the water to circulate.

    However there is nothing that says it must be done that way around, and most thermostats even cheap ones like the Flomasta 22199SX still have the N/C contact even when there is a switch so the N/O contact can be used for either central heating or cooling. So there must be some systems where the N/C contact is used.

    As to the green/yellow wire used for line, the old 17th Edition, I don't have the 18th Edition forbid the use of a single wire even if sleeved for use for anything else other than earth, however be it an over sight or intentional it was worded so you could sleeve a wire which is part of a cable, so although bad practice to sleeve a wire marked as green/yellow and extremely bad practice to use it without sleeving for anything other than earth, if sleeved it was not against regulations, although it may be now.
     
  8. JohnW2

    JohnW2

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    The 18th has not changed anything. I suspect that what it says was intentional, otherwise they would presumably not have gone to the trouble of explicitly forbidding the over-sleeving of G/Y singles.

    However, I certainly agree that, even in multiple-conductor cables, it is an undesirable practice. Furthermore, in the case of things like thermostats (which usually have only one cable going to them) it is probably usually non-compliant with the regs, which require a CPC to be taken to 'every point' in a circuit. If there is only one cable going to the thermostat (or whatever), and that has no CPC, then the thermostat 'point' will not be getting a CPC from anywhere - hence non-compliant.

    Kind Regards, John
     
  9. stem

    stem

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    Assuming that your particular boiler is a combi boiler (and not a heat only boiler connected to an S-Plan or Y-Plan system) and that it actaully has 'Ls' and 'Lr' terminals. (not all do, 'Ls' and 'Lr' are generally Worcester / Bosch terminal identifiers) then:

    Ls = Live Supply
    Lr = Live Return

    These terminals would indeed normally go to the thermostat 'Com' (Common contact) and 'NO' (Normally Open contact). When the thermostat calls for heat 'Com' and 'NO' are simply an on/off switch that switches the boiler on by effectively joining 'Ls' and 'Lr' together.

    That then leaves the unknown wire in NC. The NC contact (Normally Closed) is very rarely used in the UK, I think I have only seen two installations that have used it out of over possibly hundreds and neither were combi's. So for anyone to advise further, as a minimum, you would need to tell us, as @Taylortwocities has already asked, what the other end of the black wire in NC is actually connected to. But it would also be helpful to know what was on the other end of all the wires. As you will have gleaned from the above posts, the choice of wire colours would have been made at the discretion of the original installer, it is what is connected to each end of each wire that determines what it does.

    As a word of caution. To access the wiring terminals at many boilers, it involves opening up a room sealed compartment. If this is the case with yours, then it should only be done by an RGI, (Gassafe) who can check it is properly sealed up again afterwards.
     
  10. Taylortwocities

    Taylortwocities

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    Really? I have never come across a boiler that has that level of access complexity.
    But it academic, the OP needs to find the wiring centre, not look in the boiler.
     
  11. stem

    stem

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    Seems to apply to most modern combi's, and has been quite a common topic over in the Plumbing and Central heating forum for a few years now, usually pops up when DIYers want to install a smart thermostat themselves.

    A quick search revealed a few threads with examples here, here, here, and here.

    As my background is electrical, I can work out what needs doing electrically from the wiring diagrams, However, as I can’t be sure about the situation with regards to the boiler casing for every make / model of boiler without seeing it physically, hence the comment.
     
    Last edited: 18 Mar 2019
  12. owe

    owe

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    Indeed you are correct it is a Worcester Bosch boiler (Greenstar 30i.)

    I have the wiring diagram for the stat... but it means nothing to me
    [​IMG]

    I had hoped that seeing as it was a Salus RT500 that had stopped working that the newer RT510 would be a straight replacement... chance would be a fine thing. Seems there has been multiple revisions in the last 12 years so the wiring is different.

    So do I need to contact a spark or a RGI to get the replacement fitted (don't have either so will have to take pot luck out of the yellowpages!)? Do they need to access inside the boiler??
     
  13. stem

    stem

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    There is not sufficient information to give a definitive answer over a forum, it really needs someone to physically examine the entire system as it stands to understand what has been done in order to advise with certainty. So in this case it would probably be better to get a professional to install it for you.

    Wiring a room thermostat to a combi, is usually one of the most straight forward things to do. The thermostat is a simple on/off switch that connects to two terminals at the boiler, so the need for having multiple revisions to the wiring seems a bit unusual.

    Apart from the question about the purpose / function of the black wire in NC, as I said before, I assumed that you have a combi boiler based on the instructions you posted earlier that the stat supplier provided, however you haven't actually confirmed that, and as the WB Greenstar is available in both combi and heat only versions.....

    Regarding whether you need to open up a sealed enclosure to access the wiring terminals for your boiler is probably a question for the RGI's over on the plumbing and central heating forum. But I think it would be quite likely you would. If you don't have the combi version of the boiler but have the heat only, it's irrelevant because the thermostat would connect to a motorised valve and not the boiler anyway.
     
  14. Taylortwocities

    Taylortwocities

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    Yes. It seems like only a decade ago that I suggested that the stat cable should be traced, to see if it goes to a wiring centre.

    I don’t think the OP has even done that.
    Let’s try another question to @owe

    Do you have a separate controller where you can set times for heating and hot water?
     
  15. JohnW2

    JohnW2

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    Fair enough, but it seems very odd - I wonder if there is a sensible explanation?

    ... quite apart from the fact that that it will inevitably lead to an appreciable number of instances of room-sealed compartments being opened by non-gas trained people, I would have thought that there were good reasons for not having wiring terminals (or anything more electrical unless absolutely unavoidable) within the room-sealed-compartment.

    Have you any idea of the thinking behind this. It is surely not just a matter of 'ease of manufacture' (without consideration of the risks it introduces) is it?

    Kind Regards, John
     
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