its not hard to explain jus pipe your feed pipe to the expancion pipe and prob solved. it is not piped up right. thats all. its comming back througe the feed pipe and expanding. if it was the cylinder the expantion tank would be filling all the time with water
the thing is we dont give boilers away so y would you have too boilers too pumps and not give a reason y. and when we fit a system you can be assured we will be there a week later not on the anser macine saying sorry im on my horse.