Got Backflow Problems? REAL Plumbers Give an Overview

Backflow occurs when the pressure in a main's incoming water pipe reduces to the point where there is more pressure from an external medium such as a tank and hence the water flows in the reverse direction back into the main's water system. It can also happen through Syphonage. In America in the late 1960s wine flowed through the taps of the local residents of Cincinnati when a local winery forgot to shut off the mains inlet valve after flushing its fermentation tanks. Backflow can also be dangerous, causing drinking water contamination... or worse. For example, a maintenance man connected a large liquid petroleum gas tank to a water main to wash it out. Unbeknown to him, there was still gas under pressure in the tank. By opening the water supply valve, he back pressurized the water supply main with gas, causing three nearby houses to explode. These examples are extreme. But backflow is a real threat and adequate precautions should be taken. It is a major health & safety issue. You can use double check non return valves to eliminate this possibility, these devices splice into the pipe-work from the mains and only allow flow to occur in one direction. You can also use a reduced pressure zone (RPZ) valve. It has spring check valves that create an intermediate zone of pressure that is lower than the supply pressure. If back pressure builds up and the outlet check valve fails to hold, the intermediate zone will pressurize and cause a relief valve to open, draining the entire assembly, thus preventing possible contamination back into the supply. Prior to these RPZ valves you sometimes had to have a break tank with a ball valve and pump to then increase the pressure again. These were costly and also high maintenance.
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