Vaillant Replaces Parts Unnecessarily, Not Solving The Problem

They replaced the expansion tank (with a price tag 4x more expensive than retail), but the problem occurred again after a few hours. Then, this time they came with a pump, saying this is probable since the equipment is 7 years old. So, why did they change the first part in the first place if the second part is broken? They make you believe that your problems will be solved by replacing parts that are not necessary to replace. In any case, you'll need to buy the other part too. So, they sell you one more part which would not be possible to sell otherwise. Thanks.