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Dodgy central heating wiring noted whilst replacing stat!

Discussion in 'Plumbing and Central Heating' started by Rob3D, 15 Jan 2015.

  1. Rob3D

    Rob3D

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    Location:
    Cheshire
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    United Kingdom
    Hi guys. I've been assessing my CH recently since the pump ground to a halt for the 3rd time and the boiler repeatedly overheated and cut out. I've temporarily resolved this issues but whilst investigating I found the wiring of the ancient electronic CH controller, wall and tank stats, pump and 3 way valve actuator is a mess to say the least.

    It all works but probably not as it should. I've just replaced the tank stat with a Danfoss ATC, which seems simple enough, but my old Honeywell stat was using 3 wires rather than 2, I've had to copy this config with the new stat or else when I turn the tank stat down the CH cuts out, I think wire #3 ( I can't remember what stat connection number at this precise moment) is feeding power back into the system on satisfying the heat requirement. If I don't use this 3rd wire the system stops when the tank heat is satisfied regardless of the wall stat setting. With the 3rd wire connected (to the grey wire on the Danfoss valve actuator I think), I can turn the stat down, the system stops momentarily, the valve adjusts itself and clicks the system back on again.

    It seems to me that something is wired in series where it should be in parallel or vice versa. The CH should only switch off when both wall and tank stat are satisfied.

    First question, does this sound like incorrect operation?
    Second Q, should the tank stat ever use more than 2 connections?
    Third, it works and isn't causing a massive problem, should I do something about it?
    Fourth, is there any potential problem/danger with this setup?

    Note that I had to replace the mid position valve actuator, perhaps the old one was damaged due to incorrect wiring?

    As an aside, my tip for starting a seized Grundfos 15-60 is to take the screw cap off the front, get a hammer drill with a phillips screwdriver bit installed and give it some stick!! Worked a treat for me :D although the pump is knackered now :( . Standard disclaimers apply ;)

    Cheers
    3D!
     
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  3. TCCHeating

    TCCHeating

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    You have a y plan. So yes! The cylinder stat needs 3 wires. Com, hw satisfied and hw unsatisfied.

    Sounds like the system is working as it should.
     
  4. Rob3D

    Rob3D

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    LOL - ok, thanks for your help, I'll leave it alone then. Yes, I had come to the conclusion it was a Y-plan.

    Just need to replace the pump now :((

    Cheers!
     
  5. DIYnot Local

    DIYnot Local

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