If you connect a rainwater harvesting system to the indoor plumbing of a home that is also connected to a municipal water supply, you are creating a potential public health hazard. Without proper safeguards, unfiltered rainwater from your tank can be sucked or pushed backwards into the clean city water pipes, contaminating the drinking water of your entire neighborhood. This phenomenon is called "backflow."
How Backflow Happens
Water always flows from an area of high pressure to an area of low pressure. There are two primary ways rainwater can enter the city supply:
- Back-Siphonage: If a water main breaks down the street, or the fire department opens a fire hydrant nearby, the sudden drop in pressure in the municipal pipe creates a powerful vacuum. If your rainwater system is tied to the city line without protection, that vacuum will suck the water out of your tank and into the city grid.
- Back-Pressure: If the electric pump driving your rainwater system turns on and generates 60 PSI of pressure, but the city water pressure at your home is currently only 40 PSI, the higher pressure of your pump will actively force the rainwater backward against the city pressure and into the main lines.
The Solution: Backflow Preventers
To prevent this, plumbing codes strictly mandate the installation of backflow prevention assemblies at the boundary where the two water sources meet. There are two primary recognized methods for rainwater systems:
1. The Physical Air Gap
This is the universally foolproof method. An air gap is exactly what it sounds like: an unobstructed vertical space of free air between the municipal water supply outlet and the flood level rim of the rainwater tank it is filling. Because there is no physical pipe connecting the two, water absolutely cannot jump backward across the gap, even in a total vacuum. When setting up a "top-up" system where city water fills an empty rainwater tank, an air gap is the standard.
2. Reduced Pressure Zone (RPZ) Assembly
When an air gap is impossible (e.g., in a pressurized dual-supply switch-over valve), an RPZ valve is required. This is a complex mechanical device containing two independent check valves and a pressure differential relief valve that opens to the atmosphere. If the city pressure drops or back-pressure occurs, the valve physically dumps the water out onto the ground rather than letting it flow backward. RPZ valves must be tested annually by a certified plumber to ensure they haven't failed.
The Law
Do not attempt to bypass backflow prevention. If a cross-connection event occurs and a neighbor becomes ill from E. coli originating from the bird droppings on your roof, you are legally liable. Always consult with a licensed plumber and pull the necessary municipal permits when tying rainwater into a house grid.