outlet, so that the volume of water that must be
cleared out of the pipe at the beginning of the
draw is very small.
Figure 3, on page 63, shows a structured
plumbing system that uses the cold water line as
a temporary return. This is the most common configuration in retrofit applications, where you are
unlikely to improve the layout of the plumbing.
Why on-demand pumps? They are the most
energy-efficient way to have the benefits of a hot
water circulation system.
Once we have improved the hot water-distribution system, it is possible to make the way
we use hot water significantly more efficient as
described above. There are several interactions
that we need to account for. If the hot water delivery is not improved first, the time-to-tap could
easily double, triple or even quadruple! Likewise,
if we install a low-flow-rate showerhead (less than
1.75 gpm) or reduce the hot water temperature,
the hot water flow rate will drop below that needed to trigger the typical gas tankless water heater.
In addition, installing a drain-water heat-recovery
system on the showers can capture roughly half of
the temperature rise from incoming cold water to
the mix-point temperature of the shower. Showers represent 75 to 80 percent of a household’s hot
Don’t believe that the
energy is free because we
have installed a solar water
heater! It makes sense to
minimize the waste, because
if it is small enough, we may
be able to install a smaller
solar system and get more
value from the water
water use, so this is a good strategy. However, a
drain-water heat-recovery system will reduce our
demand for hot water, so we must consider that
effect in our distribution system strategy, as well as
on the choice of supplemental water heater — will
it work properly with low flow rates?
Wringing out the wastes and using water more
efficiently as we’ve described can reduce overall
hot water demand by 25 to 50 percent. That
means the actual amount of hot water desired
for use by the typical household of three could be
30 to 45 gallons (114 to 170 liters) per day. Sure,
we’re likely to see some take-back effect when
people have a “free” source to heat their water.
But reducing the amount of hot water needed
also reduces the size of a solar water-heating sys-
tem required for the facility or home.
Living With — or Without — Backup
Will you have a supplemental heater, or will
you live within the means the sun can supply?
Imagine that you have a hot date Saturday night.
It involves the big tub, soft candlelight and a special meal. You awake Saturday morning and realize that you barely had enough hot water for your
shower. You remember that it has been partly
cloudy all week, and it looks like it will be overcast
all day. Are you keeping the date, or will you postpone it until the sun is more accommodating?
If you answered, “I am keeping that date!”
you need a supplemental heater.
In the November/December issue, we look at
what type of supplemental heater is the best technical match for your solar water heating system and
how to size it. ST
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WE MEAN BUSINESS
84 September/October 2010 SOLAR TODA Y solartoday.org
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