At what temprature is the thermostat suppose to open on this specific model, Corsa Classic 16. Cde 4door Sedan?

The heater control valve ( working on vacuum suction)fitted with a diaframe keeps blowing out and leaking water from it. I have replaced it 3 times already. Having it on by pass causes the vehicle to have over heating problems. It has got the thermostat that has like a goose neck conecting to the radiator via a pipe housing. This pipe swells up and when the thermostat do open it returns to normal size. My findings is that the thermostat does not operate correctly and causes to much pressure build up in the cooling system causing the diaframe to pop on the heater control valve. Please at what temprature shuold it open.