A stopcock, or stop valve or stop tap, is an inline valve which controls the water supply to a property. It is usually located on or near the rising main, and usually under the kitchen sink, although in some homes it may be located in a pantry or hallway. In addition to an internal stopcock, some homes may have an external stopcock, which in some cases, is shared with adjacent properties. An external or outside stopcock is usually located under a service hatch. Its operation usually requires a long metal tool known as a stopcock key, as the stopcock itself may be two or three feet below ground level in order to avoid frost.

Stopcocks are available in a range of pipe diameters. They are usually made out of brass, and consist of a spindle with a washer attached perpendicularly at the end. When the spindle is turned via the handle on top, the washer is lowered onto a seating inside, which shuts off the flow of water. Stopcocks should not be confused with gate valves, which function differently and are used on the low pressure side of a plumbing system. A gate valve should never be used as an alternative to a stopcock, as stopcocks are specifically designed to handle water at mains pressure.

Turning off the stopcock will stop the flow of water to everything supplied by the mains, such as the kitchen tap, and any cisterns in the loft, such as the cold water storage cistern and the feed and expansion cistern. Anything whose source of water is the cold water storage cistern, such as the hot taps, or the bathroom toilet and bathroom cold taps in an indirect system, will continue to work and flush until the water in the cold cistern is exhausted.

It is extremely advisable for homeowners to know where their stopcock is located, should the mains water supply need to be closed in an emergency. While in many homes, the stopcock may be hidden behind the cleaning products stored in the cupboard under the kitchen sink, in some cases it may have been completely obscured or hidden by subsequent kitchen fitting or DIY work.


Possible faults

There are two main faults which can occur with stopcocks: they leak internally, and do not completely shut off the water; they leak externally, often from the gland nut or the spindle, or they seize up, and the spindle cannot be turned.

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