Water Heating Renewables
When you have a mixture of heat
sources that have different limitations
and different hot water temperature
outputs, you must make sure they are
not fighting each other
– Yan Evans
So it is important to consider the management of the
risk of legionella carefully, says Evans: “For example,
cycling at midnight might sound like a logical time to
An example of a
do it. But if you have a very well insulated solar cylinder
solar collector
and there is no draw off overnight, come six o’clock in
the morning, when you have run your legionella cycle
at midnight, there is no load to offer the solar on that
day when the sun comes up.”
When it comes to ‘clean’ or ‘dirty’ energy sources, Ant
Wilson of consultancy Faber Maunsell | AECOM insists
that consultants should “think electric” if they’re serious
about sustainability. “At the moment, we say electricity is
not sustainable because it is a secondary energy (because
we use gas and coal to generate it). But if we didn’t use
these fuel sources, electricity would not be seen as dirty,”
he says. “You might say electricity is too expensive to heat
water, but if you went onto, say, a half-hourly tariff, you
> the vessel that helps the heat transfer.” But it has to could take advantage of cheap electricity.” That implies
be done in accordance with UK water and health and a need for water storage because it would involve using
safety regulations, he points out. electricity directly, like a kettle.
If you are preheating a direct-fired storage water There is an alternative way of using electricity,
heater, then the water heater will be storing the water says Wilson: “You could use an air-to-water heat
at 60
0
C. Typically once a week the solar control unit is pump. If we can put in one unit of electricity and
made to switch on a shunt pump that moves the hot get out three units of heat – and some of the newer
water at 60
0
C between the storage inside the water heat pumps allow this – then surely a heat pump
heater to the bottom of the preheat cylinder to kill any solution driven by electricity would be a good way
legionella that may be present. to heat hot water. Linking this with a large store
However, Evans adds: “Some people get a little to flatten out the demand for electricity could then
paranoid about legionella. They like to run legionella save money.” l
cycles every day. If you take your solar thermal up to
temperature via that route once a day you are effectively The CIBSE Solar heating design and installation guide
taking the load away from the solar thermal system, (Domestic Building Services Panel) is available from
negating its effectiveness.” the CIBSE Bookshop at www.cibse.org
Dry cycling Saving energy through intelligent control
Even when standard controls and a building from the boiler, without contributing to the “Intelligent boiler load optimisation is
management system are in place, many building heating and hot water demand. able to recognise and identify dry cycling by
boilers waste energy through dry cycling, Willis says: “Taking effective control constantly monitoring the boiler’s thermal
according to Tony Willis (pictured), technical of dry cycling can reduce the boiler’s fuel response to changing loads every 10
sales director with Sabien consumption by as much as 25 per cent, the seconds.
Technology. However, operative word being ‘effective’. To do so it’s “Two digital temperature probes measure
he claims that his necessary to optimise the system through and monitor the boiler flow and return
company has a solution. intelligent analysis of the boiler’s flow and temperatures. The onboard software
Dry cycling occurs return water temperatures. This can be calculates the temperature gradient over
when boilers fire to achieved by retrofitting an ‘intelligent boiler time and determines when the boiler should
compensate load optimisation’ system that will minimise fire for true building/heating demand and
for standard dry cycling while ensuring the boiler’s when it should remove/inhibit unnecessary
heat losses designed set point temperature. boiler firing and energy consumption.”
48 CIBSE Journal April 2009 www.cibsejournal.com
CIBSEapr09 pp44,45,46,48_water.indd 48 2/4/09 15:49:22
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