Main Combi 24he boiler heating won't turn off!

Discussion in 'Plumbers' Talk' started by Robdob, Aug 2, 2015.

  1. Robdob

    Robdob New Member

    Hi! Main 24he combi boiler. When the timer switches the boiler on for heating it won't turn off when the timer tries to turn it off. I have an external timer so I disconnected it, problem still there and I tested the timer with a multimeter and it was fine.
    So problem must be in the boiler somewhere? No loss of pressure, boiler heats up rads perfectly ok and internal thermostat works fine as it turns the boiler on and off when the circuit temperature is high enough, but the heating stays on and the pump runs.
    As far as I can tell the boiler sends a signal for the pump to start running, this in turn operates a flow switch which then fires the boiler up after the flue pressure is tested. So am I right in thinking that it is the signal to the pump in the first stage that is staying in the "on" setting? If it is that I am assuming it's a new PCB which is needed but I don't want to spend £100 on a part of it's something cheaper. Could it be the pump itself?
    I am good with electrical systems so can test stuff myself.
     
  2. Do you have any zone valves fitted on this system?
     
  3. Robdob

    Robdob New Member

    No, no zones at all. Basic 8 rad closed system.
     
  4. tom.plum

    tom.plum Screwfix Select

    could well be the pcb, could also be the microswitch on the heating side of the diverter, I'd get one of those first, they're about £8,
     
  5. Is that how these boilers work, Mr Plum - the programmer sends a signal to the boiler, the boiler turns on the pump first and then a flow switch fires up the boiler? I guess that makes good sense - the pump will have an over-run?
     
  6. Robdob

    Robdob New Member

    Can I test the current microswitch if I take it out? I assume so.
     
  7. No reason why not. But also check to see that's it's akshully being physically moved by the flow mech.
     
  8. tom.plum

    tom.plum Screwfix Select

    on second thoughts, if it was the microswitch coming in, it would soon shut down, the pump activeas the microswith and if the pumps not going, the boiler will soon shut off,,, the pcb's looking like the fav, but hey, its your £115 :oops:
     
  9. Glad its Friday

    Glad its Friday Active Member

    To confirm the pcb, put your meter across the board. If there is no signal telling the boiler to fire then fair bet it will be the board.
     
  10. Unfortunately what the OP seems to have is a PCB with a permanently 'on' signal...

    If you need to replace the PCB, you can very likely get a reconditioned one for around the £50 mark. With warranty.
     
  11. Robdob

    Robdob New Member

    Thanks all, I think I'll be able to get a brand new PCB for around £95 - need it in the next couple of days so I'll buy one locally.
     

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