Combi boiler short cycling.

Discussion in 'Plumbers' Talk' started by Johnreb, Mar 7, 2020.

  1. Johnreb

    Johnreb New Member

    Combi boiler has been working perfectly since installation over 4 years ago. About 3 weeks ago there was a problem with boiler failing to ignite and a gas safe engineer was called out to fix the problem. Parts were replaced but eventually it turned out the gas meter was faulty and the regulator had to be replaced.
    Since then the boiler fires up OK and the hot water and radiators get very hot. There is no problem with the hot water but on the C.H. the boiler runs correctly up to the boiler thermostat setting range then starts short cycling excessively. The burner ignites ok, dies back a bit on the fan, then there is a click and the burner re-ignites. This is constant with re-ignition every 8 seconds. Only after the temperature has gradually dropped below the thermostat range will it run OK for a few minutes then the short cycling will start up again.
    Does anyone know what is most likely causing this problem.
     
  2. The Teach

    The Teach Screwfix Select

    have you got a boxt boiler'or similar tech for repair
     
  3. Johnreb

    Johnreb New Member

    This present fault only happened after the attempts to repair the previous problem. That problem has now been sorted out and I do have a suspicion that the engineer has caused this new fault by all the probing about in the PCB and various sensors within the boiler. He is definately gas safe registered and I have since contacted him about the short cycling issue but he has suggested monitoring it for a period to see if it will correct itself.
     
  4. sam spade

    sam spade Active Member

    If the regulator has been replaced on the gas meter, the gas pressure may have changed. So the gas valve in the boiler should have been adjusted to make sure it meets boiler manufacturer's specification. Did the gas safe engineer do this?
     
    Johnreb likes this.
  5. Johnreb

    Johnreb New Member

    That is a very good point. The GSE did make some adjustment to the gas valve at maximum rate but not at minimum rate. I actually asked him about that at the time and he replied that once the maximum rate has been adjusted the minimum rate will sort itself out. I have checked the manufacturers specification and they say both the maximum and minimum rates must be set. The max being 8.7% and the min 8.4%. That could also be why it runs perfectly at high rate (Hot water) but short cycles so much on minimum rate (CH)
     
  6. Johnreb

    Johnreb New Member

    The boiler has been very satisfactory installed and running perfectly for years.
    One can only engage a gas safe registered engineer to sort out the problem and hope he will be able to get it fixed.
     
  7. sam spade

    sam spade Active Member

    If the minimum can sort itself out, it makes one wonder why they bother to fit an adjusrment screw specifically for setting the minimum. The max and min adjusters do interact so the GSE may have to spend a little time getting it right, Your GSE is lazy!
     
    Johnreb likes this.
  8. Johnreb

    Johnreb New Member

    You are correct and it looks like I shall have to engage a different GSE to come in and adjust the valve properly. Thanks for your help.
     
  9. The Teach

    The Teach Screwfix Select

    Tbh,some central systems are absolute pants and should be exchanged. There is a pathway to make it a little better but its getting a gas heating technician involved who can set the boiler pcb-etc to supply heat to match the actual system varying demands.

    Gas/oil boilers will fire up when requested,often on-off-on-off every 10 minutes. Some boilers have parameters which can be adjusted to add a n extended heating cycle or delay.

    The cheap option during boiler cycling would be to physically turn down the room thermostat and when you feel cold turn it up enough to just trigger the boiler back into action-then turn down the thermostat once your body is heated.

    there is an expensive option but its just the winter months that you will need to interact with the room thermostat.

    Happy heating :cool:
     
  10. TheMorg

    TheMorg Active Member

    I'd guess the flame is failing on low flame because he didn't bother setting it up at minimum output. I've had this fault before when I've adjusted the gas valve a bit too keenly before realising my mistake.
     
  11. Johnreb

    Johnreb New Member

    We do not use a room stat, CH controlled on boiler stat and timer. Have contacted another GSE to come in and set the new gas valve at min rate then proceed from there. Unfortunately he is very busy at this time of year and we shall just have to wait until he can fit us in. We only wish to get it back to how it was.
     
  12. Johnreb

    Johnreb New Member

    I think you may be right and we are waiting for another GSE to come and do just that. The only thing that is causing a doubt in my mind is that when the CH temperature drops down below the thermostat setting range it will run normally until the temperature gets back up to the setting range then it will start short cycling excessively again.
     

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