@m1kegibson 

This will show you how to check and adjust DHW settings.  He waffles a bit to about half way through then gets to the important bit so far as you are concerned.  Hope this helps.

The settings for DHW are the defaults with the target set to 50 and drop set to 10. The issue is that even when the water temp drops well below 40 it still does not kick in and start heating the water. My daughter is now trained to hit the Heat Now button on the MELcloud app after her morning shower until we figure out why this is happening. 

Posted by: @m1kegibson

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The settings for DHW are the defaults with the target set to 50 and drop set to 10. The issue is that even when the water temp drops well below 40 it still does not kick in and start heating the water. My daughter is now trained to hit the Heat Now button on the MELcloud app after her morning shower until we figure out why this is happening. 

What's the 40C shown on the display?

It might be worth changing settings and changing them back, it feels like something might be out of sync between ftc and odu, or as @ashp-bobba suggests, calling mitsubishi.

@m1kegibson Apologies if I'm teaching grandma to suck eggs but have you confirmed the max temp drop in the controller is set to 10C?  To do that you hold down the menu button for a few seconds till the hot water/heating menu appears then under the tap symbol press the pen/notepad symbol then there you will see the options for the max temp set that should be 50, as that is what Melcloud is reporting, rhe max temp drop value, which you should check is set to 10C.  Then there is the max operation time that will stop hot water cycles if reached regardless of the set temp so check that is set to a couple of hours.  Lastly, there is the time limit that prevents it restarting even if its demanding heat but that shouldn't be an issue at this time of year.

If you are happy to post screen shots of those values then at least we can confirm its all set correctly.

I don't believe its a scheduling issue as your screen shot shows a paused symbol by the hot water symbol and nothing else if it was on a schedule there would be a clock and if it was disabled completely there would be a stop sign so I do believe you are in constant hot water heating mode the unit just believes the temperature is satisified.

Hep. Max drop is 10C. Tried changing and it 5C made no difference. As you say, DHW set to Constant, target is 50C with 10C max drop. I think I will try setting a schedule and switching to timed operation when next round at her flat to see if that makes any difference. She is waiting for another engineer visit but all they do is turn it off and on and then say “there, it’s working now”!

@m1kegibson did you try calling Mitsubishi as suggested by @ashp-bobba, it may be the easiest fix!

Not yet. I am away at present so will do so when I am in front of the controller. 

Posted by: @jamespa

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What's the 40C shown on the display?

Its the flow temperature for the heating circuit, it means its running a fixed flow temp and is not on weather compensation, get that changed come the heating season especially with UFH its way too high for all but the depths of winter and in a flat may not even need to get that high

@m1kegibson 

If you are able to access the service menu settings and navigate to the thermistor readings, I suspect you may find the answer as to why the system re-heat is not behaving as you are expecting it to based on the information displayed to you by the FTC.

On the right of this screen, THW5A and THW5B are thermistor readings for the hot water tank (however slightly older systems, and the videos Mitsubishi have posted which are of the FTC4, only show one value). 5B is for a thermistor placed low on the tank, 5A for one placed higher up - around 2/3 up perhaps, it will depend on the tank design. 5B, the lower of them, will limit the heating cycle - i.e. when the bottom part of the tank has reached the target temperature the cycle will be ended, the upper one, 5A, is the one that will trigger the reheat cycle when the temperature at that level has dropped by the value that has been set.

The issue here is that it is the value of 5B that is displayed by the FTC as the tank temperature, and that can be many degrees below the temperature at 5A, the one that will trigger the reheat cycle. Currently on my tank 5A is at 40° and 5B is at 31°, and this can drop into the low twenties while still delivering hot water out of the top of the tank.

You don’t say what brand of tank is installed, but I am going to guess it is not a Mitsubishi pre-plumbed one. The screen shot of the daily temperatures you posted shows a gradual decline of tank temperature as cold water cools the bottom of the tank during the day when water is being used but otherwise a very stable temperature overnight. (For comparison my Mitsi tank records a steady loss of 6 or 7 degrees over 24hrs even when no water draw is introducing cold water into the tank.) So I would infer that the hot water at the top of the tank, above the placement of the thermistor 5A, may well stay close to the target temperature for a long time even as the bottom of the tank gets quite cold.

If your daughter’s hot water usage is relatively low a full tank could last for a few days before requiring a heating cycle. Switching on and off, or using a daily timed off/on will force a hot water cycle but the warmer the water remaining in the tank the less energy efficient that cycle will be. So tweaking the reheat settings to get the balance right may need some monitoring of the value of the thermistor 5A.