We have had this problem now for over 5 years. We have a combi boiler an Alpha CB28. Initially we thought the actual boiler was leaking (as we discovered small leaks from it) - plumbers also did extensive searches for leaks elsewhere and found none. It was advised that a new boiler should be installed. Which we did. The problem is still the same. The plumbing around the house are all drop downs with no pipes buried. The other strange thing that happens when we have a prolonged wet weather (ie the water table rises) we have to draw water off the system as the pressure rises. We have spoken to Alpha, numerous heating installers & plumbers and no one can give an answer. Can any one out there help, many thanks