Plumbing Systems: Domestic Storage, Rising Main, Stopcock and Drainage

Domestic Storage

Modern domestic water services are based on provision of a cold-water storage cistern, usually placed in the loft. The size of cistern, usually of between 227 litres to 363 litres capacity, depends on the needs of the household. A family of four requires a minimum capacity of 227 litres.

The storage cistern is there to balance, at any one time, the load on the mains supply in the district. For example, if a group of households were together to draw off a large amount of water, this could lead to a possible loss of mains pressure. As the storage cistern can only recharge at an even rate, and serves most of the household needs, this ensures that there can be no excess burden on the mains.

This system utilising loft storage is known as the ‘indirect system’. Older, direct-to-tap systems, known as the ‘direct system’ are not now approved.

The main advantage of the indirect system is that it provides a reservoir of water, even if there is a break in supply, or the water has to be shut off. Also, mains supply comes in at a very high pressure and causes more wear on pipe bores and fittings and is more likely to produce ‘water hammer’, a noisy vibration of pipework.

As nearly all the internal services are served from the storage cistern at low pressure, there is little likelihood of noise or wear. The main basic service which may be supplied directly from the rising main is the kitchen, providing a source of fresh, non-storage water at the cold-water tap.

The consumer’s responsibility for the domestic service starts beyond the company stopcock, usually located in the pavement outside the home. Because the access points can get covered in during pavement resurfacing or other work, it is a good idea to provide your own stop cock just inside the premises if it is at any time necessary to shut down the supply to the home.

Rising main

The supply pipe to the house from the company stopcock should be buried deeply enough in the ground to be clear of damage from garden implements and also be below what is called the ‘frost line’-a depth of 460mm. The supply in the home is called the ‘rising main’ and is usually provided by a pipe of 15mm bore, unless the house is rather larger than average or if you live in a low-pressure area, where a 22mm-bore pipe may be needed.


A stopcock should also be fitted into the rising main just inside the home. A common position is beneath the kitchen sink. Wherever it is fitted, it should be accessible so that water can be turned off quickly in an emergency.

The stopcock is known as a non-return valve, which means that water can flow in only one direction; this is marked on the body of the stopcock with an arrow. It is important to fit this in the correct position, or water will not flow.

To avoid any possibility of pollution, there must be no prospect of back-flow to the mains from the domestic system, hence the non-return function of the stopcock.

After supplying the kitchen sink, the rising main should be taken by the most direct route possible to supply the storage cistern. In most modern plumbing systems, domestic hot-water needs are met from a hot-water cylinder, which may be heated in a variety of ways. Draw-off hot water is replenished from the storage cistern in the loft.

This cistern must be capable of being drained down faster than it can be filled, so that it can in no circumstances overflow. Supply to the cistern and to toilets is controlled by a ball valve, in which a plastic or copper airtight ball floats on the water (called a float) and rises with the water to lift an arm controlling an inlet valve. The arm is capable of some adjustment to set the level of water.

Cisterns must also be fitted with overflows, the outlets of which should be located in a prominent position outside the house to give an ‘early warning’ of trouble.

Shut-off or gate valves should be incorporated into the plumbing circuit so that, in particular, the cistern and the supply to the hot cylinder can be isolated where necessary.


The other essential half of the plumbing system is drainage. This falls into two categories – that above and that below ground. The system above ground consists of pipework from sinks, WC’s, baths and gutters. Below ground, the services are known collectively as ‘drains’.

Traps, which act as water seals, are fitted to outlets going into the drainage system; these prevent foul sewage air from percolating the home. Traps consist of two types-tubular and bottle; these are made in shallow-seal and deep-seal versions for differing circumstances. Traps have a tubular plug or a detachable bottle to facilitate the removal of any blockages.

WC pans have one or two types of trap-the ‘p’ or the V trap, roughly resembling these letters. The type used depends on whether the outlet connection is through the floor or goes through a wall. Left-hand and right-hand wall and back-wall exits are made.

Traps known as trapped yard gullies are used to connect kitchen and bath waste outlets to the main drainage and have a ‘u’ bend to provide the water seal.

Under modern building regulations main pipes, normally with a diameter of 102mm, which connect baths, toilets and basins, to the drains must be located inside the house structure. This presents a less unsightly exterior appearance but was introduced as a safeguard against freezing up in cold weather.

As water seals prevent noxious gases from entering the home, they have to be vented to the atmosphere. This is provided through a vent pipe positioned about lm above the highest window in the home or where there is no likelihood of permeation of smells.

10. November 2011 by admin
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