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Backflow from WC cisterns investigated

The water industry has investigated incidents in recent months where water from toilet cisterns has been siphoned back into customers’ plumbing because the WC float valve and flushing device were wrongly adjusted.

In one incident, three properties in one street had blue water at kitchen and bathroom taps shortly after a burst main was repaired. An incorrectly adjusted WC cistern at a house in the same road caused water containing blue cleaning chemical to be drawn back from the toilet cistern into the water main when the water pressure fell due to the burst. Fortunately, some people haven’t completely stopped blue cistern blocks. This has allowed the risk of backflow to be identified.

To stop water flowing back from the cistern into the supply pipe, the overflow water level, controlled by the overflow or warning pipe, must always be sufficiently below the inlet discharge point. The water fittings regulations and Scottish water by-laws require an effective air gap between the inlet valve discharge point and the overflow water level in the cistern.

The regulations require the vertical dimension of the air gap to be 20mm or twice the internal bore diameter of the pipe feeding the inlet valve, whichever is greater. For a cistern with traditional side-entry overflow (warning pipe), the overflow level is the centre line of the overflow pipe.

The air gap is the vertical measurement up from this overflow level to the lowest point of discharge on the inlet valve above it. Other types of inlet valves and overflow arrangements are allowed.

Backflow occurred in the recent incidents because the inlet and overflow arrangements had not been installed at the correct heights to maintain the air gap. Commonly, the problems occur in WC cisterns with adjustable-height bottom-entry inlet valves and drop valve-flushing devices.

It’s especially important with these that after installing the components and checking for leaks, the installer remembers to make the final check and adjustment of the height of the inlet or overflow. Some manufacturers of bottom-entry inlet valves assist by marking the inlet discharge point on the valve body with a datum line (CL mark).

Dr Steve Tuckwell is technical adviser at WRAS

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