In article ,
writes:

I wonder how long fernox lasts? I top up the system manually, and probably
add no more than 5L per year of water.

The fact you top up at all means you have a leak, and air will
be leaking in (quite possibly at a faster rate than the water
leaks out). Topping up once a year is not serious enough to
rip up all the floorboards to find it, but the leak will be
slowly using up the inhibitor.

Do the nail test I mention in my other post.

--
Andrew Gabriel