In article ,
Andy Hall writes:
Yes you should.

There is a grain of truth in what he says in so far as since the
system is not exposed to the air there is a limit on oxygen content
for corrosion.

That depends how often you have to top it up, i.e. how well he
managed to make it a "sealed system" ;-)

However, some corrosion can still happen, and for the few pounds that
a tube of Fernox gel costs, it isn't worth not doing it.
I would avoid the discussion and just do it later.

ISTR it was a condition of the manufacturer's guarantee for most
boilers when I was trawling though their installation instructions.

--
Andrew Gabriel