2 or 3 wire thermostat ?

Discussion in 'Plumbers' Talk' started by peajay, Aug 18, 2007.

  1. peajay

    peajay Member

    I have purchased a new programmable room thermostat which has only 2 wires to connect, but the wiring diagram in my boiler manual show 3 wires to connect. Can I get a 3 wire type ? and what does the third wire do ? The boiler is a fairly recent Elm Leblanc.
  2. devil's advocate

    devil's advocate New Member

    Hi peajay.

    Is your new stat battery operated? If so, that's why it doesn't need the third - neutral - connection.

    Traditional mechanical stats required a 'neutral' connection to power a heating - 'accelerating' - resistor inside which helped to compensate for the sluggish operation of the mechanical (bimetallic strip) heat sensor.

    Electronic prog stats use electronic sensors which don't have this 'lag'.

    You can get an electronic stat which requires a neutral, but this is so that it becomes mains powered rather than using batteries, which is a pretty good thing. It's called 'Heatmiser' - try a Google.

    Basically, there are 2 main connections in a stat - a live supply, and the switched live (ie: switched by the stat) which then goes to the boiler.

    Usually they are connected to: live supply goes to 'common' (or 'C' or 'com') and the switched live leaves from 'NO' (normally open).

    Make absolutely certain you know what you are doing before wiring up!

    But, your existing stat should be ok.
  3. peajay

    peajay Member

    Yes it is a battery powered stat. There are two different connector blocks on the boiler for thermostat connection, one has 9 connections and shows the stat requiring three wires and a second block which says for 230\V connection and requires two wires on terminals Ls & Lr which currently have a jumper. I think it is the Ls & Lr connections which take the stat wires ?
  4. devil's advocate

    devil's advocate New Member

    Hi peajay.

    I THINK you are right, but I don't know for certain.

    If you read through your boiler's manual, you should find reference - in the stat section - to wiring it to a stat with 'volt-free' contacts ; that's what your stat is - a pair of switch contacts which have no voltage on them (unlike the old mechanical stats which have 240V going through them).

    I'd really need to see the actual instructions before being sure - perhaps someone else on here knows?
  5. Spyder

    Spyder New Member

    Some older thermostats do indeed have three connections: 2 for switching and one (neutral feed)to power a heating resistor to act as an "anticipator".

    However if you are using two 2-port motorised valves, then you need the third wire to isolate the valves from each other. Have a look here for details of the different circuits.


    If your boiler manual shows a neutral feed to the thermostat then it is probably ok to leave this wire unconnected.
  6. peajay

    peajay Member

    I connected as I thought and the thermostat is working fine. Thanks to everyone for the advice
  7. devil's advocate

    devil's advocate New Member

    Cool - thanks for letting us know.

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