Rising main

The rising main is a pipe in the home which delivers fresh mains water to the highest part of the plumbing system, most likely the cold water storage cistern in the loft. It will also supply the feed and expansion cistern, if there is one (which there will be if the home has an open vented central heating system).

The rising main may also supply other outlets, depending on the cold water system. In a direct system, the bathroom cold taps are fed by the mains. In an indirect system, the bathroom cold taps are fed by the cold water storage cistern. In both systems, the mains feeds the kitchen tap and any other device in the vicinity, such as a water softener or an outside tap.

The rising main can be susceptible to what is known as water hammer: vibration, banging, or shuddering noises from pipes. When pressurised water or indeed any fluid in a pipe is stopped suddenly, the pressure in the pipe suddenly increases. This pressure surge creates resonance, which can manifest in these unwelcome vibrations and sounds. This can of course happen to any section of pipe carrying mains water. The cause may be a tap or a ballcock closing too suddenly, or an improperly secured pipe.

The advantages of an indirect system are that there is a reserve of water to mitigate any disruption to the mains, and that the system is less likely to be vulnerable to water hammer, due to the lower pressure water. The primary advantage of a direct system is that water from outlets fed by the mains is usually drinkable.

rising main can also refer to a pipe through which sewage is pumped so that it can gravitate to a lower point.

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