It regulates the filling of the main tank, acting as a mini pressure-breaker and level controller
Rather than connecting a high-pressure mains water supply directly into the large solar storage tank, the water goes through this small assistant tank first. Inside the assistant tank is a mechanical float valve As water flows into the assistant tank and reaches a pre-set level, the float rises and closes the inlet valve to stop the flow.
When hot water is drawn from the system by the household, the water level inside the main storage tank drops. Water from the assistant tank immediately flows downward to fill the void: Because non-pressurized solar tanks cannot withstand the expansion stress of high-pressure city water lines, this tank ensures cold water enters only via natural gravity pressure, keeping the entire setup safe from bursting.