Fig 2 shows the case where the homeowner starts the pump, and when NO fire is nearby, the 2-Port Valve is normally in a condition where the water is NOT being sent to the Spray Hose. The water is pumped thru the Pressure Relief Valve and returns to the water tank, The PVC pipe used to return the water has a small diameter and little water flows back to the tank. This causes the pressure out of the pump to be high, and the Pressure Relief Valve allows the water to pass thru and reenter the water tank.
The gasoline tank on the recommended pump allows the pump to run for
Since the fire may not arrive for many hours, a larger gasoline supply may be added. I could not find a place to purchase such a tank for the recommended pump, but there are many YouTube videos that show how to build one.
When the fire comes close enough to be detected by a thermistor, or other fire sensor, the 2-Port Valve opens and water flows into the spray hose. This is shown in the Fig 3 (shown below) The feedback to the Water Tank has been removed in Fig 3 because when the flow increases significantly into the Spray Hose, the pressure out of the pump drops enough for the Pressure Relief Valve to close, and all of the water is sent into the Spray Hose.
A low water detect signal to the Control Circuit closes the 2-Port Valve and all of the water goes back into the water tank. This is done so that the pump does not run without any water. This is necessary to avoid damage from overheating to the pump volute parts.
The low water detect signal can be used to turn off the pump by opening the pump ignition circuit.