If you mean the header tank is overflowing to outside the house, then either the ball valve in the header tank is weeping or the hot water cylinder heat exchanger has failed and water is transferring from the DHW side to the CH circuit.
Think you may find the boiler change is a red herring although disturbing the system may have caused the final failure. Was your system gravity fed before?
You need to confirm this is indeed the case before an undergoing an expensive repair.
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