Air in hot water pipe for kitchen - bungalow property

Imagine, if you can, introducing an air bubble at your kitchen tap. With no taps open would that bubble be able to work its way back towards the hot water cylinder and, when it got to the joint with the 22mm hot pipe, rise and blow out of the vent just above the cold tank? If not then you have just defined your problem....

...or you could post some photos

im not sure this pic helps but it shows the pipe coming through the ceiling ( 22 into 15 and back to 22 ) and on the long run to the kitchen

strange that only the bends are 15