SOLAR TODAY - September/October 2010

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
it produces.

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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