No hot water Worcester 24i junior

Discussion in 'Plumbers' Talk' started by jitmonlee, Jan 6, 2021.

  1. jitmonlee

    jitmonlee New Member

    Hi all,

    I've looked at previous threads and can't find the answer. My central heating seems to be ok but when I try hot water the burner goes out. I have opened my boiler to take a look but I'm not entirely certain what's going on but have figured a few things out.

    Heres what I know.
    - My central heating works fine (seemingly).
    - If I open the hot water tap, the flue fan starts spinning and the boiler fires up - the flue fan then turns off, the burner stays on for a while and the flames go a little wild and then it cuts off (presumably a safety feature). Its probably on for 3-4 seconds after the flue fan shuts off.

    I feel like the flames go wild because the exhaust has effectively shut off as the fan stops spinning. Could this be a faulty PCB not continuing to spin the flue fan? Or could something else be going on? Perhaps I just need to clean the flue?


    PS - before I figured the above out, I called a plumber (Homecure) and they wasted my time, he basically said he doesn't know what the issue is then said maybe its the water pressure sensor - he also charged me more than the fixed callout price and I paid even though I really shouldn't have. They really messed me about. He was pushing for an entire new boiler replacement the moment he walked into my home before even looking at the boiler so I have zero trust in him and his diagnosis.

    Thanks in advance!
     
  2. Mike83

    Mike83 Screwfix Select

    Could be a pcb fault.
    Is the air pressure switch in ok condition or is it melted a little bit?
     
  3. Mike83

    Mike83 Screwfix Select

    I’m guessing this is just a 24 junior and not a greenstar junior.
     
  4. jitmonlee

    jitmonlee New Member

    thanks @Mike83

    Air pressure switch looks fine. Used my multimeter to check it’s switching correctly and it is as far as I can tell.

    do you think I should just go ahead and replace the pcb or is there more discovery to do?
     
  5. Mike83

    Mike83 Screwfix Select

    If the burner stays on after the fan turns off this would usually indicate a faulty pcb.
    Under no circumstances should the flame ignite if the fan isn’t spinning.
    As the burner stays on it can melt the air pressure switch.
     
    jitmonlee likes this.
  6. jitmonlee

    jitmonlee New Member

    thanks! I’ll give this a go :)
     
  7. Shanno Grp Ltd

    Shanno Grp Ltd New Member

    Hello Jitmonlee

    We can help you with this fault, however if your not a gas safe engineer, I strongly advise do not attempt repairs your self. You may well be able to diagnose the fault, but you cant confirm the boiler is creating CO due to faulty components or leaking gas as you wont have the equipment to do so. Please be careful.

    THE FAULT
    The 24i junior runs the pump in heating mode. In Hot water mode the pump stops and only the fan runs.
    If the burner extinguishes in hot water mode, several faults may happen. as follows

    The boiler heat exchanger is blocked causing flame lift. Flame lift means the electrode cannot sense the burner and it shuts off.

    The air pressure switch may seem ok, however if the diaphragm inside is split the air pressure switch drops out causing boiler lockout. You cannot test this as APS's work with very low pressure and you need a tool to test this.

    The fans venturi is blocked causing the air pressure switch to drop out.

    The silicone tubes off the fan to the APS ( air pressure switch ) are blocked, kinked, have a split or melted.

    PCB RELAY is defective causing false reading of the APS

    Finally the boiler is overheating due to scale build.

    Hope this helps

    Kind regards
    Shanno Grp Ltd
     
  8. jitmonlee

    jitmonlee New Member

    Update in case anyone else finds this post, I replaced the PCB and so far it’s working great. I’ll post again if I have any difficulties in the near future but it seems like it has solved the problem for now. Thanks again @Mike83 !
     
  9. martinwinlow

    martinwinlow New Member

    @Shanno Grp Ltd - (Warnings re GasSafe noted)... I have the identical fault also on a Worcester Junior 24i (not Greenstar) apparently installed about 10 years ago. We are a small, remote island location (hence no GS engineer!) and therefore the combi is probably ageing more quickly than normal in our corrosive marine environment (everything else does). Anyway, with heating only on, the boiler is constantly modulating, ie firing up for a bit and then almost going out (looking at the flame through the peephole with cover off) - I don't know if that is what it should do - but it initially seemed to me a bit like the gas supply (propane) was not good enough but it is attached to a full cylinder via a Continental Auto-change-over regulator) and gas is getting to the boiler (cracked open the supply compression fitting and gas comes out - vented room thoroughly before recommencing...!). With heating on and boiler firing as above, as soon as a hot tap is opened, the boiler gives up and the status light does the 4 flashes/S thing. I have the service manual and have performed Test E and get as far as checking for 230V on connector ST1 (fan connector - L & R terminals on PCB) and, after reseting the boiler, initially (whilst the boiler is thinking about it) I get about 2.0VAC which drops a bit when the boiler tries to fire up. The status light then reverts to 4FPS. Test E says the lack of 230V at ST1 means new PCB required.

    Can you confirm this sounds correct and that it can't be something more simple? What does lack of 230VAC at ST1 imply is wrong with the PCB? Could it be that there is a PCB-based relay that supplies the fan with 230VAC and this has failed? Is the centre connection of ST1 a feedback connection... and if the fan does not run (due to no 230V) the boiler goes into lockout? If so, is it feasible to simply replace the relay rather than the whole PCB?

    Thanks in anticipation.

    MW
    Isle of Colonsay
     
  10. martinwinlow

    martinwinlow New Member

    Replacing the PCB wth a reconditioned off eBay one for £30 fixed my fault although think there is something else going on as well as the user of the boiler report occasional locking out. Time will tell...
     

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