I have a mains pressure cylinder that's started leaking out the overflow pipe outside ,what can it be{its stated to really pour out}

Linkback: https://www.plumbers.nz/q-and-a-hot-water-cylinders/44/mains-pressure-cylinder-leaking/1673/

yea i thought that too but the water coming out of the overflow is cold so im thinking maybe not the thermostat but maybe the cold water expansion valve ,i wont do the work myseld just want toknow the costs of the job and to know the costs need to know which valve to replace