Page 294 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Materials for domestic cisterns

Almost all new installations use cisterns made from plastics such as
polyethylene, polypropylene and glass-reinforced plastic (GRP).
Most cisterns manufactured today are made from polypropylene because this allows:

● lightweight construction
● strength
● hygiene
● resistance to corrosion
● flexibility, as they can easily be handled through roof space openings.
Cisterns are available either square, rectangular or circular in shape, and are
produced in black to prevent the growth of algae. However, because they are
flexible, the base of the cistern must be fully supported throughout its entire
HEALTH AND SAFETY length and width.
On no account must any Holes for pipe connections should be cut out using a hole saw, and not by using
linseed oil-based jointing
compounds be used as a heated section of copper pipe and using it to make a hole in the cistern. Doing
this also breaks down the latter alters the molecular structure of the plastic and will result in the
the plastic and provides cistern cracking. The joint between the cistern wall and fitting should be made
a culture where micro- using plastic or rubber washers.
biological growth such as
Legionella pneumophila Galvanised steel cisterns were used for many years, but these were notorious for
(Legionnaires’ disease) corrosion. They are still manufactured and can still be used, provided the inside
can occur. of the cistern is protected by the use of a special paint that is registered by the
Water Fittings and Materials Directory as safe to use with potable water.

Warning and overflow pipes

Simply put, the difference between a warning pipe and an overflow pipe lies
in the fact a warning pipe has a smaller diameter than an overflow pipe. It is
intended to act as a warning that the float-operated valve has malfunctioned
and the cistern is about to overflow.
An overflow pipe has a larger diameter than a warning pipe and should be able
to carry the excess water that would be present if the inlet valve (float-operated
valve) fails completely and lets in the maximum amount of water possible. This
should ensure that the inlet valve will never become submerged in water and
the cistern will not flood the area in which it is situated.

Warning and overflow pipes should run to a point outside of the building that is
clearly visible and below the level of the storage cistern. Warning pipes should
be situated below the overflow pipe so it is obvious which pipe is which.

Overflow and warning pipe requirements for cold water cisterns vary with the
storage volume of the cistern installed.

Cisterns up to 1000-litre capacity
Cisterns below 1000-litre capacity require a single combined warning and
overflow pipe. The bottom of the combined warning and overflow pipe should
be a minimum of 25 mm above the water level of the cistern.

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