Water exchange in F&E tank via feed pipe to system

[John.g]

I only open the F&E tank filling valve every few months (to check for any leaks) and I have never ever heard a drop of water making up which I find extraordinary in a OV system, unfortunately my tank is so high up that I have no head room to look into it, I must rig up a periscope to have a look in., the water level is ~ a meter above the top of the attic converted bedroom rad top which runs with a very slight negative pressure but still gets perfectly hot and has never caused any problems as > 35 years old.