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Electric underfloor heating thermostat wiring - HELP

Discussion in 'Electrics UK' started by greyrider, 2 Feb 2015.

  1. greyrider

    greyrider

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    The old thermostat stopped working so I pulled it, and binned the thing (ooops) instead of keeping it for reference. The wiring was a single black and a twisted pair of reds to two, separate terminals. It's an old system, some 30+ years, on E7 and works fine.
    I took a known working, Honeywell, thermostat from the hallway, but it had three wired terminals.

    Choices are:

    Term 3 20(4) @ 240v
    Term 4 6(4) @ 240v

    N:2
    L:1
    E:4
    Switch:3

    I've tried the black to N and the twisted pair to L.
    The black to 3.
    Split the pair and ran one to L and the other to 3.
    The black to 3 and the pair to 1.

    No joy and our living room is COLD.
    Any help much appreciated.
     
  2. deadshort

    deadshort

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    Hi,

    sorry you're information is unclear. If you post pictures that may help.

    Regards,

    DS
     
  3. greyrider

    greyrider

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    Cables I have, one black and twisted together, two red.

    Label from thermostat case,


    [​IMG]
     
  4. stem

    stem

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    Do you mean

    A. The twisted reds went to two separate terminals, or

    B. the black went to one terminal and the two twisted reds to the second?

    Wiring colours don't confirm anything, it all depends on how the original installer used them. We would need to know what is connected at the other end of each wire.

    The thermostat cover looks to be from a Honeywell T6060, if it is

    N = Neutral
    L = Live
    3 = Switched live for heating load

    With this thermostat, it is possible for it work without the neutral connection, which is used to operate a small internal heater known as an 'accelerator' to improve the accuracy of mechanical thermostats. If so, and there are only two connections, terminals L and 3 would be used.

    However, it is important to check that you do not have a neutral wire to the thermostat, if you inadvertently wire it so that they are connected together when the thermostat operates, you will create a short circuit, blow a fuse and possibly cause damage.
     
  5. deadshort

    deadshort

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    Hi,

    L = two reds plus a loop wire to terminal 3 (it must be minimum 1.5mm)
    terminal 4 =black

    Your thermostat may not be compatible if it requires a neutral.

    DS
     
  6. winston1

    winston1

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    Perhaps your old thermostat was not faulty and the fault is somewhere else.
     
  7. greyrider

    greyrider

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    B. two reds twisted together went to one terminal and the single black to the other.

    I'll be out from 12.30 'till around the same time tomorrow so apologise in advance if I can't reply - it's not that I'm ignoring anyone's help.

    Thanks all.
     
  8. stem

    stem

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    If you are sure that there were only two connections made at the original thermostat, then they will be the live and a switched live. If so, terminals 1 and 3 on the thermostat shown in your photograph are the ones required.

    It doesn't really matter which way around they go, but we'll assume the two reds are the live (1) and the black the switched live (3)

    However, as you say you have tried this combination and it didn't work, it would suggest that something else may be at fault, which may require test equipment to check. Does the thermostat still click as you turn it up & down? Has a fuse blown or MCB tripped? Is the timeswitch on?
     
  9. greyrider

    greyrider

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    Yep, thermostat is a proven working item and dutifully makes "click, click" sound when turned.
    Thanks all for the help, phoned home and they've gone for 1 & 3. Find out tonight/tomorrow if it's a goer.
     
  10. greyrider

    greyrider

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    Well, I doff my, virtual, cap to you and raise a, virtual pint, it only works! Reports from home are toasty toes this morning. A thousand thanks for all answers and help.
     
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