DESIGN: DEAR MR PLUMBER
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maximum ‘dead leg’ lengths. This is a very sensible proposal that
will work much better. It will be necessary to interpret this dead-leg
length correctly, though, because the dead leg may consist of more
than one pipe size.
The dead leg from a geyser may consist of a length of 22mm
diameter up to the shower, and from there onwards, it could consist
of a 15mm diameter pipe to the furthest fixture, such as a WHB or a
sink. The total quantity of hot water in the combined 22mm/15mm
diameter pipe shall not be more than four litres.
The maximum dead-leg length of 15mm diameter pipe is 27.6m
long and is 0.145ℓ per metre. The maximum dead-leg length of
22mm diameter pipe is 12.5m long and is 0.320ℓ per metre.
The designer or plumber must therefore be careful and calculate
the correct combined length of 15mm and 22mm with four litres of
water, so that not more than four litres of water is wasted before it
is at 55°C, as per the legionella requirements.
Please note that the dead-leg length of a 15mm diameter pipe is
more than double the length of a 22mm diameter dead leg, but
both have the same capacity of water. Most of the geysers that are
mostly used in housing and buildings that do not require central
hot water systems do not have connections for a circulation system
and, therefore, it can only be a dead-leg system.
In a dead-leg system, the requirement of 55°C at the furthest
fixture is not a problem, as the total length of the piping from the
geyser is four litres, and more than one geyser will be required
in a large house, flat buildings, or in office buildings where single
geysers are used.
If we are really serious about water and energy conservation,
then we must make it compulsory that all geysers must have
connections for a flow and return hot water reticulation system li