Mechanical Backup Valves automatically maintain a pre- set water level near the bottom of a rainwater tank when there is insufficient rainfall to meet the water demand. As the water level drops, a weighted float slides down a long cord until it reaches a stop at the end. When the water level continues to drop, the weight of the float pulls the valve arm, allowing the backup water supply to flow into the tank. The water level rises and lifts the float, causing the valve arm to rise and shut off the water flow. Two options are available: a 1/2” low-flow valve and a 1” high-flow valve. At typical municipal water pressures, the 1/2” valve supplies less than 4 gpm, so it is recommended for small-scale systems where the rate of water use is relatively low. In contrast, at typical municipal water pressures the 1” valve supplies more than 60 gpm which is sufficient to match the extraction rate of large rainwater harvesting systems. All parts are corrosion free plastics and stainless steel for trouble-free performance. Other features include extra- long mounting stems to accommodate thick-wall tanks, an integral mounting nut and sealing gasket that eliminate the need for a tank bulkhead, and a blocking device that enables the valve to be locked in the closed position. Internal check valves are also standard, but to prevent cross-contamination the valves outlet should be located several inches above the high water level of the tank.