@ashp-bobba I am most grateful to you and I am so pleased that you were able to summarise it and also come up with a simple alternative.

When the contractor originaly visited, the young man dealing with this aspect coupled up good quality looking equipment to the PUHZ, that maybe included a pump and gas container. He abandoned the attempt when he could not get confirmation of the procedure after frequent abortive attempts to contact Mitsubishi Portugal.

I now hope that I can present your description to them and make progress on the pipe renewal before the winter.

Sincere thanks once again.

So this is essentially an AC split with a hydro box which then converts the refrigerant energy to a plate-heat exchanger but in the hydro box end (so like taking that part out of a mono block and placing it in its own box inside the house) "i have over simplified this description" therefore as far as the refrigerant side goes this can be treated like an AC. I am sure if you switch the unit off, recover the refrigerant, complete the changes, calculate the length of the run and weigh the correct gas back in and switch back on after completing all of the test it will just run up again.

I must admit I have never seen a Mitsubishi hydro box system in the Uk and they are not in my catalog so perhaps they don't sell them here or as a main option anyway but I have worked on a lot of hydro box systems and it is often for example on a Daikin or Hitachi hydro system you need to dip switch one switch to open the valves to pull all of the gas out but if the same as AC, Mitsubishi was one of the only brands you never needed to dip switch to remove the refrigerant as their EEV's relax open and not closed like all other brands. This will all be covered in the manual, also Mitsi Portugal should be able to confirm.

One more tip, if you cant get anywhere with Mitsi Portugal just call Mitsi UK tech, they are very good and can guid you through anything you need to pump down, reclaim and so on. You just must use a qualified F-Gas tech.

@ashp-bobba Thanks for the updated comments. I certainly had the impression that the engineer was seeking something like flicking a dipswitch before proceeding.

Could I impose further please? I have been scouring the various Mitsu manuals again and I have found a couple of pages in OCH755 Sept 2020 which look relevant and are rather better written than the last that I forwarded. Could you have a look at these two pages: 19 & 20, and see if this adds to your earlier comments. If it looks more useful, I will translate it and send it through to the contractor iin the hope that it will get him moving again.

Two other points:

1 - The EHSC Hydrobox was made in UK but the PUHZ in Japan.

2 - I have previously contacted Mitsu UK and generally have found them friendly and helpful, but when you reveal that you are in Portugal, they sometimes switch off. Mitsu Portugal have been dismal in response to any enquiry.

Thanks 

So the manual is correct and looks fine to me, to abbreviate the sequence and take out the things you don't need, you should do the following:

Attache you gauges

Wind in the liquid stop valve on the side of the condenser or inside the cover (this is the small pipe)

Turn the system on and wait 3 mins

Switch the dip switches as instructed (hopefully the unit start a test run in cooling)

Watch your gauge drop until it reaches just below 0 (maybe 1 or 2" vacuum "the reason for this is to make sure you pull all of the gas back) 

As soon as it reaches 0 above while the compressor is still running close the other gas (suction) stop valve and turn off the unit (mitsi say this will stop running after 3 mins anyway 

Note: if the pipework is long and the system has been trimmed with additional refrigerant  the system may not be able to pump down all of it, I think on average they can pump down 20% more than their original standing charge but thats not a rule some can do more.

I still think if you are not sure your best bet although slightly more expensive is to play it safe and recover the refrigerant to a bottle as no sequences are needed for this.