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Vitodens 100-W control problem

Discussion in 'Plumbing and Central Heating' started by taff224, 2 Dec 2011.

  1. taff224

    taff224

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    Evening all,

    I have a Vitodens 100-w (WB1a I think) the 4 pipe setup which is controlled by a Honeywell controller, one room thermostat and 1 cylinder thermostat.

    The problem I have is that the boiler is not recognising the second demand placed on it.

    i.e. If I start the DHW at 05:00 and the CH at 05:30 then the CH never comes on....even if the cylinder thermostat isn't demanding any supply. The same occurs in reverse. The only way to guarentee hot water and heating is to independantly set the times so they never overlap. It is so extreme that even if the DHW turns off at 06:00 the CH will still not turn on unless it is turned off and back on again.

    The individual systems work fine with the thermostats - so DHW will kick in and out according to the thermostat demand so will the CH.

    I think it is a control issue in the boiler as the LCD screen doesn't display both symbols at once...just the first one demanded..or nothing if the first one is turned off.

    Anyone got any ideas? I have mailed Viessmann Tech support but I won't get an answer until next week probably and the plumber who fitted never picks up the phone on weekends.

    I don't remember having this issue when it was installed (in 2008) and think it is a recent issue. Could there have been a loose connection in the wiring that I can check?

    Thanks

    Chris
     
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  3. mysteryman

    mysteryman

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    The boiler has an integral diverter valve. It gives priority to hot water. Is the cylinder thermostat reaching a satisfied position, and taking the dhw demand off the boiler - the tap symbol must disappear? If and when this happens, the boiler can then give central heating, and the radiator symbol will show. If it is a WB1A, the DHW live is terminal LS on plug 56 and the CH live is LR. If both are live, it will do DHW only until the cyl 'stat is satisfied, then go to CH. Check that our cyl 'stat is physically fixed properly on the cyl, and is not set too high - 50/55C is usually enough.
     
  4. taff224

    taff224

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    Just retested the process to check I wasn't getting anything wrong...the following process.

    DHW stat set to 50 - demanding heat (I hear the click as I bring it up from bottom)
    CH stat set to 20 - demanding heat as current temp is 19

    Turn on DHW - boiler starts and provides DHW - tap symbol
    Turn on CH - boiler still providing DHW - tap symbol only

    Turn DHW stat down to bottom setting - boiler stops providing heat but keeps pumping with tap symbol showing.
    After a short period of time (1 min) the pumping stops and the tap symbol dissappears - no CH symbol showing and no activity.

    Turn off DHW and the CH symbol appears and I get CH heating (this is slightly different from previous statement)

    Repeat other way round:

    CH stat at 20 - demands heating
    DHW stat at 50 - demanding heating

    CH on - CH symbol and heating.
    DHW on - no DHW - CH symbol showing and still heating

    Turn down CH stat and heating stops but CH symbol still there with pump running

    I've waited 5 minutes and the CH symbol isn't vanishing - no heating just pumping noise...I'll check tomorrow if it goes off after longer than 5 minutes but it doesn't look like it.

    Turn the CH off and the DHW comes on...

    Basically you can't have both on at once because the only one that works is the first that turned on....even if the stat isn't demanding heat.

    Any thoughts?

    Thanks

    Chris
     
  5. mysteryman

    mysteryman

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    We need to know what controls you have other than the boiler.
    Timer? Cylinder thermostat? Room thermostat? Any 2 port valves?
     
  6. taff224

    taff224

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    Controller/timer is a Honeywell ST9400A
    One Cyl stat
    One room stat
    There is a junction box between the controller/stats and the boiler

    The junction box has 240V mains and the 2 stats entering at the base, a single cable coming out to the controller (I assume power), one cable back from the controller, one cable going to the rear left corner of the Boiler and one to the front right.

    There are no external valves or pumps all that is inside the 100-W.

    The boiler only has one control on it - that is the water temperature dial - if you turn it to 0 it give presure in the LCD screen, otheriwse the screen shows water temp and which system is being demanded and flame state.

    Think that's all I can see without opening something up.

    Thanks for the help

    Chris
     
  7. mysteryman

    mysteryman

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    Look for the tap or radiator symbols on the LCD screen. If it is showing one or the other and not producing heat, it is likely to be a boiler fault. If you are seeing temperatures in the 80s or above, the boiler probably needs a powerflush. If it is showing neither a tap nor rad, it is a problem with your other controls.
     
  8. DIYnot Local

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