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Discuss Combi CH seems to be randomly working, HW is fine, Help! in the Central Heating Forum area at

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  1. Pintsize

    Pintsize New Member

    Hi, Viessmann Vitodens 100-W, with Vitotrol 100 wireless digital thermostat. I'm not a plumber and just trying to help an old friend out if I can.

    I've bled air out of the boiler a couple of times, as it was very noisy, this seems to of quietened it down. Some radiators don't get warm, but a couple of the first ones did get hot. All radiators were bled, but had no air out of any of them 14 in total, 12 with TRV's and all valves appear to be working freely.

    I had started balancing the radiators, but boiler doesn't run long enough to do it?

    Tried 2 options of asking for heating to come on, Hot Water is always fine

    Method 1
    CH is set to Manual and temp is set to 40 degrees (have tried sensible temps as well) Comfort temp is set to 26 degrees. Boiler fires up for a random amount of time then stops, radiators a little warm, but boiler just quietly sitting there and fires up randomly.

    Method 2
    Program 1 set for each phase to run continuously for a 24 hr period 7 days a week. Comfort setting temp 26 degrees, some radiators get a little warm, boiler just sitting there again. Fires up randomly, for probably less than a minute most of the time.

    I'ts probably fired up 4 times in the past half an hour.

    Any suggestions on things to try, only thing left for me to try out of desperation is to drain the system, flush it through and fill it properly, hoping that air or sludge could be the problem? Have spent hours on the manual, youtube and general searches, I'm out of ideas and on the verge of telling the old girl she will have to get a professional to it. Help greatly appteciated.
  2. bacon_sandwich

    bacon_sandwich GSR

    You need to determine if its switching off because of over temp or no call for heat from thermostat. e.g. if the pump has failed boiler temp will rise very quickly and the randomness is the time it takes to cool back down ready for another go. If its a thermostat fault boiler will hardly be getting warm. Try looking at the Ch temp on the display and see how quickly it rises, 25 degrees to 70 degrees in a minute means water is not flowing.
  3. Pintsize

    Pintsize New Member

    Thanks bacon_sandwich,
    Ch takes the best part of 2 minutes to get to the high 68 degrees before giving up, no fault on the display. this would indicate the thermostat?

    Hw was okay last night for running tap for over 2 minutes, but now getting an over temp fault in seconds as the boiler rises to 112 degrees very quickly then faults. Pump fault?

    When I look at the vitorol 100 in manual mode, only the option to switch off DHW is available, should the Ch not be on there as well?
    If I do select off for DHW, makes no difference, boiler keeps heating it until its in fault?

    Could this be more than 1 fault? Do I need to get her to call a professional, or is there anything I could do?
  4. bacon_sandwich

    bacon_sandwich GSR

    TBH, 2 mins is still fast to get from 25-70, one way or another heat is not getting out of the boiler. Unless you have huge trapped air or have turned all rads off, it means an engineer is going to have to work the problem , find out what is faulty and replace it. As its a gas boiler , its going to need to be checked after anyway for safety so may as well call out someone now whilst its still warm
  5. Pintsize

    Pintsize New Member

    Thanks bacon sandwich, I'll do just that
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