• This topic has 8 replies, 6 voices, and was last updated 5 years ago by Bear.
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  • Is it possible to connect a thermostat backwards?
  • onewheelgood
    Full Member

    We had a new boiler fitted in the spring – a system boiler, not a combi, with a thermostat on the cylinder. It’s never quite worked properly. The symptoms are as follows:

    The hot water (if we get any, see later) is always at the temperature that the boiler is set to. Since the installers left that at 85°C that meant we could make coffee with water out of the tap – we have turned it down to 65°.

    Some days we don’t get any hot water at all, but if you put the immersion heater on it will start working again after a while

    On the days we don’t get hot water, the controller and the app insist that the hot water is on, but the boiler doesn’t fire up

    Yesterday was a no hot water day and I discovered that if I turned the thermostat down to a point below the actual temperature of the water, the boiler fired up.

    This leads me to believe that the thermostat is wired so it only demands hot water if the water is already hotter than the temperature to which it is set. Is this possible?

    bigyinn
    Free Member

    Or the temp sender in the hot water cylinder is wired wrongly?

    giant_scum
    Free Member

    Sounds like the cylinder stat is wired wrong.

    There is most likely 2 options for connection in it either (NO) Normally Open or NC (Normally Closed).

    As explained in this link

    https://www.flameport.com/electric/central_heating/hot_water_cylinder_thermostat.cs4

    HTH

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    onewheelgood
    Full Member

    Thanks @giant_scum , that makes sense.

    Cougar
    Full Member

    Isn’t that, like, really dangerous?  The boiler will never switch off once it fires?

    In any case – it’s an installation issue, get the people who installed it to come fix it?

    5lab
    Full Member

    there’s probably an upper limit (or it’d have exploded already :))

    whats wierd is that if it was just wired backwards, surely it’d never work (once it is full of cold water, it never goes above 60deg or whatever to keep it heating further).

    does sound buggered either way

    onewheelgood
    Full Member

    Isn’t that, like, really dangerous?  The boiler will never switch off once it fires?

    No, the boiler primary circuit temp is set at 65°, so that limits the firing.

    whats weird is that if it was just wired backwards, surely it’d never work (once it is full of cold water, it never goes above 60deg or whatever to keep it heating further).

    This is the bit that really puzzled me. My current theory is that it’s a big cylinder, sized for a house with 3 bathrooms, but the kids have left home and there’s just two of us, who tend to use the water during the hours the boiler is on. So the temp never gets below the thermostat setting, and it usually works. But the other day, my daughter was staying and she had a long shower while the hot water was switched off. This pulled the temp down enough that the boiler didn’t fire when it next came on. Then, I put the immersion heater on and the temperature got above 60° and the boiler fired up. This was actually the clue that led me to the answer, after many months of puzzlement during which I couldn’t identify the trigger that led to the hot waterless days. Intermittent problems are always a pain to diagnose.

    fossy
    Full Member

    85c hot water, good god, that’s dangerous.  Get the installer back.

    Bear
    Free Member

    Yup cylinder stat can make or break on temp rise. Sounds like it is the wrong way round.

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