1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Boiler only fires for CH

Discussion in 'Plumbing and Central Heating' started by timbim, 5 Jul 2013.

  1. timbim

    timbim

    Joined:
    27 Dec 2010
    Messages:
    95
    Thanks Received:
    7
    Location:
    Sussex
    Country:
    United Kingdom
    Odd issue with friend's hot water system. He's just moved into the house, so no idea how long it's been like this. System is a standard closed loop combined HW and CH with hot cylinder. Controller is a Honeywell ST6400C.

    Basically, the boiler won't fire at all on the HW cycle of the controller, only on CH or CH and HW. On CH and HW, the tank heating loop is active, so the tank will heat. I've fiddled with the cylinder stat, it's currently set at 60 but adjusting makes no difference to anything.

    Any ideas as to what's wrong or the next thing to test?

    Thanks.
     
  2. Sponsored Links
  3. 45yearsagasman

    45yearsagasman

    Joined:
    14 Mar 2010
    Messages:
    6,094
    Thanks Received:
    1,093
    Location:
    Norfolk
    Country:
    United Kingdom

    Got any thing like these near the cylinder?
     
  4. timbim

    timbim

    Joined:
    27 Dec 2010
    Messages:
    95
    Thanks Received:
    7
    Location:
    Sussex
    Country:
    United Kingdom
    Yep, two two port motorised valves on the output side of the pump. One into CH circuit and one into cylinder heating loop. Both Honeywell brand.

    The cylinder one can't have failed closed as the tank heats when the CH is on. Is there any way of determining the valve position from outside the casing?
     
  5. 45yearsagasman

    45yearsagasman

    Joined:
    14 Mar 2010
    Messages:
    6,094
    Thanks Received:
    1,093
    Location:
    Norfolk
    Country:
    United Kingdom
    Not possible without removing the heads. If yours have a raised dimple on the grey case then it can be removed from the body. You can then check the spindles are free to move and you can also see the motor working.
    I suspect a failed motor and/or a seized valve. Work with the power off while removing covers etc.
     
  6. timbim

    timbim

    Joined:
    27 Dec 2010
    Messages:
    95
    Thanks Received:
    7
    Location:
    Sussex
    Country:
    United Kingdom
    What leads you to that conclusion, and how is it consistent with the cylinder heating when the boiler is on?
     
  7. 45yearsagasman

    45yearsagasman

    Joined:
    14 Mar 2010
    Messages:
    6,094
    Thanks Received:
    1,093
    Location:
    Norfolk
    Country:
    United Kingdom
    Why not do as I suggest first? If I were on site, I would do those checks first given the symptoms.
     
  8. oilboffin

    oilboffin

    Joined:
    26 Apr 2011
    Messages:
    1,665
    Thanks Received:
    247
    Location:
    isle of Lewis
    Country:
    United Kingdom
    Lock the heating valve open see if the boiler comes on that's the best way it's what I would do
     
  9. DIYnot Local

    DIYnot Local

    Joined:
    3 Sep 2019
    Country:
    United Kingdom

    If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

    Select the supplier or trade you require, enter your location to begin your search.


    Are you a trade or supplier? You can create your listing free at DIYnot Local

     
  10. Sponsored Links
Loading...

Share This Page