These thermosiphons usually have a restrictor in them.. a disc with small hole in middle.. to restrict flow.. something in the range 2-3mm. From what I gleaned, it sounded like you had the issue with the upper pipe that runs from the boiler to the group. This is where the restrictor is located in my TS. However, on mine the restrictor is at the end and obvious.. But wonder if your restrictor is a bit further down the pipe in yours. Easy for scale to get in that opening.. I'd use a flashlight to see if you can find the restrictor.

This assumes you have a restrictor..(yours less than 5 years old.. so likely) .. and I've understood what you meant in previous posts..

I reamed my restrictor out with a drill and that improved some performance issues I had..

Glad you solved it.

Never heard of or used sulphamic acid.
Always used citric acid. It has the nice property of running blue while working and clear when no more scale.

For the next time:
To descale just the HX:

1. Remove wire from boiler fill sensor and let pump run for about 10s to overfill boiler slightly
2. Ground boiler fill wire so 3 way valve does not trigger
3. Fill HX w descaler. When it stops running blue, it's clear. Flush HX with clean water until there is no blue tinge and no sour taste.
4. Attach boiler fill wire to probe and bleed some water from boiler until pump starts

Thank you CafeIke and Bluenoser for all the input.
I didn't find a restrictor, the pipe is all clear, so if at all it would be at the bend connecting to HX.

In any case it's all sorted now, thanks again for the advice!
Nicolas

Just for reference in future, your restrictor is inside part #17 in your diagram from your previous post. (a union elbow)

Thank you Dave. I love how every question finds an answer on this forum!
Nicolas