Appreciate the great write up @majordennisbloodnok and i'm glad the documentation was clear enough that you got it all working! 😊 

Few points from me:

Step 13 & 14 shouldn't actually be required, when you send:

"active": true

If you aren't already in Fixed Flow, it will perform the swap for you.
This will only happen on the transition from "active:" false to "active:" true though, so probably why you mentioned it here as a required step.

I should also point out that if everything goes wrong and you can't make head nor tail of it all, all you need to do is change the heat pump back to "Heating Compensation" mode instead and the Mitsi WC curve will be obeyed again and the dongle's WC curve ignored. It's a quick and easy get-out clause.

This is correct, you can only write a Flow Setpoint in a Fixed Flow mode, so changing away from fixed flow it cannot write anymore.
You can also press that button around the LED to perform a soft restart, while you will remain in Fixed Flow mode it will de-activate by setting "active": false

There is a proposed change in the next firmware release that making that switch to a mode other than Fixed Flow will also perform the de-activation.

Finally, as i mentioned previously, this is the most complex feature on the device and there are a lot of steps involved. Home Assistant doesn't have a particularly accessible way to do this, so you do need to go through it line by line.

For MELPump subscribers i mentioned previously that this is coming to a nice web based design tool in the coming weeks, you can draw out your curve points in the App and it generates what is described manually above and sends it over to the adapter.

However, I've found the documentation doesn't work exactly as laid out and I needed to make a tweak.

Please let me know what this was so I can update the documentation 

Posted by: @f1p

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For MELPump subscribers i mentioned previously that this is coming to a nice web based design tool in the coming weeks, you can draw out your curve points in the App and it generates what is described manually above and sends it over to the adapter.

Logically, I'll fall into that category so the natural step would be to wait for it for the functionality to become available to do this.

However, I suspect there's a great learning opportunity from trying to follow the technical steps.  Whether I'll ever use that knowledge again remains to be seen, but I've come this far down the rabbit hole already, so I might as well burrow a bit further for now.

It's hard to believe that this started a few weeks ago with the simple plan to try Mel Pump's free trial via my MELCloud account for a better output of data, and has somehow descended (or perhaps ascended is more appropriate, given the additional functionality I've discovered from there) into me being a purchaser of a Home Assistant Green box, plus the CN105 dongle, discovering from there that I can tap into the Open Energy Monitor reporting ecosystem, and am now clinging onto the coat-tails of people doing funky things with their combined coding expertise.

Posted by: @sheriff-fatman

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I've had a quick read through it, and the immediate take-away from it was that the operating mode will show as being in constant flow, rather than curve mode, on the analysis within Mel Pump, which was my immediate observation when I saw the same thing under Havenwise control.  I suspect that Havenwise must be interfacing in a similar way via whatever means it uses to do so (as it doesn't use the dongle for this) to do something similar, albeit much more complex, to make the frequent changes to the constant flow temperature requirement that I noted in a previous post on a different thread.

...

That's a good spot and a valuable insight, @sheriff-fatman. I don't use Havenwise and so was not aware of this but it makes plenty of sense since if it didn't remove the responsibility for varying flow temperature from the Mitsi inbuilt coding there would necessarily be a fight between the two systems for control. Of course, with Havenwise being a cloud-based solution this does mean any loss of your heat pump's Internet access would probably mean it continuing in fixed flow temp mode at the last temperature it was told to use until it regained access to the Internet, whereas in your and my case the heat pump will continue to be controlled according to the custom curve for as long as the dongle is visible to the FTC whether Internet is available or not.

Posted by: @sheriff-fatman

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It's hard to believe that this started a few weeks ago with the simple plan to try Mel Pump's free trial via my MELCloud account for a better output of data, and has somehow descended (or perhaps ascended is more appropriate, given the additional functionality I've discovered from there) into me being a purchaser of a Home Assistant Green box, plus the CN105 dongle, discovering from there that I can tap into the Open Energy Monitor reporting ecosystem, and am now clinging onto the coat-tails of people doing funky things with their combined coding expertise.

It's quite possible you've now caught HPDHD (Heat Pump Deficit Hyperactivity Disorder) - a condition first officially identified by @cathoderay - and I'm sorry to say that it's not a condition that normally clears up on its own.

I'd disagree, though, about the clinging onto coat tails; that's not quite how things work. Melpump only started by providing an app that repackages data your heat pump sends to Mitsubishi's servers. @f1p's dongle only reads the data your heat pump is already designed to store and then passes that data onwards. By combining the two, Melpump is then able to do its simple things on more detailed data and so the whole is (apologies for the cliche) far greater than the sum of its parts. By adding in an extra bit of code to provide a custom weather compensation curve, @f1p has extended the dongle in a simple way which, when combined with the other improvements once again reinforces the cliche. In other words, the overall solution is a combination of simple things - a technical lego set, if you will - rather than some amazingly complex and impenetrable edifice.

You, by spotting that similarity of constant flow use by Havenwise, have once again added a small extra piece to the lego set. That now allows the rest of us to better understand how Havenwise - and quite possibly Homely - may operate and potentially increases our collective ability to produce similar solutions of our own in Home Assistant or similar. @cathoderay may yet (if he hasn't already) incorporate that understanding into his bespoke python modbus scripts. In other words, you're not hanging on anyone's coat tails; you're along with the rest of us for the ride and contributing like us all when you spot something. Welcome to the party.

N.B. By talking about Melpump and the dongle in terms of "...it's only...", I'm not playing down their effectiveness or value. Their lack of underlying complexity is in fact a strength. KISS principle applies.

@majordennisbloodnok I think the HPDHD diagnosis may be accurate.

For information, this is the 4 week old post where I included some observations of the Havenwise running mode, including the constant flow mode of operation.

https://renewableheatinghub.co.uk/forums/postid/53002/