To build presure you will need the filling key. (A white plastic T shaped which allows to fill up the system). Once this is inserted you can fill up. If this problem keeps happening then the overflow valve may be faulty. I have just had to have ours replaced along with the pump. Here is a link to the manual http://www.worcester-bosch.co.uk/cache/file/186/user-instructions---customer-care-guide.pdf or it