Unit 315 session 4 Storage cisterns

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The installation of large-scale cisterns differs somewhat from the cisterns in domestic properties. Large cisterns must be installed in accordance with the Water Supply (Water Fittings) Regulations 1999

Regulation 5 states that the water undertaker must be notified before the installation of large cisterns begins and it is important to remember that the correct backflow protection must be present in relation to the

fluid category of the contents of the cistern. 

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Large cisterns over 1,000 litres are made from a range of materials that can either be constructed in one piece or in sections. 

​Sectional cisterns can be tailored to fit any space or capacity and are assembled on site, usually in 1m² sections, then joined together with stainless steel bolts. 

requirements of large-scale cisterns

​Overflows for large cisterns are quite different
from those fitted to cisterns for domestic purposes.

The objective is the same – to warn that the float

operated valve is malfunctioning and to remove water

that may otherwise damage the premises. But with

larger cisterns, the potential for water wastage and

water damage is far greater.

Therefore, the layout is

different.

​The overflow/warning pipe on large-scale cisterns must:

● contain a vermin screen to prevent the ingress of insects and vermin

● be capable of draining the maximum inlet flow

without compromising the inlet air gap

● contain an air break before connection to a drain

● not be of such a length that it will restrict the flow of water, causing the air gap to be compromised

● discharge in a visible, conspicuous position.

​Alternatively, the warning pipe may be discarded

provided a water level indicator with an audible

or visual alarm is installed that operates when

the water level reaches 25 mm below the invert

of the overflow pipe.

​The size of the overflow pipe will depend

upon the type of air gap incorporated into the cistern and this will depend upon the fluid category of the cistern contents. It must be remembered that:

● if a type AG air gap (fluid category 3) is fitted, the

overflow diameter shall be a minimum of twice the

inlet diameter

● if a type AF air gap (fluid category 4) is fitted, the

minimum cross-sectional area of the overflow pipe

must be, throughout its entire length, four times the

cross-sectional area of the inlet pipe.