Overflow pipe/warning pipe – difference

Overflow pipes and warning pipes are key components of any cistern, such as those found in your loft. In the event of ball valve failure, they can prevent serious structural damage to a property, while alerting the occupant to a fault. But what actually is the difference between an overflow pipe and a warning pipe? Is there even a difference?

What is the difference between an overflow pipe and a warning pipe?

The answer to this question lies primarily on the size of the cistern on which the pipe is fitted. On cisterns with a capacity of less than 1000 litres, the overflow pipe and warning pipe are the same thing and have the same purpose: to discharge excess water safely while alerting the resident that a cistern is overflowing. On such cisterns, the water level should be set to at least 25 mm below the overflow/warning pipe. The overflow pipe itself should be:

  • At least 19mm in diameter
  • Fitted at a constant fall
  • With a discharge point that is clearly visible

However, cisterns with a capacity greater than 1000 litres but less than 5000 litres require two separate pipes: a warning pipe and an overflow pipe, each with their own purpose. As the name implies, the purpose of the warning pipe is merely to alert people to a fault and to an impending overflow. The warning pipe should have a diameter of at least 25 mm, and should be at least 25 mm above the water level. It is typically smaller than the overflow pipe, which should be capable of evacuating all of the excess water under maximum fault conditions. The purpose of the overflow pipe is not only to prevent structural damage but also to prevent the inlet valve from becoming submerged. It should be positioned at least 25 mm above the warning pipe.

At the point of exposure outside a building, the warning pipe should be positioned below the overflow pipe as per the position of the pipes in the cistern, making it clear which pipe is which.

Note that cisterns with a capacity greater than 5000 litres have different regulations. For example, an electric alarm can be fitted in lieu of a warning pipe.