Page 383 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 6 Hot water systems

Occasionally, the cold feed may be 28 mm if there is more than one bathroom
in the property. The cold feed should be fitted with a drain-off valve at the
lowest point to allow complete drain down of the hot water storage cylinder.
The connection of the cold feed to the cistern must be at least 25 mm above
any cold distribution pipework to ensure that, in the event of mains cold water
failure, the hot water runs out first.

The hot water draw-off should have a gradual rise towards the vent and must
be a minimum of 450 mm in length to prevent parasitic circulation occurring.
The vent pipe must rise vertically, terminate inside the cold water storage
cistern and be sealed by means of a rubber grommet; it should have no valve
installed anywhere along its length. The vent pipe and draw-off must be
installed in a minimum of 22 mm size pipe. Magnesium sacrificial anode
brazed to the bottom of the
The hot distribution pipework must be a minimum of 22 mm pipework to any hot water storage cylinder
large-volume appliances such as baths, but can be reduced in size to 15 mm
to supply kitchen sinks, washbasins and shower valves. It is good practice
to install isolating valves at the appliances, although it is not a requirement
of the Water Supply Regulations. The pipework should have a gradual p Figure 6.15 Sacrificial anodes
incline towards drain-off valves to permit total draining of the system for
maintenance and repair.
Where double feed indirect cylinders are installed, the primary system
must contain a separate feed and expansion cistern or expansion vessel and
disconnectable filling loop, which separates the primary water from the secondary
water. The cold feed to the primary system from the F and E (feed and expansion)
cistern can be installed in 15 mm pipework and must not contain any form of
isolation valve. The vent from the primary system must be installed in 22 mm
pipework. It should rise vertically and terminate over the F and E cistern. The
height of the vent pipe above the F and E cistern should not be less than 150 mm
plus 40 mm for every metre in height from the overflow level to the lowest point
of the cold feed pipe.

ACTIVITY
To calculate the height of the vent pipe above the feed and expansion cistern,
we must first determine the length from the overflow pipe to the lowest part
of the cold feed pipe at the cylinder. If the distance between them is, say,
4 m then the calculation is as follows:
4 × 40 + 150 = 310
So, the vent pipe must be taken above the overflow level 310 mm.
Now try it for yourself:
1 There is a distance of 6 m between the overflow level and the cold feed
connection on the cylinder. What is the recommended height of the vent pipe?
2 There is a distance of 3 m between the overflow level and the cold feed
connection on the cylinder. What is the recommended height of the vent pipe?
3 The height of the vent pipe above the F and E cistern is 350 mm. What
is the distance between the overflow level and the cold feed connection to
the cylinder?

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