When you commit to hooking your toilets and washing machine up to a rainwater tank, you inevitably face the "dry spell" problem. If it doesn't rain for three months, your tank will run dry. When that happens, you still need to flush the toilet. The solution is a dual-supply system that seamlessly transitions from rainwater back to the municipal grid.
The Top-Up Method
Before discussing complex valves, the simplest "dual-supply" system is a tank top-up. A float valve (exactly like the one inside a toilet tank) is installed near the bottom of your main rainwater tank and connected directly to a city water line with a proper air gap.
When the tank level drops to 10%, the city water clicks on and trickles just enough water into the tank to keep the indoor pump from running dry. The house continually runs off the pump. This is physically the safest method against cross-contamination (due to the air gap), but it means you are using electricity to pump municipal water.
In-Line Changeover Valves
A more sophisticated approach involves a specialized valve installed inside the house or near the pump. This valve is hooked up to both the high-pressure city line and the high-pressure rainwater pump line. It feeds a single output pipe running to the toilets.
Manual Switch-Over
A manual switch is a physical, three-way lever. When you notice the rainwater tank is empty, you walk down to the basement, flip the lever from "Rain" to "City," and turn off the rainwater pump so it doesn't burn out. This is cheap, incredibly reliable, and involves almost zero complex parts to fail. The downside is that you will inevitably discover the tank is empty in the middle of the night when the toilet fails to refill.
Automatic Switch-Over
Automatic switch-over valves (often called mains diverter valves or hydraulic valves) are the gold standard for indoor integration. These are typically hydraulic mechanisms (meaning they don't even require electricity) that detect water pressure.
While the rainwater pump is running and providing 50 PSI, the valve holds the city water shut. When the tank runs dry and the pump shuts off, the pressure on the rainwater side of the valve drops to zero. The 50 PSI city water instantly pushes the internal diaphragm over, seamlessly supplying the toilets without you ever noticing a pause.
When it finally rains and the pump reactivates, the higher pressure of the pump pushes the diaphragm back, shutting off the city water and resuming rainwater use.
Important Code Considerations
Because automatic in-line switch-over valves physically intersect the city water and the rainwater under pressure, they require extremely robust backflow prevention (usually an RPZ valve) installed immediately behind the device on the city side. This ensures that if the diaphragm inside the switch-over valve fails, the rainwater cannot be pushed back into the municipal grid.