Most boilers have a control stat that limits temperature to 82deg C and the limit stat would operate at 95-100deg C. If the limit stat is above this level (from manufacturers data) it probably means that the boiler has a high thermal mass and will continue to transfer heat from the heat exchanger to the water even after the boiler has ceased firing (thermal inertia). Take the difference between the control stat and high limit stat upper operating temperatures and add this to the high limit stat operating temperature, if this figure exceeds 114deg copper should be used (i.e Hi limit stat 110deg - Boiler stat 90 deg = 20 + 110 = 130 total = unsuitable).