Within ten years you will need a new condensation boiler. I asked my boiler service man about new boilers as ours (an oil fired no gas here) is 16 years old he said that the expected service life of a new condensation boiler is under ten years and for the extra 2% or less increase in efficiency I might expect to see it would be better to stick with the boiler I have as it would last at least as long again.
That's what I'll be doing. Isolation comes first.
Our present boiler was 80% efficient new, but being 20 years old, this has probably gone down to about 75%.
The best condensation boilers get 95/98%, so the jump is worth it in our case.
But, knowing much more about boilers than the average consumer (lots of boilers on commercial vessels) I'll probably have to drive to Germany to get models only sold there that are dual source.
AND have a 10 to 15 year warranty (not to mention that my gramp, two uncles and a cousin are/were in the heating business and can give me tips).
And only cost 10 to 15% more than your De Dietrich. But I'll have to Pay 21% VAT instead of 8%, mysteries of Belgian tax system.
As I side note, I used to build shops, and do the heating/air conditioning for those.
In a shop the heating system gets a WAY harder workload than a house system (wide doors that open and close all the time, big windows heat sinks), yet we were able to warranty these for 10 years of near nonstop usage.
So as a consumer, if you don't ask for the same, then you are getting shafted.