Honeywell Room Thermostat 2 or 3 wires help

Discussion in 'Electricians' Talk' started by Vinnyc, Apr 23, 2019.

  1. Vinnyc

    Vinnyc New Member


    Could really do with some advice and help with this one.

    I recently moved into a new house and it has very old room thermostat and timer programmer. I need to replace the thermostat which is a old Honeywell thermostat that has 3 wires into it.
    I want to replace it with a new Honeywell DT90E but my current wiring layout is confusing me somewhat, according to the diagram on the plate of my current thermostat the yellow wire is live in? I think the red might be live out and the blue a neutral?
    Ran a voltmeter on the cables and they all register a live voltage when the heating is on.
    The heating/water system has a Randall 4033 switch/timer system and the boiler is a British Gas 330 (Glowworm)

    I honestly don't know where to begin on this one.

    I have posted photos of the thermostat and the wiring diagram for the new DT90E

    Any help would be greatly appreciated.

    Many thanks

    Attached Files:

  2. Bazza

    Bazza Screwfix Select

    from where to where did you measure voltage?

    Firstly, measure from earth to the three wires. There should be only one that has a 230v AC voltage. That's your permanent live. (1 on the honeywell). Note, make sure that heating is selected to ON at the programmer or you will get zip.

    Now you know that, you will probably find that there is 230VAC between the (known) live and one of the other wires. Probably the blue. That's the neutral (2 on the honeywell). The third cable will be call for heat and goes to 3 on the Honeywell.

    PS. Look inside the lid of the old stat. There's usually a diagram of the connections there!
  3. Vinnyc

    Vinnyc New Member

    Hi thanks for the reply

    I tested them to ground and the yellow wire (1) is live, I have attached a photo of the original wiring diagram that is on the back of the old thermo cover.
    The top blue wire (2) reads 50v when the switch is off so does that make this the call for heat wire?

    Many thanks
  4. Bazza

    Bazza Screwfix Select

    No pic vinny
  5. Mike83

    Mike83 Screwfix Select

    What about the red wire?
  6. Mike83

    Mike83 Screwfix Select

    As the new stat is powered by batteries it just seems that you’ll be using red and yellow.
    Yellow A
    Red B.

    Just join the red and yellow together and when the boiler fires this will confirm these 2 wires are the ones you need.
  7. Vinnyc

    Vinnyc New Member

  8. Bazza

    Bazza Screwfix Select

    Ah. I see what is going on now. Mike83 is correct.
    Make sure you put the unused blue wire in a but of insulated terminal block.
  9. Vinnyc

    Vinnyc New Member

    Hi thanks for the help guys.

    What is the top blue wire for and how come its not needed?

    Many thanks
  10. Mike83

    Mike83 Screwfix Select

    The new stat doesn’t require a Neutral.
    It’s that simple.

    Some stats require it, others don’t.

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