Nowt to worry about.

Remember the 15mm feed is typically referred to as the F&E - feed &
expansion. Depending on the layout and size of the system this may
cause the F&E tank to become warm, after all the increase in volume of
the water as the temperature increases has to go somewhere. If the vent
is cold then it can't be overpumping.

Personally if I were you I'd seriously consider going sealed, saves a
lot of hassle for a very little cost.