Over expansion continuous flow on HWT

Hot water is flowing continuously from the over expansion 7 bar valve on the HWT. There are 2 cold water inlets off the meter into the house, one which serves hot water regulated at 3 bar and the other entering the house separately to serve cold taps which is unregulated. When i isolate hot water to the house the flow out the expansion valve stops. There are 2 shower mixers in the house so i removed them and capped the ends, turned on the water and no flow out the expansion valve or upstairs hot taps. Then replaced one of the mixers and turned on water and the expansion valve flows. Repeated for other mixer and was the same. Then turned on hot tap and there was a slow flow through the tap. I assume the mixers are both broken and so are leaking cold into the hot water creating a build up of pressure resulting in the expansion valve opening on the HWT. What can i do to resolve this and would new mixers fix this or could it be the unregulated cold supply vs 3bar hot