Bosch Optiflow 26 Professional not fit for purpose

My Gas supplier installed four Bosch Optiflow 26 Professional units at my house a year ago. We have had nothing but problems with them all. Bosch have sent someone round twice. We have changed the GAS, the regulators, the pipes on their recommendations. Eventually we swapped one of the units for a different maker's unit, we do not have problems with that unit but continue to have problems with all the other Bosch units.