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Calculating Cold Water Storage Requirements

CIBSE Guide G (2004) Public Health Engineering gives data for calculating cold water storage requirements for various buildings.

Table 2.2 gives 24 hour storage requirements based on various fittings, e.g. Shower 140-230 litres, Bath 900 litres, WC 180 litres, Basin 90 litres, Sink 90-180 litres, Urinal 110 litres.

Where water storage is to be located in each domestic dwelling, this should be provided by a cold water storage cistern mounted in the roof space or similar area with a storage to water line of at least 227-300 litres.

The cistern should be protected from frost and designed to maintain the water quality.Table 2.3 gives recommended minimum Storage of Cold Water for hot and cold water services as shown below.

BS6700 (2006) also gives Recommended minimum storage of cold water for domestic purposes (hot and cold outlets) in Table 1.

Example 1

In a house with 1No. bath, 1No. sink, 1No.basin, 1No. shower and 1No.WC the cold water storage could be sized using Table 2.2 as detailed above.

Using minimum storage requirements for a 24 hour supply of cold water gives; Bath = 900 litres, Sink = 90 litres, Basin = 90 litres, Shower = 140 litres WC = 180 litres. Total = 1400 litres

This 1400 litres is too much storage for a house.

The older CIBSE B (1986) Table B4.2 gives storage at 90 litres per person in a house for 24 hours.

Storage Required = 90 x 5 people = 450 litres

From Institute of Plumbing Guide Table A11, the nearest tank size is SCM 680, which has an actual capacity of 491 litres to the water line, 680 litres is the nominal capacity of the tank.

The dimensions of this rectangular tank are 1092mm x 864mm x 736mm high.Table A12 shows the equivalent tank as a circular Polythene or Polypropylene cistern. The cistern in this case would be PC 100 with an actual capacity of 455 litres and a height of 760mm.

The statement in CIBSE Guide G (2004) section 2.4.3.1 gives domestic storage at 227-300 litres.

A typical circular storage tank is shown below.

Circular Cold Water Storage Tank

An air vessel is shown in the above diagram on the mains water pipe to the tank. This is used to reduce water hammer as shown below.

The consumption of cold water in buildings is based on the number of occupants. For most dwellings a cold water storage of 227 litres capacity is sufficient.

Example 2

To size the cold water storage tank(s) for a commercial/industrial building the method usually adopted is based on Table 2.3 in the CIBSE guide G (2004) as shown above. For example for a hotel with a maximum of 50 occupants the water storage would be: 135 x 50 = 6750 litres.

The largest galvanised steel tank capacity in the Institute of Plumbing guide table A11 is 3364 litres.

Therefore: 2No. steel tanks of capacity each 3364 litres could be used giving a total water capacity of:

2 x 3364 = 6728 litres

Each tank dimensions are (A11) :

2438mm long x 1524mm wide x 1219mm high.

Note:

It is possible to have a non-storage system for cold water in a building although there would be no security of supply of cold water.

This means that there is no cold water storage tank in the building.

There would be no 24 hour storage capability so that if the water main was turned off, then WC’s could not flush and other sanitary items would be inoperable.

This is only possible in a small building where the mains water system is secure and is not turned off regularly and water is not required for important reasons.

A non-storage system is not recommended for commercial buildings but has been tried in some domestic systems.

There are some advantages in not storing water in a building, one is that there is no risk of frost damage and another is that there is less risk of water borne bacterial growth.

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