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Toilets and cisterns

A running cistern wastes more water than any other fault in a house, and it is usually two parts, both of which are standard.

The cistern is simpler than it looks

Inside a cistern there are only two mechanisms that matter. The inlet valve lets water in and shuts off at a set level. The outlet valve (the flush valve, sitting over the hole in the bottom) holds water in until you press the button. Nearly every cistern complaint is one of those two.

Water trickling into the pan continuously means the outlet valve seal is not sealing — grit, a perished seal, or a warped flush valve. Water running out of the overflow means the inlet valve is not shutting off, or the level is set too high. A button that does nothing is usually the button assembly or its actuating rods, not the valve. A slow, weak flush is often the level set too low, or a partially blocked rim.

A blocked toilet is a different problem entirely — and an important distinction: if only the toilet backs up, the blockage is usually local to the pan or its branch. If the toilet backs up and the shower or floor waste gurgles or fills at the same time, the blockage is further downstream in the main line, and plunging the pan will not fix it. That second case is where tree roots and collapsed pipes live; see blocked drains.

One safety line worth stating plainly: never mix drain chemicals, and never use a caustic drain product and then attempt to plunge — you are splashing a corrosive back at yourself.

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Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.