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Discharges from un-vented
cylinders |
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| The 1986 Model Water Bye-laws stipulate that unvented
storage water heaters should incorporate a means of accommodating
expansion, and a factory-fitted temperature relief valve. This
requirement has not changed with the new Water Regulations.
Paul Rivett, Technical Director, Heatrae Sadia Heating Limited
discusses the issues. |
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| Before 1987. cistern-fed (vented) water heating
systems were the norm in this country whereas in many parts
of the world, including Europe and North America, un-vented
cylinders which fed directly from the cold water mains were
generally used. The superior performance of the un-vented systems
in terms of pressure and flow was apparent to any visitor to
those countries where they were permitted. |
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The Model Water Bye-laws made it permissible.
for the first time, to connect hot water systems directly to
the cold water supply in Britain. However, Bye-law 91 called
for a means of accommodating expansion as the water heats up.
This measure was introduced to prevent waste. In contrast, the
European approach permitted -and still permits -expanded water
to be discharged to waste. British manufacturers responded by
developing systems which allow the water to expand against a
cushion of air using either an external expansion vessel or
an internal air-gap.
Over the intervening years, the need for water conservation
has become increasingly important. Nevertheless. there was
substantial pressure on the Water Regulations Advisory Committee
to change this requirement and permit European-style expansion
to waste. |
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Water conservation
A figure of 1000 litres per year is widely used as the volume
of water that would be discharged from the average un-vented
water heater in a year if it was allowed to discharge to waste.
This is based on one heating cycle per day in a typical family
home of three people. The water use by this family is taken
as 250.000 litres per year.
In practice. one cycle per day is probably an unrealistic
figure. Furthermore, any leakage through the expansion valve
could increase the discharge rate to as much as 10,000 litres
per year. It is often argued that 1000 litres per year is
small compared with the amount of water consumed by garden
sprinklers, or wasted by incorrectly adjusted toilet siphons.
However, in a climate where the public is constantly being
exhorted to avoid waste from dripping taps and inefficient
showers, and where appliance manufacturers are urged to design
their products to avoid waste, any relaxation in the requirement
to conserve expanded water would have been untenable.
If the 4 -5 million new homes due to be built over the next
decade all had European-type un- vented water heaters discharging
1000 litres a year, the loss would be equivalent to the average
water requirements of a town of 50,000 people! |
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Un-vented system with expansion to
waste
The term 'operational discharge' has been coined to describe
the discharge of expanded water from European type unvented
systems. Whatever term is used, it cannot alter the fact that
the water is discharged to waste! The term does, however,
underline the fact that the expansion valve is being utilised
in a completely different way. In British un-vented water
heaters the expansion valve is a safety device that only opens
under exceptional conditions (for example if the expansion
air-gap collapses) to allow a direct discharge. In European
systems the valve is in daily use. |
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This daily passage of water through the expansion
valve could result in a build-up of lime scale, or erosion
of the valve seating, causing continuous leakage through the
valve. Annual maintenance, and possibly replacement of the
expansion valve, would be essential. Without regular maintenance,
the discharge through the expansion valve could increase ten-fold.
Safety and reliability
There are also safety implications associated with the European
approach especially in very cold weather when water, trickling
to waste through the discharge pipe, could freeze and block
the pipe. As a result water would back up in the pipe until
it overflowed at the tundish. With the British system the
likelihood of a discharge coinciding with freezing conditions
is an acceptable risk; with the European system, discharge
takes place daily throughout the year. |
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| British v European
practice |
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The differing approaches to water heating in
Britain and Europe are considerable, European householders
are more accustomed to regular maintenance of water heating
systems, Annual maintenance of the expansion valve, and replacement
when it is worn, is therefore accepted. In Britain, we are
not used to annual maintenance of water heaters. The risk
that expansion valves will be neglected is high.
The cost of replacing an expansion valve could be expensive
and should be taken into account when considering the economics
of .operational discharge' versus accommodation of expanded
water .
Another difference between Britain and Europe relates to
the location of water heating cylinders. In Europe, it is
common to install the cylinder in visible locations in basements
or utility rooms. Internal spillage is likely to be noticed
and even it is not, the damage will be minimal. In Britain
it is common practice to conceal the cylinder in an airing
cupboard on the first floor of a dwelling, sometimes directly
above the electricity meter and consumer unit. The damage
to decorations, structures. electrical systems and building
contents could be extensive if there was internal spillage.
In some parts of Europe, there is increasing interest in
the British approach, in the interests of water conservation. |
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| Factory fitted safety devices |
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Another major difference between British and
European practice is the fitting of safety devices. Britain
is the only county where it is mandatory to supply un- vented
water heaters with factory-fitted safety devices. There have
been a number of fatalities in Europe where the appropriate
safety devices were not installed correctly.
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| It is sometimes argued that a temperature relief
valve designed to open before 95 degrees C is an unnecessary
expense because there are already adequate safety devices. Heatrae
Sadia believes that the added protection is justified and that
a combined temperature and pressure relief valve (typically
operating at 90 degree C or 10 bar) is even better. This view
is apparently shared in Australia and the USA where both temperature
and pressure relief are mandatory. |
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| Conclusion The British approach
to unvented water heating makes sense on grounds of safety
and water conservation. There are a number of parallels to
be drawn between this and the situation with the 13A plug
and socket system where, again British industry resisted efforts
to force the UK to change to an inferior European system. |
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