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One area
which has not been discussed much is the use of domestic hot water (DHW), and
how to reduce energy used to heat the water. However, its role is getting more
and more important, especially in residential buildings. The share of energy
used to heat the DHW has been increasing significantly as the thermal
performance of building envelope has been improving. The heating energy for DHW
depends on the use of water, which has decreased significantly during the last
few decades. Twenty five years ago the average use was around 200 litres
per day per occupant in residential buildings; now it is 120–140 litres
(113 to 133 litres per day per person in Germany – article by Seybold et
al in this issue). The reduction is partly due to water saving faucets, new
washing methods and even more importantly, the common use of a shower instead
of a bath tub for better personal hygiene. About 40% of the domestic water is
used as hot water. It can correspond to 30–40 kWh/m² of energy use in
residential buildings, which is significant (25%) in relation to energy use in
the EU buildings that is typically in the range of 100–150 kWh/m². The share is even higher in low energy and nearly zero energy
buildings: about 50% in single family buildings and more than 50% in
multifamily buildings.
An
interesting article by Agudelo-Vera et al in this
issue reports that the water systems are oversized, and that new design rules
should be established. Water use, including hot water, depends, of course, on
the occupants, their behaviour and customs. Not much information is available
on the variation of real use between countries. However, the European
Commission has based the sizing of water heaters on water use profiles. An
article by Klobut in this issue gives an overview of
the contents of the recently adopted ecodesign
regulations for water heaters. Occupant behaviour can be influenced and DHW
saved by charging the actual heating cost from occupants based on metered use.
Metering can be installed in new buildings as required by EPBD, but is a
problem in existing buildings.
Water usage
profiles depend on the day, time and month in addition to the building type and
occupants. The information on profiles is becoming more and more important for
the efficient use of renewable energy sources for heating. Solar heating of DHW
is, in most cases, the most cost effective use of renewable energy. Water use
profiles affect the sizing of the collector, storage tanks and back up heating.
An important issue to keep in mind with solar water heating systems is the
possibility of low temperature level (below 55°C) in the system, which may
allow the growth of Legionellabacteria in the plumbing system.
Water use
can be further reduced by user behaviour and wider use of water saving
technology, like easy to operate and automatic faucets and well-designed shower
heads. The reduction in water use may have an effect on the waste water system.
The sewage pipes and their slopes are designed for a minimum water flow to stay
clean. The reduced water flow may have an effect on sewage system design. An
opposite trend, however, is the luxury bathrooms with private jacuzzis and rain showers. Domestic hot water systems can
also ‘leak’ heat through the uninsulated or
insufficiently insulated piping, and the bath room or
towel heaters which circulate high temperature hot water year around.
Heat
recovery is a widely used technology; it is also possible with waste water. An
article by Seybold et al focuses on that and reports significant saving
potential, but also points out the problems with the fouling of heat transfer
surfaces and the need to develop automatic cleaning methods. The reduced sewage
water temperature may also have an effect on the performance of sewage
treatment plants.
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