HW hot working, CH is!

Discussion in 'Plumbers' Talk' started by walton909, Feb 16, 2020.

  1. walton909

    walton909 New Member

    Hello all,

    My setup:

    I have a gravity fed, S-plan system with an Vaillant egotech boiler (model unknown, I can dig around if needed). A year or so ago I installed a nest thermostat. Everything has been working fine until a couple of days ago.

    It is _possible_ that the problem started (but not sure) when I had to turn off the CH gravity feed so I could drain upstairs rads and replace a valve. Not knowing the system, I ended up turning off HW/CW gravity feed too. But obviously I turned that all back on. I added some inhibitor into the CH tank too. Unsure how that could be related... :)

    Problem:

    When the timer calls for HW, the HW zone valve opens, but the boiler does NOT turn on (or sometimes does for only a short amount of time).
    When the timer calls for CH, the CH zone value opens, and the boiler DOES turn on.

    Currently, to get HW, I make sure both HW and CH are calling for heat (as the boiler will stay on to satisfy the CH call) ... not an ideal solution. I'm not sure how to diagnose the problem - any ideas would be greatly appreciated.
     
  2. Mike83

    Mike83 Screwfix Select

    It sounds like the HW zone valve isn’t making the internal microswitch when opening.
    This is easy to test with a multimeter or similar.
     
  3. PandA3

    PandA3 Active Member

    yep, zone valve microswitch. The colour of the return cable for boiler demand is usually orange. Check you get voltage on the orange when valve opens.
     
  4. walton909

    walton909 New Member

    Thanks guys, I'll have a poke around :)
     
  5. walton909

    walton909 New Member

    @Mike83, @PandA3, so I measured the boiler demand voltage when CH was being called, and when HW was being called. Results were:

    CH: 240v
    HW: 145-150v

    This is all new to me, but going off your earlier posts and the difference in voltages, I'm guessing your original theory is correct...that the HW zone valve has gone? If so, I'm guessing it's easier to replace the whole thing rather than repair?

    Thanks again!
     
    Last edited: Feb 16, 2020
  6. Mike83

    Mike83 Screwfix Select

    A picture or name of the valve will help.
    Some valves are a complete unit but others the actuator can be replaced.
     
  7. walton909

    walton909 New Member

    Is it "normal" for a valve to break like this? E.g the voltage to be boiler from the valve is present, but not 240v as you'd expect. Id have thought it would have been all or nothing?

    Anyway, as requested here are images of the zone valve.

    20200216_183122.jpg 20200216_183325.jpg

    Thanks again
     
  8. Mike83

    Mike83 Screwfix Select

    Normal. No.
    But I have seen a reduced voltage from the valves switch numerous times.
    As you said you would expect all or nothing but it does happen.
     
  9. PandA3

    PandA3 Active Member

    I would say Yes, it is common for the microswitch to fail. The voltage you are measuring is probably just induced voltage from the system wiring being in close proximity, and intertwined. The voltage will have no current and therefore does not have any power behind it. (Not recommending, but If you Earthed it, then it would reduce to zero volts without any adverse affects) it's enough for your multimeter to pick up a reading.
    If you have tested for 230v going IN to the microswitch and tested that 230v Doesn't come out on the Orange when it is driven to it's ON position, then it's pretty much conclusive that the valve switch is knackered. To prove even further, You could bridge the microswitch (probably brown and orange) and see that the system works and boiler fires.
     
  10. walton909

    walton909 New Member

    Has anyone any experience knowing if it is possible to swap the heads on this model of valve? A little bit of googling proved inconclusive.
     
  11. PandA3

    PandA3 Active Member

    I believe Boss Therm are same as Siemens/Honeywell (BSS Home brand) so you just need to find the equivalent part number. It is probably Siemens CZV222 rather than BZV222.
    Take the lid off and have a look if its screwed to base. Also if you take lid off you might see why the microswitch isn't getting pressed by the moving Cam and be able to fix it.
    Here's a probably replacement but check it first...
    https://www.heatingcontrolsonline.co.uk/siemens-dva25-czv222-actuator-p-308.html
     
  12. PandA3

    PandA3 Active Member

    Put czv222 in an eBay search
    there are a few for less than £20.... and the box says 'demountable' so happy days!
     
  13. walton909

    walton909 New Member

    Thanks for looking. I'll poke around the old one and see what I find. For £20 it's worth a punt! :)
     
  14. walton909

    walton909 New Member

    @Mike83 @PandA3

    Last update - Small delay as during all this the bloody shower pump died and needed replacing! Typical. But today, finally, the replacement head came. I just finished installing and HW is now working independent of CH.

    Thanks for taking the time to school me through this :)
     
    WillyEckerslike likes this.
  15. PandA3

    PandA3 Active Member

    Thanks for taking the time to school me through this :)[/QUOTE]

    No prob. Glad you managed to sort yourself, and took the time to leave a concluding comment.
     

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