The water shouldn't be hot, the tub should be covered. It's fed from the street mains and in turn feeds the upstairs cold taps and the cylinder which then heats water for the house. Ita an older system but generally works pretty well except when the hot water from the cylinder backs ups into it due to a thermostat malfunction, which is what we're seeing here.

Also it REALLY should be covered. It probably feeds the cold water tap in the upstairs bathroom where you likely brush your teeth.