In this article I will tell you a bit more about the cold water cistern used in cold water plumbing. As you probably already know, the rising main and the cold water cistern are connected through a ball-valve. A ball-valve has a ball of either metal or plastic that floats on the water surface and closes off and/or turns off the valve depending on water levels. A cold water cistern has an over-flow pipe, a so called warning-pipe and this can be seen sticking out from the house up by the eaves. If you notice a drip or flow coming out from this pipe, something is wrong and the issue should be looked in to. Because this ball-valve is connected to the mains, it has to be a high-pressure one. This differs from a WC ball-valve used in an indirect system.
A cold water cistern should be kept out of light and fitted with a lid so no insects can get it. It should however not be airtight. Since the cistern will provide the entire house with water it will usually have two mains feeds. There is one feed for your cold water taps (except from the one in the kitchen) and it also goes to the WCs and another feed for the hot water system. You can see these feeds as pipes coming from the cistern and sometimes you will notice further feeds, fitted for a bidet or shower. Each feed coming from the cold water cistern should have its own valve and this is so you can easily switch them of, should you need to do any work on them. Unless this is done, you will have to drain the entire system before starting the work. Cold water feeds should be fitted with gate-valves near the cistern whilst the cold feed leading to the hot water system should be fitted by the hot water cylinder instead. Make sure that all feed pipes are 22 mm to allow for a good flow. The reason behind having separate feeds is purely for safety reasons. A lack of separate feeds is most often noticeable when the water in the shower gets uncomfortably hot if someone is flushing the toilet or running the cold water tap in the kitchen.
The recommended size for a cold water cistern is 230 litres, or 50 gallons if you so will, for the actual capacity of water it can hold. This correlates to a nominal capacity of 320 litres, or 70 gallons. The reason for this quite generous capacity is so that an average household can continue normal day-to-day life, should there be a mains failure. If you come across a cistern that is too small, you can have a second cistern installed next to it. The cisterns then have to be connected at low level, by two 28 mm pipes.