water overflowing from expansion tanks

I have recently had a new boiler fitted. (baxi solo). When heating on water seems to backflush into large cold water tank in loft. whichwarms the water up and constantly fills up then overflows.
When the heating is turned off , water stops going into large tank, but now water seems to backflush into the small tank in loft which fills up and overflows.
turned the pump down to number 2, but still does the same.
please can anyone help
thank you

would say you have a split coil in the cylinder.

cheers mate. confirmed my diagnosis

cragglerock said:

cheers mate. confirmed my diagnosis

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