Posting a message to the forum will remove the above advertisement
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

Discuss S plan probable wiring fault? in the Central Heating Forum area at

  1. Best

    Best Trusted Plumber

    I replaced a programmer on an S plan system in a small bungalow and when checking zone valves operation, I discovered an existing problem.
    There is the usual 2 port MV for radiators, (controlled with hall stat), plus a second 2 port MV for hot cylinder, (controlled with cylinder stat).

    Programmer on hot water only setting, it works fine and cyl stat turns it on and off.
    Problem is on radiators only setting, the rads work but power is also then to the cylinder MV. Cylinder stat then becomes nonfunctional.

    Customer had said her hot water was always very hot.
    Wiring fault I guess in wiring box?
    Work is all very amateurish looking.
    I can’t easily open the wiring box due to some idiot piping across the cover.
  2. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    Sometimes better all round just to start again
    • Like Like x 4
    • Agree Agree x 4
  3. Millsy 82

    Millsy 82 Plumber GSR Top Contributor!!

    Not on gravity on programmer? Whip the programmer off and put a link inbetween the live and heating and see if you still get power to hot water side, if not I would be looking faulty programmer.
    • Useful Useful x 2
    • Like Like x 1
  4. Last Plumber

    Last Plumber Trusted Plumber GSR

    Have you tried disconnecting the brown to both MV's and testing switched live to each in turn from the Programmer?
    • Like Like x 2
  5. Best

    Best Trusted Plumber

    Am going to have to rethink this.
    I completely forgot that there would be a gravity setting in the programmer! :oops:
    So will check that first.

    The cylinder would have been very hot previously in retrospect due to the cylinder stat not strapped to cylinder. :)
    Although I suspect it also was constantly heating when rads were on with old programmer
    • Like Like x 2
  6. CBW1982

    CBW1982 GSR Top Contributor!!

    Ah the joys of something clicking into place, I love that feeling, although not after making the air blue and tearing my hair (or lack of ) out :D
    • Funny Funny x 1
  7. Chuck

    Chuck Well-Known Member

    A common 'gotcha', you're in good company.

    If that is the problem and you convert to independent CH and DHW, don't forget to check whether the modified system needs an ABV fitted/adjusted because the cylinder loop will no longer be providing a bypass.
    • Like Like x 2
  8. Best

    Best Trusted Plumber

    That is what is great about a second opinion from here, - it means I approach it again differently.
    You know what it is like on a job, - I had half an hour to check it all and decided not to get too involved in the wiring.
    Job didn’t give me much confidence when you see the standard of work, - cyl stat not strapped to cylinder and just lying on floor and a copper pipe fitted in front of the wiring box cover. :eek:
    • Agree Agree x 1
  9. Best

    Best Trusted Plumber

    That’s a good point!
    I think there is no bypass valve anywhere. Doubt if the installers knew what it is.
    • Like Like x 1
  10. Coby kenny AGUILAR

    Coby kenny AGUILAR Member

    Two ports have a permanent live at all times which is usually a grey wire

    That cyl start won't work because is isn't receiving power from the programmer

    Time (programmer) - temperature (stat) - control (MV)

    Everything has a sequence
    • Useful Useful x 1
  11. Best

    Best Trusted Plumber

    Yes, I knew that. Had no time to check the wiring, but will do.
    Danfoss MVs, which can be very unreliable, but both are working ok.
    • Like Like x 2
  12. Last Plumber

    Last Plumber Trusted Plumber GSR

    Probably because they're pimple sized dip switches that are almost invisible to the Human eye. Let's face it, most systems have been fully pumped for a good while now and remembering to check that is not always in the front of our minds.

    Remember the Landis & Gyr Clock? the one where you slid the selector to the far end and twist locked the CH and DHW together.
    No mistake there! Plus it was probably 50/50 which way it wanted to be, if not more toward 10 programs?
    • Like Like x 2
  13. Ready Steady Gas

    Ready Steady Gas New Member

    Just a thought,

    Check the orange cable from the CH MV has not been mistakenly linked to the grey HW MV cable in any way. If that makes sense?

    That would also be a possible cause of the fault you are describing.
    • Useful Useful x 2
Similar Threads - plan probable wiring Forum Date
Plant room insulation people Commercial Plumbing Forum Friday at 9:11 PM
System explanation from pipe work Central Heating Forum Apr 11, 2019