Taps that run by themselves

In the past we have encountered cases in which witnesses were confronted with bathroom or kitchen taps found running after having been securely turned off.

Although not common, a possible natural explanation is the formation of ‘a hydraulic lock’. This can happen in a tap with a corroded washer, or which is otherwise faulty, when water leaks upwards into the mechanism. When the tap is closed by hand the water stops and it feels as though it has been firmly turned off, but, because of water in the mechanism, it is only water-pressure that is holding off the flow; the tap itself is still physically open.

A household tap in cross-section, illustrating a hydraulic lock.

Hydraulic lock in a household tap. The blue area represents water, which is leaking upwards into the mechanism.

This is the situation shown in this photograph of a modified tap with its mechanism exposed. The blue area represents water. It can be seen that water is leaking slightly upwards past the point where it is usually confined by the mechanism of the tap. There is enough downward-pressure on the water to hold it back for now, but the mechanism is actually open right the way to the spout.

Time passes and, often, the temperature changes, which affects the density of the water – at which point there is nothing to stop the water from flowing again. Typically the tap is turned off in the evening, the temperature drops overnight, and then the water will be discovered running in the morning.

If you are experiencing self-running taps, a simple experiment can determine whether a hydraulic lock might be the cause. Place a piece of coloured tape on the affected tap, so that you can see the exact position the tap was in when you turned it off. If the phenomenon does not appear every night, it might be possible to get into the habit of turning off the tap always with the piece of tape in the 12 o’clock position, for instance.

If the hydraulic lock hypothesis is correct, then, when the tap starts to run, the tape will register zero or only a very tiny amount of rotation. But if there is significant rotation, this suggests the tap has been physically turned and another theory is required.