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Heating won't turn off unless hot water demand is present on programmer

Discussion in 'Plumbing and Central Heating' started by CTL, 24 Nov 2019.

  1. CTL

    CTL

    Joined:
    19 Jun 2013
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    Location:
    West Midlands
    Country:
    United Kingdom
    Hi

    I’ve recently had a new boiler installed, a Vaillant Ecotec Plus 415 which replaced a Potterton Suprima 30. There was some difficulty experienced by the installer and he swapped the existing programmer out with a spare. This didn’t fix the issue he was having (can’t be more specific, sorry), so returned the previously installed one (Danfos FP751SI), at which point everything seemed fine.

    While everything appeared to work correctly when he left: hot water and radiators getting hot, it’s become apparent that there is a temperamental heating issue with the rads not turning off.

    I’ve spent a lot of today with a multi-meter checking things. I’ve managed to narrow down the issue and is 100% reproducible now.

    At certain ‘random’ times, the heating wouldn’t turn off without a complete power down, meaning heat is constantly being pumped out.

    It seems that there is an issue with the 3 port valve actuator not moving to the correct position. At first I suspected it was a seized valve, but that doesn’t seem to be the case as I can move it fairly easily with my fingers once the device is removed. It was replaced a few years back too. In addition, it also doesn’t align with the rest of my testing:

    When the boiler is powered off (mains power), the actuator moves away from the manual position. It has resistance when trying to move it back to the manual position. When I do so, it returns back to the furthest point away from ‘manual’ over a few seconds.



    On the programmer, hot water is set to ON:

    o The system heats the water correctly and takes the thermostat on the tank into account, shutting off when it should

    o The radiators behave as they should: They obey the main thermostat and take the programmer into account (rads on/off)

    o In other words, I can’t find anything wrong here



    The issue manifests when there is no demand for hot water from the programmer

    o Boiler powered down -> actuator resets away from manual

    o Power up boiler and leave hot water OFF on programmer / set tank thermostat to lowest point

    o Turn central heating on – actuator moves and radiators heat correctly

    o Stop the demand for heat (either lower thermostat temp, or turn off on programmer) = heating sill continue. The actuator doesn’t move back to the furthest point from ‘manual’. This will not stop.

    o I believe this is the root cause.

    o If I now turn on the demand for hot water, the valve will correctly move – assuming there is no demand for central heating.



    While I’m familiar with electronics (more on the side of PCBs than household electronics), I’m at the limit on my knowledge here from the reading I’ve done. I’m guessing that the valve head sends a signal back to the boiler, letting it know the value has been reset. When the boiler doesn’t see this, it continues to heat?

    So, any ideas why the central heating will work fine when there is a hot water demand, but not when there isn’t, with the valve effectively getting ‘stuck’?

    The obvious thing to do would be to replace the 3 port valve electronics, but I’m confused as to why it works fine when there is a hot water demand.

    No wiring in the house was changed, apart from the standard connections to a new boiler.

    If it’s boiler related, it’s under warranty naturally so I’ll refer it back to them.

    Thanks for any input.
     
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  3. ianmcd

    ianmcd

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    Highly unlikely to be boiler related , it is more likely external controls or install related and they will possibly charge if you call them out and it is not a boiler fault, you would be better getting the installer back to check first, there is a bridge link in the boiler at 24VRT that has to be removed when you fit 240v external controls which you have so make sure that link has been removed
     
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  4. CTL

    CTL

    Joined:
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    Location:
    West Midlands
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    That's the one!

    Had the installer re-visit this morning as I was adamant the electronics in the house were fine. He did a few checks before I asked about this and then realised. Thanks ianmcd :)
     
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  5. DIYnot Local

    DIYnot Local

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