Replace Horstmann DRT2 Programmable stat with C Stat 17B

Discussion in 'Plumbers' Talk' started by CMBORO, Nov 15, 2017.

  1. CMBORO

    CMBORO New Member

    I am looking to replace my old Horstmann DRT2 thermostat with the new Horstmann C Stat 17B.

    On opening up the DRT2 I can see 2No. cables terminated as below.
    The C Stat 17b has a terminal strip showing 1-4 and earth as below

    Can anyone advise into which terminals the 2 cables should be connected ?

    upload_2017-11-15_9-14-28.png upload_2017-11-15_9-20-7.png
     
  2. If you look at your DR2, you'll see the two terminals connected are 'COM' and 'NO'. COM = common and NO = Normally Open.

    Basically, the DR2 is a 'switch', and when it 'calls for heat', it closes that switch and joins these two terminals together - ie the Normally Open connection becomes 'closed'.

    On you new stat, I would hope that the 4 terminals are also labelled in a similar way somewhere, inside the removed cover or somewhere? But, in case they are not, Terminal 1 is your 'COM' and Terminal 4 is your 'NO'.

    So, connect these two wires to 1 and 4. It doesn't matter which way around you do this - it'll work either way - but 'convention' would have it that the RED (presumably 'live') goes to COM (Terminal 1) and the BLACK (presumably 'switched live) goes to NO or Term 4 in this case.

    I don;t know if these two wires ARE 'live' on your system or not, but if they are then you should really wrap a collar of RED tape around the end of the black insulation too, to indicate it DOES become live whenever the 'stat calls for heat.

    Do you have a mains tester screwdriver? Does it light up on either of these wires when the power is on to the boiler?

    ANYHOO - don't forget to FULLY turn off the power before you start fiddling, or else you might not be able to complete the job...
     
  3. CMBORO

    CMBORO New Member


    Thanks very much for the very prompt reply.

    I can confirm that the terminals 1-4 do not make any reference to COM or NO inside the unit, which is where the confusion lies.
    But I will connect these to 1 & 4 as suggested

    I don't believe the cables are live as this is, I believe, a fully battery operated stat, but I could be wrong.

    I will however as suggested fully turn off the power just in case

    Thanks again
     
  4. Isn't there a wiring label on the removed front cover inside?

    Anyhoo, I did look for installation instructions for that model, and I am confident that these are the correct two terminals. (They are 'com' and 'call for heat')


    The fact that the room stat is battery operated has NO bearing on whether these wires carry mains voltages - so DISCONNECT!

    It all comes down to the boiler wiring - some will have 'volt-free' control (actually, it does have a voltage, but a very small one, probably around 5V) but others WILL use mains voltages for their control.
     
  5. CMBORO

    CMBORO New Member


    There is nothing on the cover itself, however in the manual the wiring diagram indicates the following, which I believe ties in exactly with what you have suggested

    upload_2017-11-15_10-13-40.png
     
    Deleted member 33931 likes this.
  6. Pollowick

    Pollowick Screwfix Select

    As the new stat is battery operated, then, with a meter, it should be possible to confirm which terminals are the Com, NO, and NC by varying the set point and waiting for the switch over
     

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