Horstmann 423 Diamond Replacement timer

Discussion in 'Plumbing and Central Heating' started by Pmbob, 27 Nov 2021.

  1. Pmbob

    Pmbob

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    Hi

    I need to change the ancient horstmann timer (h423d) as its constant ticking is making me crazy. I'm looking to replace it with a Drayton LP722 (Mk4) Programmer.

    What I want the new timer to do is programme when to heat the water on its own and when to heat the radiators (I understand that to do this it will heat the water also)

    It is a gravity fed Central heating system (boiler - water tank).

    I understand that I'll need to replace the back plate and the wiring to it is different.

    I've managed to find a wiring diagram for the H423. From what i can make out from it - Currently there are wires going to L N E (mains 3a) one wire from no2 (to burner?) , one wire from no4 (to thermostat?), and another going to same E (I'll try to attach files)

    What I want to get right is the wiring on the new back plate. Obviously L N E are the same, but does current no 2 got to new no 3, and current no 4 go to new no 4???

    I also understand there's a jumper on the rear of the new timer that needs to be moved.

    Any help will be much appreciated.

    Thanks
     

    Attached Files:

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  3. ianmcd

    ianmcd

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    post a pic of your HW cylinder, you like many are confused that a "tank fed system" as you call it is an open vent system not a gravity system, but does sound like you have a GRAVITY HW system, they are not the same
     
  4. Pmbob

    Pmbob

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    Here it is.
     

    Attached Files:

  5. ianmcd

    ianmcd

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    You dont have a gravity HW system , you have a fully pumped Y plan system
     
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  7. Pmbob

    Pmbob

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    Okie dokie.

    How will that affect the installation of the new programmer??
     
  8. Pmbob

    Pmbob

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    I've attached the installation instructions for the Drayton LP722. I see I will need to ensure the programmer is set to 'pump'
     

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  9. ianmcd

    ianmcd

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    yep simple as that set it to fully pumped, you have a far better installation than a gravity HW one, just make sure the HW off wire is correct
     
  10. Pmbob

    Pmbob

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    HW off is no1 on the new back plate right?, so where does that wire come from?

    Does current no 2 go to new no 3, and current no 4 go to new no 4 on the back plate or is that wrong? Is there additional wiring to be done with the back plate or the programmer?

    Thx btw for your help and patience with me
     
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