After 3 years the primary sealed side would need another dose of
inhibitor.

But if the system has been operating correctly, without frequent refills, 3
year old inhibitor is likely to be working well, and sludging is unlikely to
be the cause of the problems. Obviously, flushing and replacing inhibitor
every 3 years is a good thing. It's just that the DHW side of the exchanger
is more likely to have scaled up.

Christian.