The rising main is a pipe in the home which delivers fresh mains water to the highest part of the plumbing system. This is usually the cold water storage cistern or the feed and expansion cistern. Both cisterns are usually located in the loft.
Rising main overview
Water for a property comes from the mains in the street. It will then flow through the external stopcock under a service hatch. This may also contain the water meter, if the property is on a tariff that uses one.
Mains water then flows through a stopcock inside the property. This pipe is typically 15 mm copper. The pipe is tee’d off and supplies the kitchen sink with cold water. In a typical home with cisterns in the loft, it then rises up to the roof space where it supplies the cold storage cistern and/or the feed and expansion tank
Direct and indirect systems
Depending on the cold water system, the rising main may also supply other outlets in the home on its way up to the loft. 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.
However, 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 stops suddenly, the pressure in the pipe also 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 advantage of an indirect system is that there is a reserve of water to mitigate any disruption to the mains. Plus, the system is quieter and 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.
A rising main can also refer to a pipe through which sewage is pumped so that it can gravitate to a lower point.