@bami it may be to do with the position of the temperature sensor in the tank. If it’s half way down the top of the tank will be hotter than the set temperature due to stratification. 
If so do you can drop the temperature further and still have plenty of hot water

although I’ve just noticed the image but it could still be a sensor position issue as hot water will rise even after the heat pump has stopped heating for a time especially if the coil is low down in the tank. 

@bontwoody thank you oh wise one 🙏🏻 😉

@bami lol. I could be wrong 😆

@bontwoody if the tank blows up or sets on fire I'll track you down and haunt you 👻😆

@bami I feel pretty safe then 😁. As a matter of interest where abouts is the coil and the sensor ?

@bontwoody Absolutely no clue (no surprise there!).  How do I find out?

@bami is there a cylinder schematic? Failing that a make and model might be enough to look up.

@bontwoody The tank is a Kodiak Joule Smart Pre Plumb.  Can find no mention of temp sensor anywhere online or in the installation manual.  I did find this photo of the coil placement on the Joule.ie website.

@bami check the field setting values #3031 to 3033 for the booster heater (immersion) operation.

#3031 turns it on/off (I have mine set to NOT USE), #3032 sets the delay before it becomes active on a DHW cycle (if you do use it, set this high so the ASHP has sufficient time to try to reach the DHW set point temperature alone), and #3033 sets the overshoot temp (max 4C). This last setting (assuming #3031 is set to use) will allow the DHW tank set point temperature to overshoot by 4C which is what could be happening in your case?

So if #3031 is set to USE, and the delay time in #3032 is short, the immersion will kick in and help the ASHP to achieve the DHW set point temp, and then allow an overshoot of up to 4C as set in #3033.

If this is the issue, setting #3031 to NOT USE should fix it.

Posted by: @bami

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Following excellent advice on here I've now set a schedule for the DHW to come on for 2 hours in the afternoon and reduced the water temp to 43C.  But the DHW still seemed to be way too hot.

2 hours is far too long, and again could be contributing to this issue. It only needs to be long enough to reheat the DHW tank to the set point. Mine typically does this in around 30mins, so I schedule my DHW reheat to start every day at 2pm and schedule it to turn off again after 45mins, as once it's done, I know it's done until tomorrow. Keep an eye on it for a couple days to get a feel for how long it may reasonably take, and then add a bit for good measure just in case the tank is colder than usual.

@bami old scientist has a good call there. I don’t use my Samsung to heat DHW as I have a Mixergy tank so aren’t familiar with the settings.

@old_scientist Thank you so much for this info 🙏🏻  I'm still in the very early stages of getting to grips with it all.

The booster is set to Use and the delay time is 75 mins.  Overshoot is 0C.

I've decreased the time the tank heats on the schedule down to 60 mins and will see how that goes.  This time is below the delay time of 75 mins for the booster anyway, so the booster even while on shouldn't affect anything.  As the overshoot is set to 0C that can't be responsible for heating the water higher than the set point.