Unit 315 session 4 Storage cisterns

​Unit 315 Cold water systems

Session 6
Storage cisterns

Large cisterns

​

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. 

​One-piece large cisterns

​
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. 

​Sectional large cisterns 

Open Ended

What problems do you think the installation of a large storage cistern would give?

​Overflow and warning pipe

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.

Hotspot

Click on the spot where the following is relevant:

Air gap 2 × inlet diameter

Hotspot

Not less than 25 mm (2 answers)

Hotspot

Overflow pipe

Hotspot

Warning

pipe

Hotspot

Shut off

level

Open Ended

Research an 'AG air gap (fluid category 3)' and describe below.

Open Ended

Research an 'AF air gap (fluid category 3)' and describe below

Multiple Choice

Why is it important to have the correct backflow protection in large cisterns?

• To prevent water wastage

• To comply with Water regulations

• To avoid damage to the premises

• To facilitate drainage

Multiple Choice

How do you think sectional cisterns typically joined together during installation?

• Welding

• Adhesive bonding

• Stainless steel bolts

• Compression fittings

Multiple Choice

What is the primary objective of the overflow/warning pipe in large-scale cisterns?

• Increase water pressure

• Enhance drainage efficiency

• Facilitate water supply

• Warn about float valve malfunction

Multiple Choice

What specific requirement does the overflow/warning pipe on large-scale cisterns have to meet

• Contain a vermin screen

• Be made of stainless steel

• Restrict the flow of water

• Facilitate maximum inlet flow

Multiple Choice

What is the alternative solution mentioned in the text for the warning pipe in large-scale cisterns?

• Increasing the length of the warning pipe

• Installing a water level indicator with an alarm

• Using a larger fluid category

• Discarding the warning pipe entirely